V10/cmd/gcc/internals-2
File: internals, Node: Extensions, Next: Bugs, Prev: Incompatibilities, Up: Top
GNU Extensions to the C Language
********************************
GNU C provides several language features not found in ANSI standard C.
(The `-pedantic' option directs GNU CC to print a warning message if any of
these features is used.) To test for the availability of these features in
conditional compilation, check for a predefined macro `__GNUC__', which is
always defined under GNU CC.
* Menu:
* Statement Exprs:: Putting statements and declarations inside expressions.
* Naming Types:: Giving a name to the type of some expression.
* Typeof:: `typeof': referring to the type of an expression.
* Lvalues:: Using `?:', `,' and casts in lvalues.
* Conditionals:: Omitting the middle operand of a `?:' expression.
* Zero-Length:: Zero-length arrays.
* Variable-Length:: Arrays whose length is computed at run time.
* Subscripting:: Any array can be subscripted, even if not an lvalue.
* Pointer Arith:: Arithmetic on `void'-pointers and function pointers.
* Constructors:: Constructor expressions give structures, unions
or arrays as values.
* Dollar Signs:: Dollar sign is allowed in identifiers.
* Alignment:: Inquiring about the alignment of a type or variable.
* Inline:: Defining inline functions (as fast as macros).
* Extended Asm:: Assembler instructions with C expressions as operands.
(With them you can define ``built-in'' functions.)
* Asm Labels:: Specifying the assembler name to use for a C symbol.
File: internals, Node: Statement Exprs, Next: Naming Types, Prev: Extensions, Up: Extensions
Statements and Declarations inside of Expressions
=================================================
A compound statement in parentheses may appear inside an expression in GNU
C. This allows you to declare variables within an expression. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the
absolute value of `foo ()'.
This feature is especially useful in making macro definitions ``safe'' (so
that they evaluate each operand exactly once). For example, the
``maximum'' function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either A or B twice, with bad results if the
operand has side effects. In GNU C, if you know the type of the operands
(here let's assume `int'), you can define the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
Embedded statements are not allowed in constant expressions, such as the
value of an enumeration constant, the width of a bit field, or the initial
value of a static variable.
If you don't know the type of the operand, you can still do this, but you
must use `typeof' (*Note Typeof::.) or type naming (*Note Naming Types::.).
File: internals, Node: Naming Types, Next: Typeof, Prev: Statement Exprs, Up: Extensions
Naming an Expression's Type
===========================
You can give a name to the type of an expression using a `typedef'
declaration with an initializer. Here is how to define NAME as a type name
for the type of EXP:
typedef NAME = EXP;
This is useful in conjunction with the statements-within-expressions
feature. Here is how the two together can be used to define a safe
``maximum'' macro that operates on any arithmetic type:
#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for `a' and `b'. Eventually we hope to
design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.
File: internals, Node: Typeof, Next: Lvalues, Prev: Naming Types, Up: Extensions
Referring to a Type with `typeof'
=================================
Another way to refer to the type of an expression is with `typeof'. The
syntax of using of this keyword looks like `sizeof', but the construct acts
semantically like a type name defined with `typedef'.
There are two ways of writing the argument to `typeof': with an expression
or with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that `x' is an array of functions; the type described is that
of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to `int'.
A `typeof'-construct can be used anywhere a typedef name could be used.
For example, you can use it in a declaration, in a cast, or inside of
`sizeof' or `typeof'.
* This declares `y' with the type of what `x' points to.
typeof (*x) y;
* This declares `y' as an array of such values.
typeof (*x) y[4];
* This declares `y' as an array of pointers to characters:
typeof (typeof (char *)[4]) y;
It is equivalent to the following traditional C declaration:
char *y[4];
To see the meaning of the declaration using `typeof', and why it might
be a useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])
Now the declaration can be rewritten this way:
array (pointer (char), 4) y;
Thus, `array (pointer (char), 4)' is the type of arrays of 4 pointers
to `char'.
File: internals, Node: Lvalues, Next: Conditionals, Prev: Typeof, Up: Extensions
Generalized Lvalues
===================
Compound expressions, conditional expressions and casts are allowed as
lvalues provided their operands are lvalues. This means that you can take
their addresses or store values into them.
