Celestin Apprentice 2

home *** CD-ROM | disk | FTP | other *** search

/ Celestin Apprentice 2 / Apprentice-Release2.iso / Tools / MPW / GCC 1.37.1r15 / Manual / Info / gcc.info-4 < prev next >

Wrap

Text File | 1990-03-14 | 44.9 KB | 1,033 lines | [TEXT/MPS ]

Info file gcc.info, produced by Makeinfo, -*- Text -*- from input file gcc.texinfo. This file documents the use and the internals of the GNU compiler. Copyright (C) 1988, 1989 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 sections entitled ``GNU General Public License'' and ``Protect Your Freedom--Fight `Look And Feel''' are 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 sections entitled ``GNU General Public License'' and ``Protect Your Freedom--Fight `Look And Feel''' 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: Function Attributes, Next: Dollar Signs, Prev: Constructors, Up: Extensions Declaring Attributes of Functions ================================= In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls. A few functions, such as `abort' and `exit', cannot return. These functions should be declared `volatile'. For example, extern volatile void abort (); tells the compiler that it can assume that `abort' will not return. This makes slightly better code, but more importantly it helps avoid spurious warnings of uninitialized variables. Many functions do not examine any values except their arguments, and have no effects except the return value. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared `const'. For example, extern const void square (); says that the hypothetical function `square' is safe to call fewer times than the program says. Note that a function that has pointer arguments and examines the data pointed to must *not* be declared `const'. Likewise, a function that calls a non-`const' function must not be `const'. Some people object to this feature, claiming that ANSI C's `#pragma' should be used instead. There are two reasons I did not do this. 1. It is impossible to generate `#pragma' commands from a macro. 2. The `#pragma' command is just as likely as these keywords to mean something else in another compiler. These two reasons apply to *any* application whatever: as far as I can see, `#pragma' is never useful. File: gcc.info, Node: Dollar Signs, Next: Alignment, Prev: Function Attributes, Up: Extensions Dollar Signs in Identifier Names ================================ In GNU C, you may use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. Dollar signs are allowed if you specify `-traditional'; they are not allowed if you specify `-ansi'. Whether they are allowed by default depends on the target machine; usually, they are not. File: gcc.info, Node: Alignment, Next: Inline, Prev: Dollar Signs, Up: Extensions Inquiring about the Alignment of a Type or Variable =================================================== The keyword `__alignof__' allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like `sizeof'. For example, if the target machine requires a `double' value to be aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This is true on many RISC machines. On more traditional machine designs, `__alignof__ (double)' is 4 or even 2. Some machines never actually require alignment; they allow reference to any data type even at an odd addresses. For these machines, `__alignof__' reports the *recommended* alignment of a type. When the operand of `__alignof__' is an lvalue rather than a type, the value is the largest alignment that the lvalue is known to have. It may have this alignment as a result of its data type, or because it is part of a structure and inherits alignment from that structure. For example, after this declaration: struct foo { int x; char y; } foo1; the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as `__alignof__ (int)', even though the data type of `foo1.y' does not itself demand any alignment. File: gcc.info, Node: Inline, Next: Extended Asm, Prev: Alignment, Up: Extensions An Inline Function is As Fast As a Macro ======================================== By declaring a function `inline', you can direct GNU CC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. To declare a function inline, use the `inline' keyword in its declaration, like this: inline int inc (int *a) { (*a)++; } (If you are writing a header file to be included in ANSI C programs, write `__inline__' instead of `inline'. *Note Alternate Keywords::.) You can also make all ``simple enough'' functions inline with the option `-finline-functions'. Note that certain usages in a function definition can make it unsuitable for inline substitution. When a function is both inline and `static', if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GNU CC does not actually output assembler code for the function, unless you specify the option `-fkeep-inline-functions'. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined. When an inline function is not `static', then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-`static' inline function is always compiled on its own in the usual fashion. If you specify both `inline' and `extern' in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This combination of `inline' and `extern' has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking `inline' and `extern') in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library. File: gcc.info, Node: Extended Asm, Next: Asm Labels, Prev: Inline, Up: Extensions Assembler Instructions with C Expression Operands ================================================= In an assembler instruction using `asm', you can now specify the operands of the instruction using C expressions. This means no more guessing which registers or memory locations will contain the data you want to use. You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand. For example, here is how to use the 68881's `fsinx' instruction: asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); Here `angle' is the C expression for the input operand while `result' is that of the output operand. Each has `"f"' as its operand constraint, saying that a floating-point register is required. The `=' in `=f' indicates that the operand is an output; all output operands' constraints must use `='. The constraints use the same language used in the machine description (*note Constraints::.). Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand, and another separates the last output operand from the first input, if any. Commas separate output operands and separate inputs. The total number of operands is limited to the maximum number of operands in any instruction pattern in the machine description. If there are no output operands, and there are input operands, then there must be two consecutive colons surrounding the place where the output operands would go. Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means, or whether it is valid assembler input. The extended `asm' feature is most often used for machine instructions that the compiler itself does not know exist. The output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm does not support input-output or read-write operands. For this reason, the constraint character `+', which indicates such an operand, may not be used. When the assembler instruction has a read-write operand, or an operand in which only some of the bits are to be changed, you must logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) `combine' instruction with `bar' as its read-only source operand and `foo' as its read-write destination: asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand, and it must refer to an output operand. Only a digit in the constraint can guarantee that one operand will be in the same place as another. The mere fact that `foo' is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work: asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GNU CC knows no reason not to do so. For example, the compiler might find a copy of the value of `foo' in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to `foo''s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GNU CC can't tell that. Unless an output operand has the `&' constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption that the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. *Note Modifiers::. Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the vax: asm volatile ("movc3 %0,%1,%2" : /* no outputs */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5"); You can put multiple assembler instructions together in a single `asm' template, separated either with newlines (written as `\n') or with semicolons if the assembler allows such semicolons. The GNU assembler allows semicolons and all Unix assemblers seem to do so. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes that the subroutine `_foo' accepts arguments in registers 9 and 10: asm ("movl %0,r9;movl %1,r10;call _foo" : /* no outputs */ : "g" (from), "g" (to) : "r9", "r10"); If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the `asm' construct, as follows: asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:" : "g" (result) : "g" (input)); This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do. Usually the most convenient way to use these `asm' instructions is to encapsulate them in macros that look like functions. For example, #define sin(x) \ ({ double __value, __arg = (x); \ asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ __value; }) Here the variable `__arg' is used to make sure that the instruction operates on a proper `double' value, and to accept only those arguments `x' which can convert automatically to a `double'. Another way to make sure the instruction operates on the correct data type is to use a cast in the `asm'. This is different from using a variable `__arg' in that it converts more different types. For example, if the desired type were `int', casting the argument to `int' would accept a pointer with no complaint, while assigning the argument to an `int' variable named `__arg' would warn about using a pointer unless the caller explicitly casts it. If an `asm' has output operands, GNU CC assumes for optimization purposes that the instruction has no side effects except to change the output operands. This does not mean that instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register. You can prevent an `asm' instruction from being deleted, moved or combined by writing the keyword `volatile' after the `asm'. For example: #define set_priority(x) \ asm volatile ("set_priority %0": /* no outputs */ : "g" (x)) (However, an instruction without output operands will not be deleted or moved, regardless, unless it is unreachable.) It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following ``store'' instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary ``test'' and ``compare'' instructions because they don't have any output operands. If you are writing a header file that should be includable in ANSI C programs, write `__asm__' instead of `asm'. *Note Alternate Keywords::. File: gcc.info, Node: Asm Labels, Next: Explicit Reg Vars, Prev: Extended Asm, Up: Extensions Controlling Names Used in Assembler Code ======================================== You can specify the name to be used in the assembler code for a C function or variable by writing the `asm' (or `__asm__') keyword after the declarator as follows: int foo asm ("myfoo") = 2; This specifies that the name to be used for the variable `foo' in the assembler code should be `myfoo' rather than the usual `_foo'. On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore. You cannot use `asm' in this way in a function *definition*; but you can get the same effect by writing a declaration for the function before its definition and putting `asm' there, like this: extern func () asm ("FUNC"); func (x, y) int x, y; ... It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GNU CC does not as yet have the ability to store static variables in registers. Perhaps that will be added. File: gcc.info, Node: Explicit Reg Vars, Next: Alternate Keywords, Prev: Asm Labels, Up: Extensions Variables in Specified Registers ================================ GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated. * Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often. * Local register variables in specific registers do not reserve the registers. