Re: malloc attributes and realloc

[Dan Nicolaescu <dann at ics dot uci dot edu>]

Daniel Berlin <dberlin@dberlin.org> writes:

  > On Jan 2, 2004, at 11:58 AM, Joseph S. Myers wrote:
>>  (And quite what does the malloc
>> attribute do at present, given the lack of points-to analysis in GCC?)
>>
>>
  > Absolutely nothing, AFAIK. :)
  > I'm dreading the __attribute__ (malloc) related bugs that will happen
  > if I include this optimization in tree-ssa's PTA.

Are you referring to what __attribute__ (malloc) does for the tree
optimizations on the tree-ssa branch?

For RTL optimizations __attribute__ (malloc) does what the
documentation says:

     The `malloc' attribute is used to tell the compiler that a function
     may be treated as if it were the malloc function.  The compiler
     assumes that calls to malloc result in a pointers that cannot
     alias anything.  This will often improve optimization.

See the difference in the generated code for "foo" and "bar": 


extern void *myalloc_withattribute (unsigned int size) __attribute__((malloc));
extern void *myalloc_no_attribute (unsigned int size);
extern int link_error (void);

struct str {
  int *aaaa;
  int *bbbb;
};

int
foo (unsigned int SZ, struct str *test)
{
  int *aaaa;
  int *bbbb;
  
  aaaa = myalloc_withattribute (SZ*sizeof(int));
  bbbb = myalloc_withattribute (SZ*sizeof(int));

  aaaa[0] = 1;
  bbbb[0] = 1;

  /* aaaa and bbbb do not alias, so the comparisons can be resolved at
     compile time */
  if (aaaa[0] != 1)
    return link_error ();
  if (bbbb[0] != 1)
    return link_error ();

  test->aaaa = aaaa;
  test->bbbb = bbbb;
  return 0;
}

int
bar (unsigned int SZ, struct str *test)
{
  int *aaaa;
  int *bbbb;
  
  aaaa = myalloc_no_attribute (SZ*sizeof(int));
  bbbb = myalloc_no_attribute (SZ*sizeof(int));

  aaaa[0] = 1;
  bbbb[0] = 1;

  /* bbbb might alias aaaa, the comparison cannot be resolved at
     compile time */
  if (aaaa[0] != 1)
    return link_error ();
  if (bbbb[0] != 1)
    return link_error ();

  test->aaaa = aaaa;
  test->bbbb = bbbb;
  return 0;
}

The generated x86 assembly: 
(compiled with gcc -O2 -fomit-frame-pointer)

foo:
	subl	$28, %esp
	movl	%ebx, 16(%esp)
	movl	32(%esp), %ebx
	movl	%edi, 24(%esp)
	movl	36(%esp), %edi
	sall	$2, %ebx
	movl	%esi, 20(%esp)
	movl	%ebx, (%esp)
	call	myalloc_withattribute
	movl	%ebx, (%esp)
	movl	%eax, %esi
	call	myalloc_withattribute
	movl	%eax, 4(%edi)
	movl	%esi, (%edi)
	movl	$1, (%esi)
	movl	$1, (%eax)
	movl	16(%esp), %ebx
	xorl	%eax, %eax
	movl	20(%esp), %esi
	movl	24(%esp), %edi
	addl	$28, %esp
	ret

bar:
	subl	$28, %esp
	movl	%ebx, 16(%esp)
	movl	32(%esp), %ebx
	movl	%esi, 20(%esp)
	movl	%edi, 24(%esp)
	sall	$2, %ebx
	movl	36(%esp), %edi
	movl	%ebx, (%esp)
	call	myalloc_no_attribute
	movl	%ebx, (%esp)
	movl	%eax, %esi
	call	myalloc_no_attribute
	movl	$1, (%esi)
	movl	$1, (%eax)
	cmpl	$1, (%esi)
	je	.L5
	movl	16(%esp), %ebx
	movl	20(%esp), %esi
	movl	24(%esp), %edi
	addl	$28, %esp
	jmp	link_error
	.p2align 4,,7
.L5:
	movl	%eax, 4(%edi)
	xorl	%eax, %eax
	movl	%esi, (%edi)
	movl	16(%esp), %ebx
	movl	20(%esp), %esi
	movl	24(%esp), %edi
	addl	$28, %esp
	ret