For example, a compound expression can be assigned, provided the last
expression in the sequence is an lvalue. These two expressions are
equivalent:
(a, b) += 5
a, (b += 5)
Similarly, the address of the compound expression can be taken. These two
expressions are equivalent:
&(a, b)
a, &b
A conditional expression is a valid lvalue if its type is not void and the
true and false branches are both valid lvalues. For example, these two
expressions are equivalent:
(a ? b : c) = 5
(a ? b = 5 : (c = 5))
A cast is a valid lvalue if its operand is valid. Taking the address of
the cast is the same as taking the address without a cast, except for the
type of the result. For example, these two expressions are equivalent (but
the second may be valid when the type of `a' does not permit a cast to `int
*').
&(int *)a
(int **)&a
A simple assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the inner
left-hand side expression. After this is stored, the value is converter
back to the specified type to become the value of the assignment. Thus, if
`a' has type `char *', the following two expressions are equivalent:
(int)a = 5
(int)(a = (char *)5)
An assignment-with-arithmetic operation such as `+=' applied to a cast
performs the arithmetic using the type resulting from the cast, and then
continues as in the previous case. Therefore, these two expressions are
equivalent:
(int)a += 5
(int)(a = (char *) ((int)a + 5))
File: internals, Node: Conditionals, Next: Zero-Length, Prev: Lvalues, Up: Extensions
Conditional Expressions with Omitted Middle-Operands
====================================================
The middle operand in a conditional expression may be omitted. Then if the
first operand is nonzero, its value is the value of the conditional
expression.
Therefore, the expression
x ? : y
has the value of `x' if that is nonzero; otherwise, the value of `y'.
This example is perfectly equivalent to
x ? x : y
In this simple case, the ability to omit the middle operand is not
especially useful. When it becomes useful is when the first operand does,
or may (if it is a macro argument), contain a side effect. Then repeating
the operand in the middle would perform the side effect twice. Omitting
the middle operand uses the value already computed without the undesirable
effects of recomputing it.
File: internals, Node: Zero-Length, Next: Variable-Length, Prev: Conditionals, Up: Extensions
Arrays of Length Zero
=====================
Zero-length arrays are allowed in GNU C. They are very useful as the last
element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
{
struct line *thisline
= (struct line *) malloc (sizeof (struct line) + this_length);
thisline->length = thislength;
}
In standard C, you would have to give `contents' a length of 1, which means
either you waste space or complicate the argument to `malloc'.
File: internals, Node: Variable-Length, Next: Subscripting, Prev: Zero-Length, Up: Extensions
Arrays of Variable Length
=========================
Variable-length automatic arrays are allowed in GNU C. These arrays are
declared like any other automatic arrays, but with a length that is not a
constant expression. The storage is allocated at that time and deallocated
when the brace-level is exited. For example:
FILE *concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}
You can also define structure types containing variable-length arrays, and
use them even for arguments or function values, as shown here:
int foo;
struct entry
{
char data[foo];
};
struct entry
tester (struct entry arg)
{
struct entry new;
int i;
for (i = 0; i < foo; i++)
new.data[i] = arg.data[i] + 1;
return new;
}
(Eventually there will be a way to say that the size of the array is
another member of the same structure.)
The length of an array is computed on entry to the brace-level where the
array is declared and is remembered for the scope of the array in case you
access it with `sizeof'.
Jumping or breaking out of the scope of the array name will also deallocate
the storage. Jumping into the scope is not allowed; you will get an error
message for it.
You can use the function `alloca' to get an effect much like
variable-length arrays. The function `alloca' is available in many other C
implementations (but not in all). On the other hand, variable-length
arrays are more elegant.
There are other differences between these two methods. Space allocated
with `alloca' exists until the containing *function* returns. The space
for a variable-length array is deallocated as soon as the array name's
scope ends. (If you use both variable-length arrays and `alloca' in the
same function, deallocation of a variable-length array will also deallocate
anything more recently allocated with `alloca'.)
File: internals, Node: Subscripting, Next: Pointer Arith, Prev: Variable-Length, Up: Extensions
Non-Lvalue Arrays May Have Subscripts
=====================================
Subscripting is allowed on arrays that are not lvalues, even though the
unary `&' operator is not. For example, this is valid in GNU C though not
valid in other C dialects:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}
File: internals, Node: Pointer Arith, Next: Initializers, Prev: Subscripting, Up: Extensions
Arithmetic on `void'-Pointers and Function Pointers
===================================================
In GNU C, addition and subtraction operations are supported on pointers to
`void' and on pointers to functions. This is done by treating the size of
a `void' or of a function as 1.
A consequence of this is that `sizeof' is also allowed on `void' and on
function types, and returns 1.
File: internals, Node: Initializers, Next: Constructors, Prev: Pointer Arith, Up: Extensions
Non-Constant Initializers
=========================
The elements of an aggregate initializer are not required to be constant
expressions in GNU C. Here is an example of an initializer with run-time
varying elements:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
...