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. These local variables are sometimes convenient for use with the extended `asm' feature (*note Extended Asm::.). * Menu: * Global Reg Vars:: * Local Reg Vars:: File: gcc.info, Node: Global Reg Vars, Next: Local Reg Vars, Prev: Explicit Reg Vars, Up: Explicit Reg Vars Defining Global Register Variables ---------------------------------- You can define a global register variable in GNU C like this: register int *foo asm ("a5"); Here `a5' is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it. Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register `a5' would be a good choice on a 68000 for a variable of pointer type. On machines with register windows, be sure to choose a ``global'' register that is not affected magically by the function call mechanism. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register `%a5'. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified. It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand). It is not safe for one function that uses a global register variable to call another such function `foo' by way of a third function `lose' that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared). This is because `lose' might save the register and put some other value there. For example, you can't expect a global register variable to be available in the comparison-function that you pass to `qsort', since `qsort' might have put something else in that register. (If you are prepared to recompile `qsort' with the same global register variable, you can solve this problem.) If you want to recompile `qsort' or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option `-ffixed-REG'. You need not actually add a global register declaration to their source code. A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller. On most machines, `longjmp' will restore to each global register variable the value it had at the time of the `setjmp'. On some machines, however, `longjmp' will not change the value of global register variables. To be portable, the function that called `setjmp' should make other arrangements to save the values of the global register variables, and to restore them in a `longjmp'. This way, the the same thing will happen regardless of what `longjmp' does. All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions. Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register. File: gcc.info, Node: Local Reg Vars, Prev: Global Reg Vars, Up: Explicit Reg Vars Specifying Registers for Local Variables ---------------------------------------- You can define a local register variable with a specified register like this: register int *foo asm ("a5"); Here `a5' is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function. Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (*note Extended Asm::.). Both of these things generally require that you conditionalize your program according to cpu type. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register `%a5'. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers made unavailable for use in the reload pass. I would not be surprised if excessive use of this feature leaves the compiler too few available registers to compile certain functions. File: gcc.info, Node: Alternate Keywords, Prev: Explicit Reg Vars, Up: Extensions Alternate Keywords ================== The option `-traditional' disables certain keywords; `-ansi' disables certain others. This causes trouble when you want to use GNU C extensions, or ANSI C features, in a general-purpose header file that should be usable by all programs, including ANSI C programs and traditional ones. The keywords `asm', `typeof' and `inline' cannot be used since they won't work in a program compiled with `-ansi', while the keywords `const', `volatile', `signed', `typeof' and `inline' won't work in a program compiled with `-traditional'. The way to solve these problems is to put `__' at the beginning and end of each problematical keyword. For example, use `__asm__' instead of `asm', `__const__' instead of `const', and `__inline__' instead of `inline'. Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this: #ifndef __GNUC__ #define __asm__ asm #endif File: gcc.info, Node: Bugs, Next: Portability, Prev: Extensions, Up: Top Reporting Bugs ************** Your bug reports play an essential role in making GNU CC reliable. Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the important 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? * Reporting: Bug Reporting. How to report a bug effectively. File: gcc.info, Node: Bug Criteria, Next: Bug Reporting, Prev: Bugs, Up: Bugs Have You Found a Bug? ===================== If you are not sure whether you have found a bug, here are some guidelines: * If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash. * If the compiler produces invalid assembly code, for any input whatever (except an `asm' statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run. * 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 (*note 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 compiler. For example, in many nonoptimizing compilers, you can write `x;' at the end of a function instead of `return x;', with the same results. But the value of the function is undefined if `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 `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. * If the compiler produces an error message for valid input, that is a compiler bug. Note that the following is not valid input, and the error message for it is not a bug: int foo (char); int foo (x) char x; { ... } The prototype says to pass a `char', while the definition says to pass an `int' and treat the value as a `char'. This is what the ANSI standard says, and it makes sense. * 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''. * If you are an experienced user of C compilers, your suggestions for improvement of GNU CC are welcome in any case. File: gcc.info, Node: Bug Reporting, Prev: Bug Criteria, Up: Bugs How to Report Bugs ================== Send bug reports for GNU C to one of these addresses: bug-gcc@prep.ai.mit.edu {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gcc *Do not send bug reports to `info-gcc', or to the newsgroup `gnu.gcc'.