Monster Disc 2: The Best of 1992/MONSTER2.ISO/prog/djgpp/djgcc222.a04/DOCS/GCC.TEX

Text File | 1992-07-06 | 79.8 KB | 2,041 lines

ages complain of mismatched
psect attributes.  You can ignore them.  @xref{VMS Install}.

@cindex Alliant
@item
On the Alliant, the system's own convention for returning structures
and unions is unusual, and is not compatible with GNU CC no matter
what options are used.

@cindex RT PC
@cindex IBM RT PC
@item
On the IBM RT PC, the MetaWare HighC compiler (hc) uses yet another
convention for structure and union returning.  Use
@samp{-mhc-struct-return} to tell GNU CC to use a convention compatible
with it.

@cindex Vax calling convention
@cindex Ultrix calling convention
@item
On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved
by function calls.  However, the C compiler uses conventions compatible
with BSD Unix: registers 2 through 5 may be clobbered by function calls.

GNU CC uses the same convention as the Ultrix C compiler.  You can use
these options to produce code compatible with the Fortran compiler:

@smallexample
-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5
@end smallexample
@end itemize

@node Incompatibilities
@section Incompatibilities of GNU CC
@cindex incompatibilities of GNU CC

There are several noteworthy incompatibilities between GNU C and most
existing (non-ANSI) versions of C.  The @samp{-traditional} option
eliminates many of these incompatibilities, @emph{but not all}, by
telling GNU C to behave like the other C compilers.

@itemize @bullet
@cindex string constants
@cindex read-only strings
@cindex shared strings
@item
GNU CC normally makes string constants read-only.  If several
identical-looking string constants are used, GNU CC stores only one
copy of the string.

@cindex @code{mktemp}, and constant strings
One consequence is that you cannot call @code{mktemp} with a string
constant argument.  The function @code{mktemp} always alters the
string its argument points to.

@cindex @code{sscanf}, and constant strings
@cindex @code{fscanf}, and constant strings
@cindex @code{scanf}, and constant strings
Another consequence is that @code{sscanf} does not work on some systems
when passed a string constant as its format control string or input.
This is because @code{sscanf} incorrectly tries to write into the string
constant.  Likewise @code{fscanf} and @code{scanf}.

The best solution to these problems is to change the program to use
@code{char}-array variables with initialization strings for these
purposes instead of string constants.  But if this is not possible,
you can use the @samp{-fwritable-strings} flag, which directs GNU CC
to handle string constants the same way most C compilers do.
@samp{-traditional} also has this effect, among others.

@item
@code{-2147483648} is positive.

This is because 2147483648 cannot fit in the type @code{int}, so
(following the ANSI C rules) its data type is @code{unsigned long int}.
Negating this value yields 2147483648 again.

@item
GNU CC does not substitute macro arguments when they appear inside of
string constants.  For example, the following macro in GNU CC

@example
#define foo(a) "a"
@end example

@noindent
will produce output @code{"a"} regardless of what the argument @var{a} is.

The @samp{-traditional} option directs GNU CC to handle such cases
(among others) in the old-fashioned (non-ANSI) fashion.

@cindex @code{setjmp} incompatibilities
@cindex @code{longjmp} incompatibilities
@item
When you use @code{setjmp} and @code{longjmp}, the only automatic
variables guaranteed to remain valid are those declared
@code{volatile}.  This is a consequence of automatic register
allocation.  Consider this function:

@example
jmp_buf j;

foo ()
@{
  int a, b;

  a = fun1 ();
  if (setjmp (j))
    return a;

  a = fun2 ();
  /* @r{@code{longjmp (j)} may occur in @code{fun3}.} */
  return a + fun3 ();
@}
@end example

Here @code{a} may or may not be restored to its first value when the
@code{longjmp} occurs.  If @code{a} is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.

If you use the @samp{-W} option with the @samp{-O} option, you will
get a warning when GNU CC thinks such a problem might be possible.

The @samp{-traditional} option directs GNU C to put variables in
the stack by default, rather than in registers, in functions that
call @code{setjmp}.  This results in the behavior found in
traditional C compilers.

@item
Programs that use preprocessor directives in the middle of macro
arguments do not work with GNU CC.  For example, a program like this
will not work:

@example
foobar (
#define luser
        hack)
@end example

ANSI C does not permit such a construct.  It would make sense to support
it when @samp{-traditional} is used, but it is too much work to
implement.

@cindex external declaration scope
@cindex scope of external declarations
@cindex declaration scope
@item
Declarations of external variables and functions within a block apply
only to the block containing the declaration.  In other words, they
have the same scope as any other declaration in the same place.

In some other C compilers, a @code{extern} declaration affects all the
rest of the file even if it happens within a block.

The @samp{-traditional} option directs GNU C to treat all @code{extern}
declarations as global, like traditional compilers.

@item
In traditional C, you can combine @code{long}, etc., with a typedef name,
as shown here:

@example
typedef int foo;
typedef long foo bar;
@end example

In ANSI C, this is not allowed: @code{long} and other type modifiers
require an explicit @code{int}.  Because this criterion is expressed
by Bison grammar rules rather than C code, the @samp{-traditional}
flag cannot alter it.

@cindex typedef names as function parameters
@item
PCC allows typedef names to be used as function parameters.  The
difficulty described immediately above applies here too.

@cindex whitespace
@item
PCC allows whitespace in the middle of compound assignment operators
such as @samp{+=}.  GNU CC, following the ANSI standard, does not
allow this.  The difficulty described immediately above applies here
too.

@cindex apostrophes
@cindex '
@item
GNU CC will flag unterminated character constants inside of preprocessor
conditionals that fail.  Some programs have English comments enclosed in
conditionals that are guaranteed to fail; if these comments contain
apostrophes, GNU CC will probably report an error.  For example,
this code would produce an error:

@example
#if 0
You can't expect this to work.
#endif
@end example

The best solution to such a problem is to put the text into an actual
C comment delimited by @samp{/*@dots{}*/}.  However,
@samp{-traditional} suppresses these error messages.

@cindex @code{float} as function value type
@item
When compiling functions that return @code{float}, PCC converts it to
a double.  GNU CC actually returns a @code{float}.  If you are concerned
with PCC compatibility, you should declare your functions to return
@code{double}; you might as well say what you mean.

@cindex structures
@cindex unions
@item
When compiling functions that return structures or unions, GNU CC
output code normally uses a method different from that used on most
versions of Unix.  As a result, code compiled with GNU CC cannot call
a structure-returning function compiled with PCC, and vice versa.

The method used by GNU CC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar.  A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack).  The machine-description macros @code{STRUCT_VALUE} and
@code{STRUCT_INCOMING_VALUE} tell GNU CC where to pass this address.

By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted.  GNU CC does not use this method because it is
slower and nonreentrant.

On some newer machines, PCC uses a reentrant convention for all
structure and union returning.  GNU CC on most of these machines uses a
compatible convention when returning structures and unions in memory,
but still returns small structures and unions in registers.

You can tell GNU CC to use a compatible convention for all structure and
union returning with the option @samp{-fpcc-struct-return}.
@end itemize

@node Disappointments
@section Disappointments and Misunderstandings

These problems are perhaps regrettable, but we don't know any practical
way around them.

@itemize @bullet
@item
Certain local variables aren't recognized by debuggers when you compile
with optimization.

This occurs because sometimes GNU CC optimizes the variable out of
existence.  There is no way to tell the debugger how to compute the
value such a variable ``would have had'', and it is not clear that would
be desirable anyway.  So GNU CC simply does not mention the eliminated
variable when it writes debugging information.

You have to expect a certain amount of disagreement between the
executable and your source code, when you use optimization.

@cindex conflicting types
@cindex scope of declaration
@item
Users often think it is a bug when GNU CC reports an error for code
like this:

@example
int foo (struct mumble *);

struct mumble @{ @dots{} @};

int foo (struct mumble *x)
@{ @dots{} @}
@end example

This code really is erroneous, because the scope of @code{struct
mumble} the prototype is limited to the argument list containing it.
It does not refer to the @code{struct mumble} defined with file scope
immediately below---they are two unrelated types with similar names in
different scopes.

But in the definition of @code{foo}, the file-scope type is used
because that is available to be inherited.  Thus, the definition and
the prototype do not match, and you get an error.

