Strange behavior with movq

[Axel Freyn]

Hi Olivier,
On Wed, May 25, 2011 at 06:13:28PM +0200, Olivier Maury wrote:
> [...]
> I wrote a very simple example to test some movntq builtin function:
>
> #include <stdlib.h>
> #include <stdio.h>
>
> int main(int argc, char **argv)
> {
>   double *out;
>   double in = 2.0;
>   long long unsigned int *vin = (long long unsigned int *)∈
>
>   out = (double*)malloc(sizeof(double));
>
>   out[0] = 0.0;
>
>   printf("%E\n", out[0]);
>   printf("%E\n", *((double*)vin));
>
>   __builtin_ia32_movntq((long long unsigned int*)(out), *vin);
>
>   printf("%E\n", *out);
> }
>
> Since I'm compiling on a 32 bits OS I use the following compilation  
> command: gcc movntq.c -mfpmath=sse -msse3
>
> If I have a look at the assembly code I have the following code between  
> the two last printf :
>
>         call    printf
>         movl    -12(%ebp), %eax
>         movl    (%eax), %edx
>         movl    4(%eax), %ecx
>         movl    -16(%ebp), %eax
>         movl    %edx, -32(%ebp)
>         movl    %ecx, -28(%ebp)
>         movq    -32(%ebp), %mm0
>         movntq  %mm0, (%eax)
>         movl    -16(%ebp), %eax
>         movsd   (%eax), %xmm0
>         movsd   %xmm0, 4(%esp)
>         movl    $.LC2, (%esp)
>         call    printf
>
> [...]
>
> If the value stored in out[0] is ok after the movntq instruction (p  
> out[0] returns 2 in gdb) the last printf returns a NAN...
>
> Does anyone have an idea about what's going on here ? Is it a bug ?
Is think it's a bug in your code / the documentation.
See the old bug report http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28331

calling __builtin_ia32_emms(); after movntq solves the problem on my 
machine.

Axel