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Info file internals, produced by Makeinfo, -*- Text -*- from input
file internals.texinfo.

This file documents the internals of the GNU compiler.

Copyright (C) 1988 Free Software Foundation, Inc.

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

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

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



File: internals,  Node: Incompatibilities,  Next: Extensions,  Prev: Trouble,  Up: Top

Incompatibilities of GNU CC
***************************

There are several noteworthy incompatibilities between GNU C and most
existing (non-ANSI) versions of C.

Ultimately our intention is that the `-traditional' option will
eliminate most of these incompatibilities by telling GNU C to behave
like the other C compilers.

   * GNU CC normally makes string constants read-only.  If several
     identical-looking string constants are used, GNU CC stores only
     one copy of the string.

     One consequence is that you cannot call `mktemp' with a string
     constant argument.  The function `mktemp' always alters the
     string its argument points to.

     Another consequence is that `sscanf' does not work on some
     systems when passed a string constant as its format control
     string.  This is because `sscanf' incorrectly tries to write
     into the string constant.

     The best solution to these problems is to change the program to
     use `char'-array variables with initialization strings for these
     purposes instead of string constants.  But if this is not
     possible, you can use the `-fwritable-strings' flag, which
     directs GNU CC to handle string constants the same way most C
     compilers do.

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

          #define foo(a) "a"

     will produce output `"a"' regardless of what the argument A is.

     The `-traditional' option directs GNU CC to handle such cases
     (among others) in the old-fashioned (non-ANSI) fashion.

   * When you use `setjmp' and `longjmp', the only automatic
     variables guaranteed to remain valid are those declared
     `volatile'.  This is a consequence of automatic register
     allocation.  Consider this function:

          jmp_buf j;
          
          foo ()
          {
            int a, b;
          
            a = fun1 ();
            if (setjmp (j))
              return a;
          
            a = fun2 ();
            /* `longjmp (j)' may be occur in `fun3'. */
            return a + fun3 ();
          }

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

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

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

     In some other C compilers, a `extern' declaration affects all
     the rest of the file even if it happens within a block.

     The `-traditional' option directs GNU C to treat all `extern'
     declarations as global, like traditional compilers.

   * In traditional C, you can combine `long', etc., with a typedef
     name, as shown here:

          typedef int foo;
          typedef long foo bar;

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

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

   * PCC allows whitespace in the middle of compound assignment
     operators such as `+='.  GNU CC, following the ANSI standard,
     does not allow this.  The difficulty described immediately above
     applies here too.

   * When compiling functions that return `float', PCC converts it to
     a double.  GNU CC actually returns a `float'.  If you are
     concerned with PCC compatibility, you should declare your
     functions to return `double'; you might as well say what you mean.

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

     The method used by GCC is as follows: a structure or union which
     is 1, 2, 4 or 8 bytes long is returned like a scalar.  A
     structure or union with any other size is stored into an address
     supplied by the caller in a special, fixed register.

     PCC usually handles all sizes of structures and unions by
     returning the address of a block of static storage containing
     the value.  This method is not used in GCC because it is slower
     and nonreentrant.

     On systems where PCC works this way, you may be able to make
     GCC-compiled code call such functions that were compiled with
     PCC by declaring them to return a pointer to the structure or
     union instead of the structure or union itself.  For example,
     instead of this:

          struct foo nextfoo ();

     write this:

          struct foo *nextfoo ();
          #define nextfoo *nextfoo

     (Note that this assumes you are using the GNU preprocessor and
     not `-traditional', so that the ANSI antirecursion rules for
     macro expansions are effective.)



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 = this_length;
     }

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; 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.

If there are no output operands, and there are input operands, then
you should write two colons in a row where the output operands would
go.

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, or in which not all
bits are written and the other bits contain useful information, 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.

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).  For example, on
the vax,

     asm volatile ("movc3 %0,%1,%2"
                   : /* no outputs */
                   : "g" (from), "g" (to), "g" (count)
                   : "r0", "r1", "r2", "r3", "r4", "r5");

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:

     [email protected]
     {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.

   * 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.  They won't tell me anything.

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'.

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.

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