TopWare Tools/TOOLS.iso/tools/top1244/gccinfo.zoo/gccinfo/gcc.info-6

GNU Info File | 1992-02-16 | 46.4 KB | 1,010 lines

This is Info file gcc.info, produced by Makeinfo-1.43 from the input
file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

   Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU General Public License" is included
exactly as in the original, and provided that the entire resulting
derived work is distributed under the terms of a permission notice
identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the section entitled "GNU General Public
License" and this permission notice may be included in translations
approved by the Free Software Foundation instead of in the original
English.


File: gcc.info,  Node: Non-bugs,  Prev: Bug Reporting,  Up: Bugs

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.

   * 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 will
     eliminate the need for this feature.  So the feature is not
     worthwhile.

   * 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.

   * 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.

   * 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 `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 `-fsigned-bitfields' or
     `-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 `int' is the same as `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
     functions in any other object file, 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).

     (Of course, users strongly concerned about portability should
     indicate explicitly in each bitfield whether it is signed or not.)

   * Undefining `__STDC__' when `-ansi' is not used.

     Currently, GNU CC defines `__STDC__' as long as you don't use
     `-traditional'.  This provides good results in practice.

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

     Some users try to use `__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 `__STDC__'
     even though it does not have the library facilities.  `gcc -ansi
     -pedantic' is a conforming freestanding implementation, and it is
     therefore required to define `__STDC__', even though it does not
     come with an ANSI C library.

     Sometimes people say that defining `__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 are supposed to conform.  It says
     nothing about what any other compilers should do.  Whatever the
     ANSI C standard says is relevant to the design of plain `gcc'
     without `-ansi' only for pragmatic reasons, not as a requirement.

   * Undefining `__STDC__' in C++.

     Programs written to compile with C++-to-C translators get the
     value of `__STDC__' that goes with the C compiler that is
     subsequently used.  These programs must test `__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 `__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 `__STDC__' is defined.
     If `__STDC__' is not defined, they will all fail, and will all
     need to be changed to test explicitly for C++ as well.


File: gcc.info,  Node: VMS,  Next: Portability,  Prev: Bugs,  Up: Top

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.


File: gcc.info,  Node: Include Files and VMS,  Next: Global Declarations,  Prev: VMS,  Up: VMS

Include Files and VMS
=====================

   Due to the differences between the filesystems of Unix and VMS, GNU
CC attempts to translate file names in `#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:

  1. The first prefix is the `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 `GNU_CC_INCLUDE' to be a search list, where
     each element of the list is suitable for use with a rooted
     logical.

  2. The next prefix tried is `SYS$SYSROOT:[SYSLIB.]'.  This is where
     VAX-C header files are traditionally stored.

  3. 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.

  4. If the file specification is not a valid VMS filename (i.e. does
     not contain a device or a directory specifier, and contains a `/'
     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, `X11/foobar.h' is
     translated to `X11:[000000]foobar.h' or `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.

  5. If none of these strategies succeeds, the `#include' fails.

   Include directives of the form:

     #include foobar

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

     #include "FILE"
     #include <FILE>

   If you have this problem, the best solution is to modify the source
to convert the `#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:

     #define stdio <stdio.h>

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:

     #include "foobar"

is actually asking for the file `foobar.h'.  GNU CC does not make this
assumption, and instead takes what you ask for literally; it tries to
read the 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 `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.


File: gcc.info,  Node: Global Declarations,  Next: VMS Misc,  Prev: Include Files and VMS,  Up: VMS

Global Declarations and VMS
===========================

   GNU CC does not provide the `globalref', `globaldef' and
`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:

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

(The `_$$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 `GNU_HACKS.H'.  An example of the usage
is:

     int GLOBALREF (ijk);
     int GLOBALDEF (jkl, 0);

   The macros `GLOBALREF' and `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:

     typedef int intvector[10];
     intvector GLOBALREF (foo);

   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
`GLOBALDEF' macro by hand.  You may find a case where you wish to use
the `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: `{0,}', which will initialize the
entire array to `0'.

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

     int GLOBALVALUEREF(ijk);

declares the variable `ijk' as an array of type `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.  *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.

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

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

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


File: gcc.info,  Node: VMS Misc,  Prev: Global Declarations,  Up: VMS

Other VMS Issues
================

   GNU CC automatically arranges for `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.

   Under previous versions of GNU CC, the generated code would
occasionally give strange results when linked to the sharable
`VAXCRTL' library.  Now this should work.

   A caveat for use of `const' global variables: the `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.

   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 `/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 "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
`/VERBOSE' compiler option is used, the assembler will print both the
full and truncated names of each symbol that is truncated.