* Most users of GNU CC do not want to receive bug reports. Those that do, have asked to be on `bug-gcc'. The mailing list `bug-gcc' has a newsgroup which serves as a repeater. The mailing list and the 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 I need to ask for more information, I may be unable to reach you. For this reason, it is better to send bug reports to the mailing list. As a last resort, send bug reports on paper to: GNU Compiler Bugs 545 Tech Sq Cambridge, MA 02139 The fundamental principle of reporting bugs usefully is this: *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 me 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?'' Those bug reports are useless, and I urge everyone to *refuse to respond to them* except to chide the sender to report bugs properly. To enable me to fix the bug, you should include all these things: * The version of GNU CC. You can get this by running it with the `-v' option. Without this, I won't know whether there is any point in looking for the bug in the current version of GNU CC. * A complete input file that will reproduce the bug. If the bug is in the C preprocessor, send me a source file and any header files that it requires. If the bug is in the compiler proper (`cc1'), run your source file through the C preprocessor by doing `gcc -E SOURCEFILE > OUTFILE', then include the contents of OUTFILE in the bug report. (Any `-I', `-D' or `-U' options that you used in actual compilation should also be used when doing this.) 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 I can compile, all I 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. * The command arguments you gave GNU CC to compile that example and observe the bug. For example, did you use `-O'? To guarantee you won't omit something important, list them all. If I were to try to guess the arguments, I would probably guess wrong and then I would not encounter the bug. * The names of the files that you used for `tm.h' and `md' when you installed the compiler. * The type of machine you are using, and the operating system name and version number. * A description of what behavior you observe that you believe is incorrect. For example, ``It gets a fatal signal,'' or, ``There is an incorrect assembler instruction in the output.'' Of course, if the bug is that the compiler gets a fatal signal, then I will certainly notice it. But if the bug is incorrect output, I might not notice unless it is glaringly wrong. I won't study all the assembler code from a 50-line C program just on the off chance that it might be wrong. 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 mine would not. If you told me to expect a crash, then when mine fails to crash, I would know that the bug was not happening for me. If you had not told me to expect a crash, then I would not be able to draw any conclusion from my observations. Often the observed symptom is incorrect output when your program is run. Sad to say, this is not enough information for me unless the program is short and simple. If you send me a large program, I don't have time 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 me which source line it is, and what incorrect result happens when that line is executed. A person who understands the test program can find this as easily as a bug in the program itself. * If you send me examples of output from GNU CC, please use `-g' when you make them. The debugging information includes source line numbers which are essential for correlating the output with the input. * If you wish to suggest changes to the GNU CC source, send me 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 my development sources don't match those in your sources. Your line numbers would convey no useful information to me. * Additional information from a debugger might enable me to find a problem on a machine which I do not have available myself. However, you need to think when you collect this information if you want it to have any chance of being useful. 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. 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 `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 `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. Here are some things that are not necessary: * 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 I will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. I recommend that you save your time for something else. Of course, if you can find a simpler example to report *instead* of the original one, that is a convenience for me. 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 me the entire test case you used. * A patch for the bug. A patch for the bug does help me 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 I need. I might see problems with your patch and decide to fix the problem another way, or I 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 me the example, I won't be able to construct one, so I won't be able to verify that the bug is fixed. And if I can't understand what bug you are trying to fix, or why your patch should be an improvement, I won't install it. A test case will help me to understand. * 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. File: gcc.info, Node: Portability, Next: Interface, Prev: Bugs, 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. 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 `gnulib', 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 `gnulib' is not needed, but it is searched just in case. Each arithmetic function is defined in `gnulib.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, `gnulib.c' does not work if compiled with GNU CC, because each arithmetic function would compile into a call to itself!