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This is Info file, produced by Makeinfo version 1.68 from the
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This file documents the use and the internals of the GNU compiler.
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File:, Node: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs
How to Report Bugs
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 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
Please report each bug in a separate message. This makes it easier
for us to track which bugs have been fixed and to forward your bugs
reports to the appropriate maintainer.
Do not compress and encode any part of your bug report using programs
such as `uuencode'. If you do so it will slow down the processing of
your bug. If you must submit multiple large files, use `shar', which
allows us to read your message without having to run any decompression
To enable someone to investigate 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, we 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 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. (When you do this, use the same `-I', `-D' or `-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 complete file of compiler
input; 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.
Even if the input file that fails comes from a GNU program, you
should still send the complete test case. Don't ask the GNU CC
maintainers to do the extra work of obtaining the program in
question--they are all overworked as it is. Also, the problem may
depend on what is in the header files on your system; it is
unreliable for the GNU CC maintainers to try the problem with the
header files available to them. By sending CPP output, you can
eliminate this source of uncertainty and save us a certain
percentage of wild goose chases.
* The command arguments you gave GNU CC or GNU C++ to compile that
example and observe the bug. For example, did you use `-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.
* The type of machine you are using, and the operating system name
and version number.
* The operands you gave to the `configure' command when you installed
the compiler.
* 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. A description in English is not
enough--send 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.
* Details of any other deviations from the standard procedure for
installing GNU CC.
* 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 *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 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
If the problem is a diagnostic when compiling GNU CC with some
other compiler, say whether it is a warning or an error.
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.
* If you send examples of assembler code output from GNU CC or GNU
C++, 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 mention 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.
* 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.
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
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.
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
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 *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
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.
* In particular, some people insert conditionals `#ifdef BUG' around
a statement which, if removed, makes the bug not happen. These
are just clutter; we won't pay any attention to them anyway.
Besides, you should send us cpp output, and that can't have
* 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.
*Note Sending Patches::, for guidelines on how to make it easy for
us to understand and install your patches.
* 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.
* A core dump file.
We have no way of examining a core dump for your type of machine
unless we have an identical system--and if we do have one, we
should be able to reproduce the crash ourselves.

File:, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs
Sending Patches for GNU CC
If you would like to write bug fixes or improvements for the GNU C
compiler, that is very helpful. Send suggested fixes to the bug report
mailing list, `'.
Please follow these guidelines so we can study your patches
efficiently. 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.
* 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.)
* 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.
* Include all the comments that are appropriate to help people
reading the source in the future understand why this change was
* Don't mix together changes made for different reasons. Send them
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.
* 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.
* Use `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 `-c' format.
If you have GNU diff, use `diff -cp', which shows the name of the
function that each change occurs in.
* 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
Read the `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.
* When you write the fix, keep in mind that we can't install a
change that would break other systems.
People often suggest fixing a problem by changing
machine-independent files such as `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
Sometimes people send fixes that *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.

File:, Node: Service, Next: Contributing, Prev: Bugs, Up: Top
How To Get Help with GNU CC
If you need help installing, using or changing GNU CC, there are two
ways to find it:
* Send a message to a suitable network mailing list. First try
`', and if that brings no response, try
* Look in the service directory for someone who might help you for a
fee. The service directory is found in the file named `SERVICE'
in the GNU CC distribution.

File:, Node: Contributing, Next: VMS, Prev: Service, Up: Top
Contributing to GNU CC Development
If you would like to help pretest GNU CC releases to assure they work
well, or if you would like to work on improving GNU CC, please contact
the maintainers at `'. A pretester should be
willing to try to investigate bugs as well as report them.
If you'd like to work on improvements, please ask for suggested
projects or suggest your own ideas. If you have already written an
improvement, please tell us about it. If you have not yet started
work, it is useful to contact `' before you
start; the maintainers may be able to suggest ways to make your
extension fit in better with the rest of GNU CC and with other
development plans.

File:, Node: VMS, Next: Portability, Prev: Contributing, Up: Top
Using GNU CC on VMS
Here is how to use 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
* VMS Misc:: Misc information.

File:, Node: Include Files and VMS, Next: Global Declarations, 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, the name `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.
A `#include' file name cannot contain a DECNET node name. The
preprocessor reports an I/O error if you attempt to use a node name,
whether explicitly, or implicitly via a logical name.

File:, 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__
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \
globalref TYPE NAME
globaldef TYPE NAME = VALUE
globalvalue TYPE NAME = VALUE
globalvalue TYPE NAME
(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
GLOBALREF (int, ijk);
GLOBALDEF (int, 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];
GLOBALREF (intvector, 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:
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.
GLOBALVALUEREF (int, ss$_normal);
GLOBALVALUEDEF (int, xyzzy,123);
#ifdef __GNUC__
#define ss$_normal ((int) ss$_normal)
#define xyzzy ((int) xyzzy)
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__
GLOBALDEF (int, color, 0);
enum globaldef color {RED, BLUE, GREEN = 3};

File:, 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.
Although the VMS linker does distinguish between upper and lower case
letters in global symbols, most VMS compilers convert all such symbols
into upper case and most run-time library routines also have upper case
names. To be able to reliably call such routines, GNU CC (by means of
the assembler GAS) converts global symbols into upper case like other
VMS compilers. However, since the usual practice in C is to distinguish
case, GNU CC (via GAS) tries to preserve usual C behavior by augmenting
each name that is not all lower case. This means truncating the name
to at most 23 characters and then adding more characters at the end
which encode the case pattern of those 23. Names which contain at
least one dollar sign are an exception; they are converted directly into
upper case without augmentation.
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. The compiler option `/NAMES' also provides control over
global name handling.
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
#define StuDlyCapS studlycaps
These macro definitions can be placed in a header file to minimize
the number of changes to your source code.

File:, 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:, 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 Target 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;
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!