This behavior may seem silly, but it's what the ANSI standard specifies.
It is easy enough for you to make your code work by moving the
definition of @code{struct mumble} above the prototype.  It's not worth
being incompatible with ANSI C just to avoid an error for the example
shown above.
@end itemize

@node Non-bugs,, Disappointments, Trouble
@section Certain Changes We Don't Want to Make

This section lists changes that people frequently request, but which
we do not make because we think GNU CC is better without them.

@itemize @bullet
@item
Checking the number and type of arguments to a function which has an
old-fashioned definition and no prototype.

Such a feature would work only occasionally---only for calls that appear
in the same file as the called function, following the definition.  The
only way to check all calls reliably is to add a prototype for the
function.  But adding a prototype eliminates the motivation for this
feature.  So the feature is not worthwhile.

@item
Warning about using an expression whose type is signed as a shift count.

Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.

@item
Warning about assigning a signed value to an unsigned variable.

Such assignments must be very common; warning about them would cause
more annoyance than good.

@item
Warning when a non-void function value is ignored.

Coming as I do from a Lisp background, I balk at the idea that there is
something dangerous about discarding a value.  There are functions that
return values which some callers may find useful; it makes no sense to
clutter the program with a cast to @code{void} whenever the value isn't
useful.

@item
Assuming (for optimization) that the address of an external symbol is
never zero.

This assumption is false on certain systems when @samp{#pragma weak} is
used.

@item
Making @samp{-fshort-enums} the default.

This would cause storage layout to be incompatible with most other C
compilers.  And it doesn't seem very important, given that you can get
the same result in other ways.  The case where it matters most is when
the enumeration-valued object is inside a structure, and in that case
you can specify a field width explicitly.

@item
Making bitfields unsigned by default on particular machines where ``the
ABI standard'' says to do so.

The ANSI C standard leaves it up to the implementation whether a bitfield
declared plain @code{int} is signed or not.  This in effect creates two
alternative dialects of C.

The GNU C compiler supports both dialects; you can specify the dialect
you want with the option @samp{-fsigned-bitfields} or
@samp{-funsigned-bitfields}.  However, this leaves open the question
of which dialect to use by default.

Currently, the preferred dialect makes plain bitfields signed, because
this is simplest.  Since @code{int} is the same as @code{signed int} in
every other context, it is cleanest for them to be the same in bitfields
as well.

Some computer manufacturers have published Application Binary Interface
standards which specify that plain bitfields should be unsigned.  It is
a mistake, however, to say anything about this issue in an ABI.  This is
because the handling of plain bitfields distinguishes two dialects of C.
Both dialects are meaningful on every type of machine.  Whether a
particular object file was compiled using signed bitfields or unsigned
is of no concern to other object files, even if they access the same
bitfields in the same data structures.

A given program is written in one or the other of these two dialects.
The program stands a chance to work on most any machine if it is
compiled with the proper dialect.  It is unlikely to work at all if
compiled with the wrong dialect.

Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines.  These users would be
inconvenienced if the compiler treated plain bitfields differently on
certain machines.

Occasionally users write programs intended only for a particular machine
type.  On these occasions, the users would benefit if the GNU C compiler
were to support by default the same dialect as the other compilers on
that machine.  But such applications are rare.  And users writing a
program to run on more than one type of machine cannot possibly benefit
from this kind of compatibility.

This is why GNU CC does and will treat plain bitfields in the same
fashion on all types of machines (by default).

There are some arguments for making bitfields unsigned by default on all
machines.  If, for example, this becomes a universal de facto standard,
it would make sense for GNU CC to go along with it.  This is something
to be considered in the future.

(Of course, users strongly concerned about portability should indicate
explicitly in each bitfield whether it is signed or not.  In this way,
they write programs which have the same meaning in both C dialects.)

@item
Undefining @code{__STDC__} when @samp{-ansi} is not used.

Currently, GNU CC defines @code{__STDC__} as long as you don't use
@samp{-traditional}.  This provides good results in practice.

Programmers normally use conditionals on @code{__STDC__} to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation.  Since plain @samp{gcc} supports
all the features of ANSI C, the correct answer to these questions is
``yes''.

Some users try to use @code{__STDC__} to check for the availability of
certain library facilities.  This is actually incorrect usage in an ANSI
C program, because the ANSI C standard says that a conforming
freestanding implementation should define @code{__STDC__} even though it
does not have the library facilities.  @samp{gcc -ansi -pedantic} is a
conforming freestanding implementation, and it is therefore required to
define @code{__STDC__}, even though it does not come with an ANSI C
library.

Sometimes people say that defining @code{__STDC__} in a compiler that
does not completely conform to the ANSI C standard somehow violates the
standard.  This is illogical.  The standard is a standard for compilers
that claim to support ANSI C, such as @samp{gcc -ansi}---not for other
compilers such as plain @samp{gcc}.  Whatever the ANSI C standard says
is relevant to the design of plain @samp{gcc} without @samp{-ansi} only
for pragmatic reasons, not as a requirement.

@item
Undefining @code{__STDC__} in C++.

Programs written to compile with C++-to-C translators get the
value of @code{__STDC__} that goes with the C compiler that is
subsequently used.  These programs must test @code{__STDC__}
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ANSI C fashion
or in the traditional fashion.

These programs work properly with GNU C++ if @code{__STDC__} is defined.
They would not work otherwise.

In addition, many header files are written to provide prototypes in ANSI
C but not in traditional C.  Many of these header files can work without
change in C++ provided @code{__STDC__} is defined.  If @code{__STDC__}
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.

@item
Deleting ``empty'' loops.

GNU CC does not delete ``empty'' loops because the most likely reason
you would put one in a program is to have a delay.  Deleting them will
not make real programs run any faster, so it would be pointless.

It would be different if optimization of a nonempty loop could produce
an empty one.  But this generally can't happen.
@end itemize

@node Bugs
@chapter Reporting Bugs
@cindex bugs
@cindex reporting bugs

Your bug reports play an essential role in making GNU CC reliable.

When you encounter a problem, the first thing to do is to see if it is
already known.  @xref{Trouble}.  If it isn't known, then you should
report the problem.

Reporting a bug may help you by bringing a solution to your problem, or
it may not.  (If it does not, look in the service directory; see
@ref{Service}.)  In any case, the principal function of a bug report is
to help the entire community by making the next version of GNU CC work
better.  Bug reports are your contribution to the maintenance of GNU CC.

In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.

@menu
* Criteria:  Bug Criteria.   Have you really found a bug?
* Where: Bug Lists.         Where to send your bug report.
* Reporting: Bug Reporting.  How to report a bug effectively.
* Patches: Sending Patches.  How to send a patch for GNU CC.
* Known: Trouble.            Known problems.
* Help: Service.             Where to ask for help.
@end menu

@node Bug Criteria
@section Have You Found a Bug?
@cindex bug criteria

If you are not sure whether you have found a bug, here are some guidelines:

@itemize @bullet
@cindex fatal signal
@cindex core dump
@item
If the compiler gets a fatal signal, for any input whatever, that is a
compiler bug.  Reliable compilers never crash.

@cindex invalid assembly code
@cindex assembly code, invalid
@item
If the compiler produces invalid assembly code, for any input whatever
(except an @code{asm} statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.

@cindex undefined behavior
@cindex undefined function value
@cindex increment operators
@item
If the compiler produces valid assembly code that does not correctly
execute the input source code, that is a compiler bug.

However, you must double-check to make sure, because you may have run
into an incompatibility between GNU C and traditional C
(@pxref{Incompatibilities}).  These incompatibilities might be considered
bugs, but they are inescapable consequences of valuable features.

Or you may have a program whose behavior is undefined, which happened
by chance to give the desired results with another C or C++ compiler.

For example, in many nonoptimizing compilers, you can write @samp{x;}
at the end of a function instead of @samp{return x;}, with the same
results.  But the value of the function is undefined if @code{return}
is omitted; it is not a bug when GNU CC produces different results.

Problems often result from expressions with two increment operators,
as in @code{f (*p++, *p++)}.  Your previous compiler might have
interpreted that expression the way you intended; GNU CC might
interpret it another way.  Neither compiler is wrong.  The bug is
in your code.

After you have localized the error to a single source line, it should
be easy to check for these things.  If your program is correct and
well defined, you have found a compiler bug.

@item
If the compiler produces an error message for valid input, that is a
compiler bug.

@cindex invalid input
@item
If the compiler does not produce an error message for invalid input,
that is a compiler bug.  However, you should note that your idea of
``invalid input'' might be my idea of ``an extension'' or ``support
for traditional practice''.