}
File: internals, Node: Constructors, Next: Dollar Signs, Prev: Initializers, Up: Extensions
Constructor Expressions
=======================
GNU C supports constructor expressions. A constructor looks like a cast
containing an initializer. Its value is an object of the type specified in
the cast, containing the elements specified in the initializer. The type
must be a structure, union or array type.
Assume that `struct foo' and `structure' are declared as shown:
struct foo {int a; char b[2];} structure;
Here is an example of constructing a `struct foo' with a constructor:
structure = ((struct foo) {x + y, 'a', 0});
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
You can also construct an array. If all the elements of the constructor
are (made up of) simple constant expressions, suitable for use in
initializers, then the constructor is an lvalue and can be coerced to a
pointer to its first element, as shown here:
char **foo = (char *[]) { "x", "y", "z" };
Array constructors whose elements are not simple constants are not very
useful, because the constructor is not an lvalue. There are only two valid
ways to use it: to subscript it, or initialize an array variable with it.
The former is probably slower than a `switch' statement, while the latter
does the same thing an ordinary C initializer would do.
output = ((int[]) { 2, x, 28 }) [input];
File: internals, Node: Dollar Signs, Next: Alignment, Prev: Constructors, 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.
File: internals, 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: internals, 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)++;
}
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, 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.
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.
File: internals, 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 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 number of operands is limited to the maximum number of
operands in any instruction pattern in the machine description.
Output operand expressions must be lvalues, and there must be at least one
of them. The compiler can check this. The input operands need not be
lvalues, and there need not be any. The compiler cannot check whether the
operands have data types that are reasonable for the instruction being
executed.
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. For an operand that is read-write, 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. Therefore it is not necessary to substitute operand
1 into the assembler code output.
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.
GNU CC assumes for optimization purposes that these instructions have 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 %1": \
"=m" (*(char *)0): "g" (x))
Note that we have supplied an output operand which is not actually used in
the instruction. This is because `asm' requires at least one output
operand. This requirement exists for internal implementation reasons and
we might be able to relax it in the future.
In this case output operand has the additional benefit effect of giving the
appearance of writing in memory. As a result, GNU CC will assume that data
previously fetched from memory must be fetched again if needed again later.
This may be desirable if you have not employed the `volatile' keyword on
all the variable declarations that ought to have it.
File: internals, Node: Asm Labels, 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' 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: internals, 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: internals, 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.
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: internals, 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
As a last resort, snail them 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 mention a fact or leave it out,
mention 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 an exact example.
If you want 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.
In cases where GNU CC generates incorrect code, if you send me a small
complete sample program I will find the error myself by running the
program under a debugger. If you send me a large example or a part of
a larger program, I cannot do this; you must debug the compiled
program and narrow the problem down to one source line. Tell me which
source line it is, and what you believe is incorrect about the code
generated for that line.
* 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.
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.
Of course, it can't hurt if you can find a simpler example that
triggers the same bug. Errors in the output will be easier to spot,
running under the debugger will take less time, etc. An easy 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.
However, simplification is not necessary; if you don't want to do
this, report the bug anyway.
* 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, because I might see
problems with your patch and decide to fix the problem another way.
Sometimes with a program as complicated as GNU CC it is very hard to
construct an example that will make the program go through a certain
point in the code. If you don't send me the example, I won't be able
to verify that the bug is fixed.
* 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 using the debugger to find the facts. They also don't
serve a useful purpose.
File: internals, 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: internals, 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. As a
result, functions compiled with PCC returning such types cannot be called
from code compiled with GNU CC, and vice versa. This usually does not
cause trouble because the Unix library routines don't return structures and
unions.
Structures and unions that are 1, 2, 4 or 8 bytes long are returned 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 in a register. This method is faster than the one normally
used by PCC and is also reentrant. The register used for passing the
address is specified by the machine-description macro `STRUCT_VALUE_REGNUM'.
GNU CC always passes arguments on the stack. At some point it will be
extended to pass arguments in registers, for machines which use that as the
standard calling convention. This will make it possible to use such a
convention on other machines as well. However, that would render it
completely incompatible with PCC. We will probably do this once we have a
complete GNU system so 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. The routines
needed on the Vax and 68000 are in the file `gnulib.c'. You must compile
this file with the standard C compiler, not with GNU CC, and then link it
with each program you compile with GNU CC. (In actuality, many programs
will not need it.) The usual function call interface is used for calling
the library routines. Some standard parts of the C library, such as
`bcopy', are also called automatically.