   The `/NOCASE_HACK' compiler option should not be used when you are
compiling programs that use libg++. libg++ has several instances of
objects (i.e.  `Filebuf' and `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:

     #define StuDlyCapS studlycaps

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


File: gcc.info,  Node: Portability,  Next: Interface,  Prev: VMS,  Up: Top

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.

   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 `abort'.  Fortunately, the new
strategies can be added in a machine-independent fashion, and will
affect only the target machines that need them.


File: gcc.info,  Node: Interface,  Next: Passes,  Prev: Portability,  Up: Top

Interfacing to GNU CC Output
****************************

   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 (*note Machine Macros::.).

   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 `int' or `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 `STRUCT_VALUE' and
`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.

   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.

   If you use `longjmp', beware of automatic variables.  ANSI C says
that automatic variables that are not declared `volatile' have
undefined values after a `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 `longjmp', and you don't
want to write `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:

     {
       int careful;
       &careful;
       ...
     }

   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 `-msoft-float'.  Some standard parts of the C library, such as
`bcopy' or `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 `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 `libgcc.a' is not needed, but it is
searched just in case.

   Each arithmetic function is defined in `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, `libgcc1.c' does not work if
compiled with GNU CC, because each arithmetic function would compile
into a call to itself!


File: gcc.info,  Node: Passes,  Next: RTL,  Prev: Interface,  Up: Top

Passes and Files of the Compiler
********************************

   The overall control structure of the compiler is in `toplev.c'.
This file is responsible for initialization, decoding arguments,
opening and closing files, and sequencing the passes.

   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.

   Each time the parsing pass reads a complete function definition or
top-level declaration, it calls the function `rest_of_compilation' or
`rest_of_decl_compilation' in `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
`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 (*note
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 `-d' options.

   * 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.

     Constant folding and some arithmetic simplifications are also done
     during this pass.

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

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

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

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

     The file `c-common.c' is also used for all of the above languages.

   * 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.

     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.

     Optimization is done in this pass for `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 `switch'
     statements.

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

     The header files `insn-flags.h' and `insn-codes.h', generated
     from the machine description by the programs `genflags' and
     `gencodes', tell this pass which standard names are available for
     use and which patterns correspond to them.

     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).

     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 `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 `integrate.h' is also used for this
     purpose.

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

   * 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 `jump.c'.

     The option `-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 `.jump' to the input file name.

   * Register scan.  This pass finds the first and last use of each
     register, as a guide for common subexpression elimination.  Its
     source is in `regclass.c'.

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

   * Common subexpression elimination.  This pass also does constant
     propagation.  Its source file is `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 `-ds' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.cse' to
     the input file name.

   * Loop optimization.  This pass moves constant expressions out of
     loops, and optionally does strength-reduction and loop unrolling
     as well.  Its source files are `loop.c' and `unroll.c', plus the
     header `loop.h' used for communication between them.  Loop
     unrolling uses some functions in `integrate.c' and the header
     `integrate.h'.

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

   * If `-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 `-dt' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.cse2' to
     the input file name.

   * 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 `stupid.c'.

   * Data flow analysis (`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.

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

   * Instruction combination (`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 `-dc' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.combine'
     to the input file name.

   * Instruction scheduling (`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 `-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 `.sched' to the input file name.

   * Register class preferencing.  The RTL code is scanned to find out
     which register class is best for each pseudo register.  The source
     file is `regclass.c'.

   * Local register allocation (`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 `-dl' causes a debugging dump of the RTL code after
     this pass.  This dump file's name is made by appending `.lreg' to
     the input file name.

   * Global register allocation (`global-alloc.c').  This pass
     allocates hard registers for the remaining pseudo registers (those
     whose life spans are not contained in one basic block).

   * 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 `reload.c' and `reload1.c', plus the header
     `reload.h' used for communication between them.

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

   * Instruction scheduling is repeated here to try to avoid pipeline
     stalls due to memory loads generated for spilled pseudo registers.

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

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

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

   * 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
     `reorg.c'.

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

   * 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 `reg-stack.c'.

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

   * 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 `final.c' plus `insn-output.c'; the latter
     is generated automatically from the machine description by the
     tool `genoutput'.  The header file `conditions.h' is used for
     communication between these files.

   * 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 `dbxout.c' for DBX
     symbol table format, `sdbout.c' for SDB symbol table format, and
     `dwarfout.c' for DWARF symbol table format.

   Some additional files are used by all or many passes:

   * Every pass uses `machmode.def' and `machmode.h' which define the
     machine modes.

   * Several passes use `real.h', which defines the default
     representation of floating point constants and how to operate on
     them.

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

   * Several passes refer to the header file `insn-config.h' which
     contains a few parameters (C macro definitions) generated
     automatically from the machine description RTL by the tool
     `genconfig'.