@item
If you are an experienced user of C or C++ compilers, your suggestions
for improvement of GNU CC or GNU C++ are welcome in any case.
@end itemize

@node Bug Lists
@section Where to Report Bugs
@cindex bug report mailing lists

Send bug reports for GNU C to one of these addresses:

@example
bug-gcc@@prep.ai.mit.edu
@{ucbvax|mit-eddie|uunet@}!prep.ai.mit.edu!bug-gcc
@end example

Send bug reports for GNU C++ to one of these addresses:

@example
bug-g++@@prep.ai.mit.edu
@{ucbvax|mit-eddie|uunet@}!prep.ai.mit.edu!bug-g++
@end example

@strong{Do not send bug reports to @samp{help-gcc}, or to the newsgroup
@samp{gnu.gcc.help}.}  Most users of GNU CC do not want to receive bug
reports.  Those that do, have asked to be on @samp{bug-gcc} and/or
@samp{bug-g++}.

The mailing lists @samp{bug-gcc} and @samp{bug-g++} both have newsgroups
which serve as repeaters: @samp{gnu.gcc.bug} and @samp{gnu.g++.bug}.
Each mailing list and its newsgroup carry exactly the same messages.

Often people think of posting bug reports to the newsgroup instead of
mailing them.  This appears to work, but it has one problem which can be
crucial: a newsgroup posting does not contain a mail path back to the
sender.  Thus, if maintaners need more information, they may be unable
to reach you.  For this reason, you should always send bug reports by
mail to the proper mailing list.

As a last resort, send bug reports on paper to:

@example
GNU Compiler Bugs
Free Software Foundation
675 Mass Ave
Cambridge, MA 02139
@end example

@node Bug Reporting
@section How to Report Bugs
@cindex compiler bugs, reporting

The fundamental principle of reporting bugs usefully is this:
@strong{report all the facts}.  If you are not sure whether to state a
fact or leave it out, state it!

Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter.  Thus, you might
assume that the name of the variable you use in an example does not matter.
Well, probably it doesn't, but one cannot be sure.  Perhaps the bug is a
stray memory reference which happens to fetch from the location where that
name is stored in memory; perhaps, if the name were different, the contents
of that location would fool the compiler into doing the right thing despite
the bug.  Play it safe and give a specific, complete example.  That is the
easiest thing for you to do, and the most helpful.

Keep in mind that the purpose of a bug report is to enable someone to
fix the bug if it is not known.  It isn't very important what happens if
the bug is already known.  Therefore, always write your bug reports on
the assumption that the bug is not known.

Sometimes people give a few sketchy facts and ask, ``Does this ring a
bell?''  This cannot help us fix a bug, so it is basically useless.  We
respond by asking for enough details to enable us to investigate.
You might as well expedite matters by sending them to begin with.

Try to make your bug report self-contained.  If we have to ask you for
more information, it is best if you include all the previous information
in your response, as well as the information that was missing.

To enable someone to investigate the bug, you should include all these
things:

@itemize @bullet
@item
The version of GNU CC.  You can get this by running it with the
@samp{-v} option.

Without this, we won't know whether there is any point in looking for
the bug in the current version of GNU CC.

@item
A complete input file that will reproduce the bug.  If the bug is in the
C preprocessor, send a source file and any header files that it
requires.  If the bug is in the compiler proper (@file{cc1}), run your
source file through the C preprocessor by doing @samp{gcc -E
@var{sourcefile} > @var{outfile}}, then include the contents of
@var{outfile} in the bug report.  (When you do this, use the same
@samp{-I}, @samp{-D} or @samp{-U} options that you used in actual
compilation.)

A single statement is not enough of an example.  In order to compile
it, it must be embedded in a function definition; and the bug might
depend on the details of how this is done.

Without a real example one can compile, all anyone can do about your bug
report is wish you luck.  It would be futile to try to guess how to
provoke the bug.  For example, bugs in register allocation and reloading
frequently depend on every little detail of the function they happen in.

@item
The command arguments you gave GNU CC or GNU C++ to compile that example
and observe the bug.  For example, did you use @samp{-O}?  To guarantee
you won't omit something important, list all the options.

If we were to try to guess the arguments, we would probably guess wrong
and then we would not encounter the bug.

@item
The type of machine you are using, and the operating system name and
version number.

@item
The operands you gave to the @code{configure} command when you installed
the compiler.

@item
A complete list of any modifications you have made to the compiler
source.  (We don't promise to investigate the bug unless it happens in
an unmodified compiler.  But if you've made modifications and don't tell
us, then you are sending us on a wild goose chase.)

Be precise about these changes---show a context diff for them.

Adding files of your own (such as a machine description for a machine we
don't support) is a modification of the compiler source.

@item
Details of any other deviations from the standard procedure for installing
GNU CC.

@item
A description of what behavior you observe that you believe is
incorrect.  For example, ``The compiler gets a fatal signal,'' or,
``The assembler instruction at line 208 in the output is incorrect.''

Of course, if the bug is that the compiler gets a fatal signal, then one
can't miss it.  But if the bug is incorrect output, the maintainer might
not notice unless it is glaringly wrong.  None of us has time to study
all the assembler code from a 50-line C program just on the chance that
one instruction might be wrong.  We need @code{you} to do this part!

Even if the problem you experience is a fatal signal, you should still
say so explicitly.  Suppose something strange is going on, such as, your
copy of the compiler is out of synch, or you have encountered a bug in
the C library on your system.  (This has happened!)  Your copy might
crash and the copy here would not.  If you @i{said} to expect a crash,
then when the compiler here fails to crash, we would know that the bug
was not happening.  If you don't say to expect a crash, then we would
not know whether the bug was happening.  We would not be able to draw
any conclusion from our observations.

Often the observed symptom is incorrect output when your program is run.
Sad to say, this is not enough information unless the program is short
and simple.  None of us has time to study a large program to figure out
how it would work if compiled correctly, much less which line of it was
compiled wrong.  So you will have to do that.  Tell us which source line
it is, and what incorrect result happens when that line is executed.  A
person who understands the program can find this as easily as finding a
bug in the program itself.

@item
If you send examples of assembler code output from GNU CC or GNU C++,
please use @samp{-g} when you make them.  The debugging information
includes source line numbers which are essential for correlating the
output with the input.

@item
If you wish to suggest changes to the GNU CC source, send them as
context diffs.  If you even discuss something in the GNU CC source,
refer to it by context, not by line number.

The line numbers in the development sources don't match those in your
sources.  Your line numbers would convey no useful information to the
maintainers.

@item
Additional information from a debugger might enable someone to find a
problem on a machine which he does not have available.  However, you
need to think when you collect this information if you want it to have
any chance of being useful.

@cindex backtrace for bug reports
For example, many people send just a backtrace, but that is never
useful by itself.  A simple backtrace with arguments conveys little
about GNU CC because the compiler is largely data-driven; the same
functions are called over and over for different RTL insns, doing
different things depending on the details of the insn.

Most of the arguments listed in the backtrace are useless because they
are pointers to RTL list structure.  The numeric values of the
pointers, which the debugger prints in the backtrace, have no
significance whatever; all that matters is the contents of the objects
they point to (and most of the contents are other such pointers).

In addition, most compiler passes consist of one or more loops that
scan the RTL insn sequence.  The most vital piece of information about
such a loop---which insn it has reached---is usually in a local variable,
not in an argument.

@findex debug_rtx
What you need to provide in addition to a backtrace are the values of
the local variables for several stack frames up.  When a local
variable or an argument is an RTX, first print its value and then use
the GDB command @code{pr} to print the RTL expression that it points
to.  (If GDB doesn't run on your machine, use your debugger to call
the function @code{debug_rtx} with the RTX as an argument.)  In
general, whenever a variable is a pointer, its value is no use
without the data it points to.

In addition, include a debugging dump from just before the pass
in which the crash happens.  Most bugs involve a series of insns,
not just one.
@end itemize

Here are some things that are not necessary:

@itemize @bullet
@item
A description of the envelope of the bug.

Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and which
changes will not affect it.

This is often time consuming and not very useful, because the way we
will find the bug is by running a single example under the debugger with
breakpoints, not by pure deduction from a series of examples.  You might
as well save your time for something else.

