Re: malloc attributes and realloc

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

Here's another data point for what is currently implemented in GCC for
__attribute__ (malloc). 

The pointers returned by an __attribute__ (malloc) function are not
considered distinct, see the pointer comparison example in the foobar
function below. 

It seems that this might the reason that having __attribute__ (malloc)
on the realloc function in glibc has not created any problems up to now.
But if code is added to GCC to optimize the comparison in foobar, then
having __attribute__ (malloc) for realloc might not be appropriate
anymore. 

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

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

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

  /* this is not resolved at compile time at this time */
  if (aaaa == bbbb)
    return link_error ();

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

the generated x86 assembly (using -O2 -fomit-frame-pointer): 


foobar:
        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_withattribute
        movl    %ebx, (%esp)
        movl    %eax, %esi
        call    myalloc_withattribute
        cmpl    %eax, %esi
        je      .L7
        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
        .p2align 4,,7
.L7:
        movl    16(%esp), %ebx
        movl    20(%esp), %esi
        movl    24(%esp), %edi
        addl    $28, %esp
        jmp     link_error