   * Several passes use the instruction recognizer, which consists of
     `recog.c' and `recog.h', plus the files `insn-recog.c' and
     `insn-extract.c' that are generated automatically from the
     machine description by the tools `genrecog' and `genextract'.

   * Several passes use the header files `regs.h' which defines the
     information recorded about pseudo register usage, and
     `basic-block.h' which defines the information recorded about
     basic blocks.

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

   * Several passes use instruction attributes.  A definition of the
     attributes defined for a particular machine is in file
     `insn-attr.h', which is generated from the machine description by
     the program `genattr'.  The file `insn-attrtab.c' contains
     subroutines to obtain the attribute values for insns.  It is
     generated from the machine description by the program
     `genattrtab'.


File: gcc.info,  Node: RTL,  Next: Machine Desc,  Prev: Passes,  Up: Top

RTL Representation
******************

   Most of the work of the compiler is done on an intermediate
representation called register transfer language.  In this language,
the instructions to be output are described, pretty much one by one,
in an algebraic form that describes what the instruction does.

   RTL is inspired by Lisp lists.  It has both an internal form, made
up of structures that point at other structures, and a textual form
that is used in the machine description and in printed debugging
dumps.  The textual form uses nested parentheses to indicate the
pointers in the internal form.

* Menu:

* RTL Objects::       Expressions vs vectors vs strings vs integers.
* Accessors::         Macros to access expression operands or vector elts.
* Flags::             Other flags in an RTL expression.
* Machine Modes::     Describing the size and format of a datum.
* Constants::         Expressions with constant values.
* Regs and Memory::   Expressions representing register contents or memory.
* Arithmetic::        Expressions representing arithmetic on other expressions.
* Comparisons::       Expressions representing comparison of expressions.
* Bit Fields::        Expressions representing bit-fields in memory or reg.
* Conversions::       Extending, truncating, floating or fixing.
* RTL Declarations::  Declaring volatility, constancy, etc.
* Side Effects::      Expressions for storing in registers, etc.
* Incdec::            Embedded side-effects for autoincrement addressing.
* Assembler::         Representing `asm' with operands.
* Insns::             Expression types for entire insns.
* Calls::             RTL representation of function call insns.
* Sharing::           Some expressions are unique; others *must* be copied.


File: gcc.info,  Node: RTL Objects,  Next: Accessors,  Prev: RTL,  Up: RTL

RTL Object Types
================

   RTL uses four kinds of objects: expressions, integers, strings and
vectors.  Expressions are the most important ones.  An RTL expression
("RTX", for short) is a C structure, but it is usually referred to
with a pointer; a type that is given the typedef name `rtx'.

   An integer is simply an `int'; their written form uses decimal
digits.

   A string is a sequence of characters.  In core it is represented as
a `char *' in usual C fashion, and it is written in C syntax as well.
However, strings in RTL may never be null.  If you write an empty
string in a machine description, it is represented in core as a null
pointer rather than as a pointer to a null character.  In certain
contexts, these null pointers instead of strings are valid.  Within
RTL code, strings are most commonly found inside `symbol_ref'
expressions, but they appear in other contexts in the RTL expressions
that make up machine descriptions.

   A vector contains an arbitrary number of pointers to expressions.
The number of elements in the vector is explicitly present in the
vector.  The written form of a vector consists of square brackets
(`[...]') surrounding the elements, in sequence and with whitespace
separating them.  Vectors of length zero are not created; null
pointers are used instead.

   Expressions are classified by "expression codes" (also called RTX
codes).  The expression code is a name defined in `rtl.def', which is
also (in upper case) a C enumeration constant.  The possible expression
codes and their meanings are machine-independent.  The code of an RTX
can be extracted with the macro `GET_CODE (X)' and altered with
`PUT_CODE (X, NEWCODE)'.

   The expression code determines how many operands the expression
contains, and what kinds of objects they are.  In RTL, unlike Lisp,
you cannot tell by looking at an operand what kind of object it is.
Instead, you must know from its context--from the expression code of
the containing expression.  For example, in an expression of code
`subreg', the first operand is to be regarded as an expression and the
second operand as an integer.  In an expression of code `plus', there
are two operands, both of which are to be regarded as expressions.  In
a `symbol_ref' expression, there is one operand, which is to be
regarded as a string.

   Expressions are written as parentheses containing the name of the
expression type, its flags and machine mode if any, and then the
operands of the expression (separated by spaces).

   Expression code names in the `md' file are written in lower case,
but when they appear in C code they are written in upper case.  In this
manual, they are shown as follows: `const_int'.

   In a few contexts a null pointer is valid where an expression is
normally wanted.  The written form of this is `(nil)'.