Of course, if you can find a simpler example to report @emph{instead} of
the original one, that is a convenience.  Errors in the output will be
easier to spot, running under the debugger will take less time, etc.
Most GNU CC bugs involve just one function, so the most straightforward
way to simplify an example is to delete all the function definitions
except the one where the bug occurs.  Those earlier in the file may be
replaced by external declarations if the crucial function depends on
them.  (Exception: inline functions may affect compilation of functions
defined later in the file.)

However, simplification is not vital; if you don't want to do this,
report the bug anyway and send the entire test case you used.

@item
A patch for the bug.

A patch for the bug is useful if it is a good one.  But don't omit the
necessary information, such as the test case, on the assumption that a
patch is all we need.  We might see problems with your patch and decide
to fix the problem another way, or we might not understand it at all.

Sometimes with a program as complicated as GNU CC it is very hard to
construct an example that will make the program follow a certain path
through the code.  If you don't send the example, we won't be able to
construct one, so we won't be able to verify that the bug is fixed.

And if we can't understand what bug you are trying to fix, or why your
patch should be an improvement, we won't install it.  A test case will
help us to understand.

@xref{Sending Patches}, for guidelines on how to make it easy for us to
understand and install your patches.

@item
A guess about what the bug is or what it depends on.

Such guesses are usually wrong.  Even I can't guess right about such
things without first using the debugger to find the facts.
@end itemize

@node Sending Patches,, Bug Reporting, Bugs
@section Sending Patches for GNU CC

If you would like to write bug fixes or improvements for the GNU C
compiler, that is very helpful.  When you send your changes, please
follow these guidelines to avoid causing extra work for us in studying
the patches.

If you don't follow these guidelines, your information might still be
useful, but using it will take extra work.  Maintaining GNU C is a lot
of work in the best of circumstances, and we can't keep up unless you do
your best to help.

@itemize @bullet
@item
Send an explanation with your changes of what problem they fix or what
improvement they bring about.  For a bug fix, just include a copy of the
bug report, and explain why the change fixes the bug.

(Referring to a bug report is not as good as including it, because then
we will have to look it up, and we have probably already deleted it if
we've already fixed the bug.)

@item
Always include a proper bug report for the problem you think you have
fixed.  We need to convince ourselves that the change is right before
installing it.  Even if it is right, we might have trouble judging it if
we don't have a way to reproduce the problem.

@item
Include all the comments that are appropriate to help people reading the
source in the future understand why this change was needed.

@item
Don't mix together changes made for different reasons.
Send them @emph{individually}.

If you make two changes for separate reasons, then we might not want to
install them both.  We might want to install just one.  If you send them
all jumbled together in a single set of diffs, we have to do extra work
to disentangle them---to figure out which parts of the change serve
which purpose.  If we don't have time for this, we might have to ignore
your changes entirely.

If you send each change as soon as you have written it, with its own
explanation, then the two changes never get tangled up, and we can
consider each one properly without any extra work to disentangle them.

Ideally, each change you send should be impossible to subdivide into
parts that we might want to consider separately, because each of its
parts gets its motivation from the other parts.

@item
Send each change as soon as that change is finished.  Sometimes people
think they are helping us by accumulating many changes to send them all
together.  As explained above, this is absolutely the worst thing you
could do.

Since you should send each change separately, you might as well send it
right away.  That gives us the option of installing it immediately if it
is important.

@item
Use @samp{diff -c} to make your diffs.  Diffs without context are hard
for us to install reliably.  More than that, they make it hard for us to
study the diffs to decide whether we want to install them.  Unidiff
format is better than contextless diffs, but not as easy to read as
@samp{-c} format.

If you have GNU diff, use @samp{diff -cp}, which shows the name of the
function that each change occurs in.

@item
Write the change log entries for your changes.  We get lots of changes,
and we don't have time to do all the change log writing ourselves.

Read the @file{ChangeLog} file to see what sorts of information to put
in, and to learn the style that we use.  The purpose of the change log
is to show people where to find what was changed.  So you need to be
specific about what functions you changed; in large functions, it's
often helpful to indicate where within the function the change was.

On the other hand, once you have shown people where to find the change,
you need not explain its purpose. Thus, if you add a new function, all
you need to say about it is that it is new.  If you feel that the
purpose needs explaining, it probably does---but the explanation will be
much more useful if you put it in comments in the code.

If you would like your name to appear in the header line for who made
the change, send us the header line.

@item
When you write the fix, keep in mind that I can't install a change that
would break other systems.

People often suggest fixing a problem by changing machine-independent
files such as @file{toplev.c} to do something special that a particular
system needs.  Sometimes it is totally obvious that such changes would
break GNU CC for almost all users.  We can't possibly make a change like
that.  At best it might tell us how to write another patch that would
solve the problem acceptably.

Sometimes people send fixes that @emph{might} be an improvement in
general---but it is hard to be sure of this.  It's hard to install
such changes because we have to study them very carefully.  Of course,
a good explanation of the reasoning by which you concluded the change
was correct can help convince us.

The safest changes are changes to the configuration files for a
particular machine.  These are safe because they can't create new bugs
on other machines.

Please help us keep up with the workload by designing the patch in a
form that is good to install.
@end itemize

@node Service
@chapter How To Get Help with GNU CC

If you need help installing, using or changing GNU CC, there are two
ways to find it:

@itemize @bullet
@item
Send a message to a suitable network mailing list.  First try
@code{bug-gcc@@prep.ai.mit.edu}, and if that brings no response, try
@code{help-gcc@@prep.ai.mit.edu}.

@item
Look in the service directory for someone who might help you for a fee.
The service directory is found in the file named @file{SERVICE} in the
GNU CC distribution.
@end itemize

@node VMS
@chapter Using GNU CC on VMS

@menu
* Include Files and VMS::  Where the preprocessor looks for the include files.
* Global Declarations::    How to do globaldef, globalref and globalvalue with
                           GNU CC.
* VMS Misc::           Misc information.
@end menu

@node Include Files and VMS
@section Include Files and VMS

@cindex include files and VMS
@cindex VMS and include files
@cindex header files and VMS
Due to the differences between the filesystems of Unix and VMS, GNU CC
attempts to translate file names in @samp{#include} into names that VMS
will understand.  The basic strategy is to prepend a prefix to the
specification of the include file, convert the whole filename to a VMS
filename, and then try to open the file.  GNU CC tries various prefixes
one by one until one of them succeeds:

@enumerate
@item
The first prefix is the @samp{GNU_CC_INCLUDE:} logical name: this is
where GNU C header files are traditionally stored.  If you wish to store
header files in non-standard locations, then you can assign the logical
@samp{GNU_CC_INCLUDE} to be a search list, where each element of the
list is suitable for use with a rooted logical.

@item
The next prefix tried is @samp{SYS$SYSROOT:[SYSLIB.]}.  This is where
VAX-C header files are traditionally stored.

@item
If the include file specification by itself is a valid VMS filename, the
preprocessor then uses this name with no prefix in an attempt to open
the include file.

@item
If the file specification is not a valid VMS filename (i.e. does not
contain a device or a directory specifier, and contains a @samp{/}
character), the preprocessor tries to convert it from Unix syntax to
VMS syntax.

Conversion works like this: the first directory name becomes a device,
and the rest of the directories are converted into VMS-format directory
names.  For example, @file{X11/foobar.h} is translated to
@file{X11:[000000]foobar.h} or @file{X11:foobar.h}, whichever one can be
opened.  This strategy allows you to assign a logical name to point to
the actual location of the header files.

@item
If none of these strategies succeeds, the @samp{#include} fails.
@end enumerate

Include directives of the form:

@example
#include foobar
@end example

@noindent
are a common source of incompatibility between VAX-C and GNU CC.  VAX-C
treats this much like a standard @code{#include <foobar.h>} directive.
That is incompatible with the ANSI C behavior implemented by GNU CC: to
expand the name @code{foobar} as a macro.  Macro expansion should
eventually yield one of the two standard formats for @code{#include}:

@example
#include "@var{file}"
#include <@var{file}>
@end example

If you have this problem, the best solution is to modify the source to
convert the @code{#include} directives to one of the two standard forms.
That will work with either compiler.  If you want a quick and dirty fix,
define the file names as macros with the proper expansion, like this:

@example
#define stdio <stdio.h>
@end example

@noindent
This will work, as long as the name doesn't conflict with anything else
in the program.

Another source of incompatibility is that VAX-C assumes that:

@example
#include "foobar"
@end example

@noindent
is actually asking for the file @file{foobar.h}.  GNU CC does not
make this assumption, and instead takes what you ask for literally;
it tries to read the file @file{foobar}.  The best way to avoid this
problem is to always specify the desired file extension in your include
directives.

