when i compile the below source with gcc 3.2.3 20030316 (debian prerelease), or with gcc version 3.3 20030129 (Debian prerelease) the result is code which only compares the bottom half of the 64-bit value which i've asked it to compare.

notice the "movd %mm1,%esi" to get the bottom 32... and mm1 is also stored to the stack, and the bottom 32-bits are compared.  i think there's an offset by 4 missing somewhere.

Release:
gcc version 3.2.3 20030316 (Debian prerelease)

Environment:
debian unstable

How-To-Repeat:
dean@zim:~/verification$ cat gccbug.c
#include <stdint.h>
#include <mmintrin.h>


typedef union {
        uint64_t uq[1];
        __m64 m;
} mm_t __attribute__((aligned(8)));


int foo(mm_t *p, const mm_t *q)
{
        mm_t t;

        //asm("psllw $1,%0" : "=&y" (t.m) : "0" (p));
        t.m = _mm_slli_pi16(p->m, 1);
        return t.uq[0] == q->uq[0];
}
dean@zim:~/verification$ gcc -c -O3 -mmmx -Wall gccbug.c
dean@zim:~/verification$ objdump -dr gccbug.o

gccbug.o:     file format elf32-i386

Disassembly of section .text:

00000000 <foo>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   57                      push   %edi
   4:   56                      push   %esi
   5:   53                      push   %ebx
   6:   83 ec 0c                sub    $0xc,%esp
   9:   8b 75 08                mov    0x8(%ebp),%esi
   c:   0f 6f 0e                movq   (%esi),%mm1
   f:   8b 4d 0c                mov    0xc(%ebp),%ecx
  12:   0f 71 f1 01             psllw  $0x1,%mm1
  16:   8b 39                   mov    (%ecx),%edi
  18:   8b 51 04                mov    0x4(%ecx),%edx
  1b:   0f 7e ce                movd   %mm1,%esi
  1e:   0f 7f 4d e8             movq   %mm1,0xffffffe8(%ebp)
  22:   31 d6                   xor    %edx,%esi
  24:   8b 5d e8                mov    0xffffffe8(%ebp),%ebx
  27:   31 fb                   xor    %edi,%ebx
  29:   89 f0                   mov    %esi,%eax
  2b:   09 d8                   or     %ebx,%eax
  2d:   0f 94 c2                sete   %dl
  30:   83 c4 0c                add    $0xc,%esp
  33:   5b                      pop    %ebx
  34:   5e                      pop    %esi
  35:   0f b6 c2                movzbl %dl,%eax
  38:   5f                      pop    %edi
  39:   c9                      leave
  3a:   c3                      ret