GNU CC for VMS is distributed with a set of include files that is
sufficient to compile most general purpose programs.  Even though the
GNU CC distribution does not contain header files to define constants
and structures for some VMS system-specific functions, there is no
reason why you cannot use GNU CC with any of these functions.  You first
may have to generate or create header files, either by using the public
domain utility @code{UNSDL} (which can be found on a DECUS tape), or by
extracting the relevant modules from one of the system macro libraries,
and using an editor to construct a C header file.

@node Global Declarations
@section Global Declarations and VMS

@findex GLOBALREF
@findex GLOBALDEF
@findex GLOBALVALUEDEF
@findex GLOBALVALUEREF
GNU CC does not provide the @code{globalref}, @code{globaldef} and
@code{globalvalue} keywords of VAX-C.  You can get the same effect with
an obscure feature of GAS, the GNU assembler.  (This requires GAS
version 1.39 or later.)  The following macros allow you to use this
feature in a fairly natural way:

@smallexample
#ifdef __GNUC__
#define GLOBALREF(TYPE,NAME)                      \
  TYPE NAME                                       \
  asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
#define GLOBALDEF(TYPE,NAME,VALUE)                \
  TYPE NAME                                       \
  asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
    = VALUE
#define GLOBALVALUEREF(TYPE,NAME)                 \
  const TYPE NAME[1]                              \
  asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
#define GLOBALVALUEDEF(TYPE,NAME,VALUE)           \
  const TYPE NAME[1]                              \
  asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)  \
    = @{VALUE@}
#else
#define GLOBALREF(TYPE,NAME) \
  globalref TYPE NAME
#define GLOBALDEF(TYPE,NAME,VALUE) \
  globaldef TYPE NAME = VALUE
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
  globalvalue TYPE NAME = VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
  globalvalue TYPE NAME
#endif
@end smallexample

@noindent
(The @code{_$$PsectAttributes_GLOBALSYMBOL} prefix at the start of the
name is removed by the assembler, after it has modified the attributes
of the symbol).  These macros are provided in the VMS binaries
distribution in a header file @file{GNU_HACKS.H}.  An example of the
usage is:

@example
GLOBALREF (int, ijk);
GLOBALDEF (int, jkl, 0);
@end example

The macros @code{GLOBALREF} and @code{GLOBALDEF} cannot be used
straightforwardly for arrays, since there is no way to insert the array
dimension into the declaration at the right place.  However, you can
declare an array with these macros if you first define a typedef for the
array type, like this:

@example
typedef int intvector[10];
GLOBALREF (intvector, foo);
@end example

Array and structure initializers will also break the macros; you can
define the initializer to be a macro of its own, or you can expand the
@code{GLOBALDEF} macro by hand.  You may find a case where you wish to
use the @code{GLOBALDEF} macro with a large array, but you are not
interested in explicitly initializing each element of the array.  In
such cases you can use an initializer like: @code{@{0,@}}, which will
initialize the entire array to @code{0}.

A shortcoming of this implementation is that a variable declared with
@code{GLOBALVALUEREF} or @code{GLOBALVALUEDEF} is always an array.  For
example, the declaration:

@example
GLOBALVALUEREF(int, ijk);
@end example

@noindent
declares the variable @code{ijk} as an array of type @code{int [1]}.
This is done because a globalvalue is actually a constant; its ``value''
is what the linker would normally consider an address.  That is not how
an integer value works in C, but it is how an array works.  So treating
the symbol as an array name gives consistent results---with the
exception that the value seems to have the wrong type.  @strong{Don't
try to access an element of the array.}  It doesn't have any elements.
The array ``address'' may not be the address of actual storage.

The fact that the symbol is an array may lead to warnings where the
variable is used.  Insert type casts to avoid the warnings.  Here is an
example; it takes advantage of the ANSI C feature allowing macros that
expand to use the same name as the macro itself.

@example
GLOBALVALUEREF (int, ss$_normal);
GLOBALVALUEDEF (int, xyzzy,123);
#ifdef __GNUC__
#define ss$_normal ((int) ss$_normal)
#define xyzzy ((int) xyzzy)
#endif
@end example

Don't use @code{globaldef} or @code{globalref} with a variable whose
type is an enumeration type; this is not implemented.  Instead, make the
variable an integer, and use a @code{globalvaluedef} for each of the
enumeration values.  An example of this would be:

@example
#ifdef __GNUC__
GLOBALDEF (int, color, 0);
GLOBALVALUEDEF (int, RED, 0);
GLOBALVALUEDEF (int, BLUE, 1);
GLOBALVALUEDEF (int, GREEN, 3);
#else
enum globaldef color @{RED, BLUE, GREEN = 3@};
#endif
@end example

@node VMS Misc
@section Other VMS Issues

@cindex exit status and VMS
@cindex return value of @code{main}
@cindex @code{main} and the exit status
GNU CC automatically arranges for @code{main} to return 1 by default if
you fail to specify an explicit return value.  This will be interpreted
by VMS as a status code indicating a normal successful completion.
Version 1 of GNU CC did not provide this default.

GNU CC on VMS works only with the GNU assembler, GAS.  You need version
1.37 or later of GAS in order to produce value debugging information for
the VMS debugger.  Use the ordinary VMS linker with the object files
produced by GAS.

@cindex shared VMS run time system
@cindex @file{VAXCRTL}
Under previous versions of GNU CC, the generated code would occasionally
give strange results when linked to the sharable @file{VAXCRTL} library.
Now this should work.

A caveat for use of @code{const} global variables: the @code{const}
modifier must be specified in every external declaration of the variable
in all of the source files that use that variable.  Otherwise the linker
will issue warnings about conflicting attributes for the variable.  Your
program will still work despite the warnings, but the variable will be
placed in writable storage.

@cindex name augmentation
@cindex case sensitivity and VMS
@cindex VMS and case sensitivity
The VMS linker does not distinguish between upper and lower case letters
in function and variable names.  However, usual practice in C is to
distinguish case.  Normally GNU CC (by means of the assembler GAS)
implements usual C behavior by augmenting each name that is not all
lower-case.  A name is augmented by truncating it to at most 23
characters and then adding more characters at the end which encode the
case pattern the rest.

Name augmentation yields bad results for programs that use precompiled
libraries (such as Xlib) which were generated by another compiler.  You
can use the compiler option @samp{/NOCASE_HACK} to inhibit augmentation;
it makes external C functions and variables case-independent as is usual
on VMS.  Alternatively, you could write all references to the functions
and variables in such libraries using lower case; this will work on VMS,
but is not portable to other systems.

Function and variable names are handled somewhat differently with GNU
C++.  The GNU C++ compiler performs @dfn{name mangling} on function
names, which means that it adds information to the function name to
describe the data types of the arguments that the function takes. One
result of this is that the name of a function can become very long.
Since the VMS linker only recognizes the first 31 characters in a name,
special action is taken to ensure that each function and variable has a
unique name that can be represented in 31 characters.

If the name (plus a name augmentation, if required) is less than 32
characters in length, then no special action is performed. If the name
is longer than 31 characters, the assembler (GAS) will generate a
hash string based upon the function name, truncate the function name to
23 characters, and append the hash string to the truncated name.  If the
@samp{/VERBOSE} compiler option is used, the assembler will print both
the full and truncated names of each symbol that is truncated.

The @samp{/NOCASE_HACK} compiler option should not be used when you are
compiling programs that use libg++. libg++ has several instances of
objects (i.e.  @code{Filebuf} and @code{filebuf}) which become
indistinguishable in a case-insensitive environment.  This leads to
cases where you need to inhibit augmentation selectively (if you were
using libg++ and Xlib in the same program, for example).  There is no
special feature for doing this, but you can get the result by defining a
macro for each mixed case symbol for which you wish to inhibit
augmentation.  The macro should expand into the lower case equivalent of
itself.  For example:

@example
#define StuDlyCapS studlycaps
@end example

These macro definitions can be placed in a header file to minimize the
number of changes to your source code.

@ifset INTERNALS
@node Portability
@chapter GNU CC and Portability
@cindex portability
@cindex GNU CC and portability

The main goal of GNU CC was to make a good, fast compiler for machines in
the class that the GNU system aims to run on: 32-bit machines that address
8-bit bytes and have several general registers.  Elegance, theoretical
power and simplicity are only secondary.

GNU CC gets most of the information about the target machine from a machine
description which gives an algebraic formula for each of the machine's
instructions.  This is a very clean way to describe the target.  But when
the compiler needs information that is difficult to express in this
fashion, I have not hesitated to define an ad-hoc parameter to the machine
description.  The purpose of portability is to reduce the total work needed
on the compiler; it was not of interest for its own sake.

@cindex endianness
@cindex autoincrement addressing, availability
@findex abort
GNU CC does not contain machine dependent code, but it does contain code
that depends on machine parameters such as endianness (whether the most
significant byte has the highest or lowest address of the bytes in a word)
and the availability of autoincrement addressing.  In the RTL-generation
pass, it is often necessary to have multiple strategies for generating code
for a particular kind of syntax tree, strategies that are usable for different
combinations of parameters.  Often I have not tried to address all possible
cases, but only the common ones or only the ones that I have encountered.
As a result, a new target may require additional strategies.  You will know
if this happens because the compiler will call @code{abort}.  Fortunately,
the new strategies can be added in a machine-independent fashion, and will
affect only the target machines that need them.
@end ifset

@ifset INTERNALS
@node Interface
@chapter Interfacing to GNU CC Output
@cindex interfacing to GNU CC output
@cindex run-time conventions
@cindex function call conventions
@cindex conventions, run-time

GNU CC is normally configured to use the same function calling convention
normally in use on the target system.  This is done with the
machine-description macros described (@pxref{Target Macros}).

@cindex unions, returning
@cindex structures, returning
@cindex returning structures and unions
However, returning of structure and union values is done differently on
some target machines.  As a result, functions compiled with PCC
returning such types cannot be called from code compiled with GNU CC,
and vice versa.  This does not cause trouble often because few Unix
library routines return structures or unions.

GNU CC code returns structures and unions that are 1, 2, 4 or 8 bytes
long in the same registers used for @code{int} or @code{double} return
values.  (GNU CC typically allocates variables of such types in
registers also.)  Structures and unions of other sizes are returned by
storing them into an address passed by the caller (usually in a
register).  The machine-description macros @code{STRUCT_VALUE} and
@code{STRUCT_INCOMING_VALUE} tell GNU CC where to pass this address.

By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted.  This is slower than the method used by GNU CC, and
fails to be reentrant.

On some target machines, such as RISC machines and the 80386, the
standard system convention is to pass to the subroutine the address of
where to return the value.  On these machines, GNU CC has been
configured to be compatible with the standard compiler, when this method
is used.  It may not be compatible for structures of 1, 2, 4 or 8 bytes.

@cindex argument passing
@cindex passing arguments
GNU CC uses the system's standard convention for passing arguments.  On
some machines, the first few arguments are passed in registers; in
others, all are passed on the stack.  It would be possible to use
registers for argument passing on any machine, and this would probably
result in a significant speedup.  But the result would be complete
incompatibility with code that follows the standard convention.  So this
change is practical only if you are switching to GNU CC as the sole C
compiler for the system.  We may implement register argument passing on
certain machines once we have a complete GNU system so that we can
compile the libraries with GNU CC.

On some machines (particularly the Sparc), certain types of arguments
are passed ``by invisible reference''.  This means that the value is
stored in memory, and the address of the memory location is passed to
the subroutine.

@cindex @code{longjmp} and automatic variables
If you use @code{longjmp}, beware of automatic variables.  ANSI C says that
automatic variables that are not declared @code{volatile} have undefined
values after a @code{longjmp}.  And this is all GNU CC promises to do,
because it is very difficult to restore register variables correctly, and
one of GNU CC's features is that it can put variables in registers without
your asking it to.

If you want a variable to be unaltered by @code{longjmp}, and you don't
want to write @code{volatile} because old C compilers don't accept it,
just take the address of the variable.  If a variable's address is ever
taken, even if just to compute it and ignore it, then the variable cannot
go in a register:

@example
@{
  int careful;
  &careful;
  @dots{}
@}
@end example

@cindex arithmetic libraries
@cindex math libraries
Code compiled with GNU CC may call certain library routines.  Most of
them handle arithmetic for which there are no instructions.  This
includes multiply and divide on some machines, and floating point
operations on any machine for which floating point support is disabled
with @samp{-msoft-float}.  Some standard parts of the C library, such as
@code{bcopy} or @code{memcpy}, are also called automatically.  The usual
function call interface is used for calling the library routines.

These library routines should be defined in the library @file{libgcc.a},
which GNU CC automatically searches whenever it links a program.  On
machines that have multiply and divide instructions, if hardware
floating point is in use, normally @file{libgcc.a} is not needed, but it
is searched just in case.

Each arithmetic function is defined in @file{libgcc1.c} to use the
corresponding C arithmetic operator.  As long as the file is compiled
with another C compiler, which supports all the C arithmetic operators,
this file will work portably.  However, @file{libgcc1.c} does not work if
compiled with GNU CC, because each arithmetic function would compile
into a call to itself!
@end ifset

@ifset INTERNALS
@node Passes
@chapter Passes and Files of the Compiler
@cindex passes and files of the compiler
@cindex files and passes of the compiler
@cindex compiler passes and files

@cindex top level of compiler
The overall control structure of the compiler is in @file{toplev.c}.  This
file is responsible for initialization, decoding arguments, opening and
closing files, and sequencing the passes.

@cindex parsing pass
The parsing pass is invoked only once, to parse the entire input.  The RTL
intermediate code for a function is generated as the function is parsed, a
statement at a time.  Each statement is read in as a syntax tree and then
converted to RTL; then the storage for the tree for the statement is
reclaimed.  Storage for types (and the expressions for their sizes),
declarations, and a representation of the binding contours and how they nest,
remain until the function is finished being compiled; these are all needed
to output the debugging information.

@findex rest_of_compilation
@findex rest_of_decl_compilation
Each time the parsing pass reads a complete function definition or
top-level declaration, it calls the function
@code{rest_of_compilation} or @code{rest_of_decl_compilation} in
@file{toplev.c}, which are responsible for all further processing
necessary, ending with output of the assembler language.  All other
compiler passes run, in sequence, within @code{rest_of_compilation}.
When that function returns from compiling a function definition, the
storage used for that function definition's compilation is entirely
freed, unless it is an inline function (@pxref{Inline}).

Here is a list of all the passes of the compiler and their source files.
Also included is a description of where debugging dumps can be requested
with @samp{-d} options.

@itemize @bullet
@item
Parsing.  This pass reads the entire text of a function definition,
constructing partial syntax trees.  This and RTL generation are no longer
truly separate passes (formerly they were), but it is easier to think
of them as separate.

The tree representation does not entirely follow C syntax, because it is
intended to support other languages as well.

Language-specific data type analysis is also done in this pass, and every
tree node that represents an expression has a data type attached.
Variables are represented as declaration nodes.

@cindex constant folding
@cindex arithmetic simplifications
@cindex simplifications, arithmetic
Constant folding and some arithmetic simplifications are also done
during this pass.

The language-independent source files for parsing are
@file{stor-layout.c}, @file{fold-const.c}, and @file{tree.c}.
There are also header files @file{tree.h} and @file{tree.def}
which define the format of the tree representation.@refill

The source files for parsing C are @file{c-parse.y}, @file{c-decl.c},
@file{c-typeck.c}, @file{c-convert.c}, @file{c-lang.c}, and
@file{c-aux-info.c} along with header files @file{c-lex.h}, and
@file{c-tree.h}.

The source files for parsing C++ are @file{cp-parse.y},
@file{cp-class.c}, @file{cp-cvt.c},@*
@file{cp-decl.c}, @file{cp-decl.c}, @file{cp-decl2.c},
@file{cp-dem.c}, @file{cp-except.c},@*
@file{cp-expr.c}, @file{cp-init.c}, @file{cp-lex.c},
@file{cp-method.c}, @file{cp-ptree.c},@*
@file{cp-search.c}, @file{cp-tree.c}, @file{cp-type2.c}, and
@file{cp-typeck.c}, along with header files @file{cp-tree.def},
@file{cp-tree.h}, and @file{cp-decl.h}.

The special source files for parsing Objective C are
@file{objc-parse.y}, @file{objc-actions.c}, @file{objc-tree.def}, and
@file{objc-actions.h}.  Certain C-specific files are used for this as
well.

The file @file{c-common.c} is also used for all of the above languages.

@cindex RTL generation
@item
RTL generation.  This is the conversion of syntax tree into RTL code.
It is actually done statement-by-statement during parsing, but for
most purposes it can be thought of as a separate pass.

@cindex target-parameter-dependent code
This is where the bulk of target-parameter-dependent code is found,
since often it is necessary for strategies to apply only when certain
standard kinds of instructions are available.  The purpose of named
instruction patterns is to provide this information to the RTL
generation pass.

@cindex tail recursion optimization
Optimization is done in this pass for @code{if}-conditions that are
comparisons, boolean operations or conditional expressions.  Tail
recursion is detected at this time also.  Decisions are made about how
best to arrange loops and how to output @code{switch} statements.

The source files for RTL generation include @file{stmt.c},
@file{function.c}, @file{expr.c}, @file{calls.c}, @file{explow.c},
@file{expmed.c}, @file{optabs.c} and @file{emit-rtl.c}.  Also, the file
@file{insn-emit.c}, generated from the machine description by the
program @code{genemit}, is used in this pass.  The header file
@file{expr.h} is used for communication within this pass.@refill

@findex genflags
@findex gencodes
The header files @file{insn-flags.h} and @file{insn-codes.h},
generated from the machine description by the programs @code{genflags}
and @code{gencodes}, tell this pass which standard names are available
for use and which patterns correspond to them.@refill

Aside from debugging information output, none of the following passes
refers to the tree structure representation of the function (only
part of which is saved).

@cindex inline, automatic
The decision of whether the function can and should be expanded inline
in its subsequent callers is made at the end of rtl generation.  The
function must meet certain criteria, currently related to the size of
the function and the types and number of parameters it has.  Note that
this function may contain loops, recursive calls to itself
(tail-recursive functions can be inlined!), gotos, in short, all
constructs supported by GNU CC.  The file @file{integrate.c} contains
the code to save a function's rtl for later inlining and to inline that
rtl when the function is called.  The header file @file{integrate.h}
is also used for this purpose.

The option @samp{-dr} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.rtl} to
the input file name.

@cindex jump optimization
@cindex unreachable code
@cindex dead code
@item
Jump optimization.  This pass simplifies jumps to the following
instruction, jumps across jumps, and jumps to jumps.  It deletes
unreferenced labels and unreachable code, except that unreachable code
that contains a loop is not recognized as unreachable in this pass.
(Such loops are deleted later in the basic block analysis.)  It also
converts some code originally written with jumps into sequences of
instructions that directly set values from the results of comparisons,
if the machine has such instructions.

Jump optimization is performed two or three times.  The first time is
immediately following RTL generation.  The second time is after CSE,
but only if CSE says repeated jump optimization is needed.  The
last time is right before the final pass.  That time, cross-jumping
and deletion of no-op move instructions are done together with the
optimizations described above.

The source file of this pass is @file{jump.c}.

The option @samp{-dj} causes a debugging dump of the RTL code after
this pass is run for the first time.  This dump file's name is made by
appending @samp{.jump} to the input file name.

@cindex register use analysis
@item
Register scan.  This pass finds the first and last use of each
register, as a guide for common subexpression elimination.  Its source
is in @file{regclass.c}.

@cindex jump threading
@item
Jump threading.  This pass detects a condition jump that branches to an
identical or inverse test.  Such jumps can be @samp{threaded} through
the second conditional test.  The source code for this pass is in
@file{jump.c}.  This optimization is only performed if
@samp{-fthread-jumps} is enabled.

@cindex common subexpression elimination
@cindex constant propagation
@item
Common subexpression elimination.  This pass also does constant
propagation.  Its source file is @file{cse.c}.  If constant
propagation causes conditional jumps to become unconditional or to
become no-ops, jump optimization is run again when CSE is finished.

The option @samp{-ds} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.cse} to
the input file name.

@cindex loop optimization
@cindex code motion
@cindex strength-reduction
@item
Loop optimization.  This pass moves constant expressions out of loops,
and optionally does strength-reduction and loop unrolling as well.
Its source files are @file{loop.c} and @file{unroll.c}, plus the header
@file{loop.h} used for communication between them.  Loop unrolling uses
some functions in @file{integrate.c} and the header @file{integrate.h}.

The option @samp{-dL} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.loop} to
the input file name.

@item
If @samp{-frerun-cse-after-loop} was enabled, a second common
subexpression elimination pass is performed after the loop optimization
pass.  Jump threading is also done again at this time if it was specified.

The option @samp{-dt} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.cse2} to
the input file name.

@cindex register allocation, stupid
@cindex stupid register allocation
@item
Stupid register allocation is performed at this point in a
nonoptimizing compilation.  It does a little data flow analysis as
well.  When stupid register allocation is in use, the next pass
executed is the reloading pass; the others in between are skipped.
The source file is @file{stupid.c}.

@cindex data flow analysis
@cindex analysis, data flow
@cindex basic blocks
@item
Data flow analysis (@file{flow.c}).  This pass divides the program
into basic blocks (and in the process deletes unreachable loops); then
it computes which pseudo-registers are live at each point in the
program, and makes the first instruction that uses a value point at
the instruction that computed the value.

@cindex autoincrement/decrement analysis
This pass also deletes computations whose results are never used, and
combines memory references with add or subtract instructions to make
autoincrement or autodecrement addressing.

The option @samp{-df} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.flow} to
the input file name.  If stupid register allocation is in use, this
dump file reflects the full results of such allocation.

@cindex instruction combination
@item
Instruction combination (@file{combine.c}).  This pass attempts to
combine groups of two or three instructions that are related by data
flow into single instructions.  It combines the RTL expressions for
the instructions by substitution, simplifies the result using algebra,
and then attempts to match the result against the machine description.

The option @samp{-dc} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.combine}
to the input file name.

@cindex instruction scheduling
@cindex scheduling, instruction
@item
Instruction scheduling (@file{sched.c}).  This pass looks for
instructions whose output will not be available by the time that it is
used in subsequent instructions.  (Memory loads and floating point
instructions often have this behavior on RISC machines).  It re-orders
instructions within a basic block to try to separate the definition and
use of items that otherwise would cause pipeline stalls.

Instruction scheduling is performed twice.  The first time is immediately
after instruction combination and the second is immediately after reload.

The option @samp{-dS} causes a debugging dump of the RTL code after this
pass is run for the first time.  The dump file's name is made by
appending @samp{.sched} to the input file name.

@cindex register class preference pass
@item
Register class preferencing.  The RTL code is scanned to find out
which register class is best for each pseudo register.  The source
file is @file{regclass.c}.

@cindex register allocation
@cindex local register allocation
@item
Local register allocation (@file{local-alloc.c}).  This pass allocates
hard registers to pseudo registers that are used only within one basic
block.  Because the basic block is linear, it can use fast and
powerful techniques to do a very good job.

The option @samp{-dl} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.lreg} to
the input file name.

@cindex global register allocation
@item
Global register allocation (@file{global-alloc.c}).  This pass
allocates hard registers for the remaining pseudo registers (those
whose life spans are not contained in one basic block).

@cindex reloading
@item
Reloading.  This pass renumbers pseudo registers with the hardware
registers numbers they were allocated.  Pseudo registers that did not
get hard registers are replaced with stack slots.  Then it finds
instructions that are invalid because a value has failed to end up in
a register, or has ended up in a register of the wrong kind.  It fixes
up these instructions by reloading the problematical values
temporarily into registers.  Additional instructions are generated to
do the copying.

The reload pass also optionally eliminates the frame pointer and inserts
instructions to save and restore call-clobbered registers around calls.

Source files are @file{reload.c} and @file{reload1.c}, plus the header
@file{reload.h} used for communication between them.

The option @samp{-dg} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.greg} to
the input file name.

@cindex instruction scheduling
@cindex scheduling, instruction
@item
Instruction scheduling is repeated here to try to avoid pipeline stalls
due to memory loads generated for spilled pseudo registers.

The option @samp{-dR} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.sched2}
to the input file name.

@cindex cross-jumping
@cindex no-op move instructions
@item
Jump optimization is repeated, this time including cross-jumping
and deletion of no-op move instructions.

The option @samp{-dJ} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.jump2}
to the input file name.

@cindex delayed branch scheduling
@cindex scheduling, delayed branch
@item
Delayed branch scheduling.  This optional pass attempts to find
instructions that can go into the delay slots of other instructions,
usually jumps and calls.  The source file name is @file{reorg.c}.

The option @samp{-dd} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.dbr}
to the input file name.

@cindex register-to-stack conversion
@item
Conversion from usage of some hard registers to usage of a register
stack may be done at this point.  Currently, this is supported only
for the floating-point registers of the Intel 80387 coprocessor.   The
source file name is @file{reg-stack.c}.

The options @samp{-dk} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.stack}
to the input file name.

@cindex final pass
@cindex peephole optimization
@item
Final.  This pass outputs the assembler code for the function.  It is
also responsible for identifying spurious test and compare
instructions.  Machine-specific peephole optimizations are performed
at the same time.  The function entry and exit sequences are generated
directly as assembler code in this pass; they never exist as RTL.

The source files are @file{final.c} plus @file{insn-output.c}; the
latter is generated automatically from the machine description by the
tool @file{genoutput}.  The header file @file{conditions.h} is used
for communication between these files.

@cindex debugging information generation
@item
Debugging information output.  This is run after final because it must
output the stack slot offsets for pseudo registers that did not get
hard registers.  Source files are @file{dbxout.c} for DBX symbol table
format, @file{sdbout.c} for SDB symbol table format, and
@file{dwarfout.c} for DWARF symbol table format.
@end itemize

Some additional files are used by all or many passes:

@itemize @bullet
@item
Every pass uses @file{machmode.def} and @file{machmode.h} which define
the machine modes.

@item
Several passes use @file{real.h}, which defines the default
representation of floating point constants and how to operate on them.

@item
All the passes that work with RTL use the header files @file{rtl.h}
and @file{rtl.def}, and subroutines in file @file{rtl.c}.  The tools
@code{gen*} also use these files to read and work with the machine
description RTL.

@findex genconfig
@item
Several passes refer to the header file @file{insn-config.h} which
contains a few parameters (C macro definitions) generated
automatically from the machine description RTL by the tool
@code{genconfig}.

@cindex instruction recognizer
@item
Several passes use the instruction recognizer, which consists of
@file{recog.c} and @file{recog.h}, plus the files @file{insn-recog.c}
and @file{insn-extract.c} that are generated automatically from the
machine description by the tools @file{genrecog} and
@file{genextract}.@refill

@item
Several passes use the header files @file{regs.h} which defines the
information recorded about pseudo register usage, and @file{basic-block.h}
which defines the information recorded about basic blocks.

@item
@file{hard-reg-set.h} defines the type @code{HARD_REG_SET}, a bit-vector
with a bit for each hard register, and some macros to manipulate it.
This type is just @code{int} if the machine has few enough hard registers;
otherwise it is an array of @code{int} and some of the macros expand
into loops.

@item
Several passes use instruction attributes.  A definition of the
attributes defined for a particular machine is in file
@file{insn-attr.h}, which is generated from the machine description by
the program @file{genattr}.  The file @file{insn-attrtab.c} contains
subroutines to obtain the attribute values for insns.  It is generated
from the machine description by the program @file{genattrtab}.@refill
@end itemize
@end ifset

@include rtl.texi
@include md.texi
@include tm.texi

@ifset INTERNALS
@node Config
@chapter The Configuration File
@cindex configuration file
@cindex @file{xm-@var{machine}.h}

The configuration file @file{xm-@var{machine}.h} contains macro
definitions that describe the machine and system on which the compiler
is running, unlike the definitions in @file{@var{machine}.h}, which
describe the machine for which the compiler is producing output.  Most
of the values in @file{xm-@var{machine}.h} are actually the same on all
machines that GNU CC runs on, so large parts of all configuration files
are identical.  But there are some macros that vary:

@table @code
@findex USG
@item USG
Define this macro if the host system is System V.

@findex VMS
@item VMS
Define this macro if the host system is VMS.

@findex FAILURE_EXIT_CODE
@item FAILURE_EXIT_CODE
A C expression for the status code to be returned when the compiler
exits after serious errors.

@findex SUCCESS_EXIT_CODE
@item SUCCESS_EXIT_CODE
A C expression for the status code to be returned when the compiler
exits without serious errors.

@findex HOST_WORDS_BIG_ENDIAN
@item HOST_WORDS_BIG_ENDIAN
Defined if the host machine stores words of multi-word values in
big-endian order.  (GNU CC does not depend on the host byte ordering
within a word.)

@findex HOST_FLOAT_FORMAT
@item HOST_FLOAT_FORMAT
A numeric code distinguishing the floating point format for the host
machine.  See @code{TARGET_FLOAT_FORMAT} in @ref{Storage Layout} for the
alternatives and default.

@findex HOST_BITS_PER_CHAR
@item HOST_BITS_PER_CHAR
A C expression for the number of bits in @code{char} on the host
machine.

@findex HOST_BITS_PER_SHORT
@item HOST_BITS_PER_SHORT
A C expression for the number of bits in @code{short} on the host
machine.

@findex HOST_BITS_PER_INT
@item HOST_BITS_PER_INT
A C expression for the number of bits in @code{int} on the host
machine.

@findex HOST_BITS_PER_LONG
@item HOST_BITS_PER_LONG
A C expression for the number of bits in @code{long} on the host
machine.

@findex ONLY_INT_FIELDS
@item ONLY_INT_FIELDS
Define this macro to indicate that the host compiler only supports
@code{int} bit fields, rather than other integral types, including
@code{enum}, as do most C compilers.

@findex EXECUTABLE_SUFFIX
@item EXECUTABLE_SUFFIX
Define this macro if the host system uses a naming convention for
executable files that involves a common suffix (such as, in some
systems, @samp{.exe}) that must be mentioned explicitly when you run
the program.

@findex OBSTACK_CHUNK_SIZE
@item OBSTACK_CHUNK_SIZE
A C expression for the size of ordinary obstack chunks.
If you don't define this, a usually-reasonable default is used.

@findex OBSTACK_CHUNK_ALLOC
@item OBSTACK_CHUNK_ALLOC
The function used to allocate obstack chunks.
If you don't define this, @code{xmalloc} is used.

@findex OBSTACK_CHUNK_FREE
@item OBSTACK_CHUNK_FREE
The function used to free obstack chunks.
If you don't define this, @code{free} is used.

@findex USE_C_ALLOCA
@item USE_C_ALLOCA
Define this macro to indicate that the compiler is running with the
@code{alloca} implemented in C.  This version of @code{alloca} can be
found in the file @file{alloca.c}; to use it, you must also alter the
@file{Makefile} variable @code{ALLOCA}.  (This is done automatically
for the systems on which we know it is needed.)

If you do define this macro, you should probably do it as follows:

@example
#ifndef __GNUC__
#define USE_C_ALLOCA
#else
#define alloca __builtin_alloca
#endif
@end example

@noindent
so that when the compiler is compiled with GNU CC it uses the more
efficient built-in @code{alloca} function.

@item FUNCTION_CONVERSION_BUG
@findex FUNCTION_CONVERSION_BUG
Define this macro to indicate that the host compiler does not properly
handle converting a function value to a pointer-to-function when it is
used in an expression.

@findex HAVE_VPRINTF
@findex vprintf
@item HAVE_VPRINTF
Define this if the library function @code{vprintf} is available on your
system.

@findex MULTIBYTE_CHARS
@item MULTIBYTE_CHARS
Define this macro to enable support for multibyte characters in the
input to GNU CC.  This requires that the host system support the ANSI C
library functions for converting multibyte characters to wide
characters.

@findex HAVE_PUTENV
@findex putenv
@item HAVE_PUTENV
Define this if the library function @code{putenv} is available on your
system.

@findex NO_SYS_SIGLIST
@item NO_SYS_SIGLIST
Define this if your system @emph{does not} provide the variable
@code{sys_siglist}.

@vindex sys_siglist
Some systems do provide this variable, but with a different name such
as @code{_sys_siglist}.  On these systems, you can define
@code{sys_siglist} as a macro which expands into the name actually
provided.

@findex NO_STAB_H
@item NO_STAB_H
Define this if your system does not have the include file
@file{stab.h}.  If @samp{USG} is defined, @samp{NO_STAB_H} is
assumed.
@end table

@findex bzero
@findex bcmp
In addition, configuration files for system V define @code{bcopy},
@code{bzero} and @code{bcmp} as aliases.  Some files define @code{alloca}
as a macro when compiled with GNU CC, in order to take advantage of the
benefit of GNU CC's built-in @code{alloca}.


@node Index
@unnumbered Index
@end ifset

@ifclear INTERNALS
@node Index
@unnumbered Index
@end ifclear

@printindex cp
@contents
@bye