Jacub,
On Fri, Jul 1, 2011 at 12:04 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> On Fri, Jul 01, 2011 at 11:45:16AM -0700, Martin Thuresson wrote:
>> In recent versions of GCC I have seen a transformation of inline
>> assembly that I'd like to confirm is valid.
>>
>> The code in question can be found in mplayer/mp3lib/dct64_sse.c
>>
>> "movaps %0, %%xmm0\n\t"
>> "shufps $27, %%xmm0, %%xmm0\n\t"
>> "movaps %1, %%xmm5\n\t"
>> "movaps %%xmm5, %%xmm6\n\t"
>> :
>> :"m"(*costab), "m"(*nnnn)
>>
>> where nnnn is
>> static const int nnnn[4] __attribute__((aligned(16))) = { 1 << 31, 1
>> << 31, 1 << 31, 1 << 31 };
>>
>> GCC turns this into:
>> "movaps %0, %%xmm0
>> shufps $27, %%xmm0, %%xmm0
>> movaps %1, %%xmm5
>> movaps %%xmm5, %%xmm6
>> " : : "m" costab_mmx[24], *"m" -2147483648*);
>>
>> The new constant might end up in an unaligned address causing the
>> program to segfault on Intel platforms.
>
> But you said the operand is an int sized memory, while you expect
> 4 times as big data with different alignment.
> So you want "m"(*(__m128d *)nnnn) (or "m"(*(__m128i *)nnnn) ).
Ah, thanks. Glad to hear it was a source bug and not a compiler bug. I just
verified that the solution you proposed worked in my example.
>
> Your "fixed" version with "p" is not correct either, because
> you don't say to gcc that it reads the value from that memory address, e.g.
> if there is
> int foo (void)
> {
> int p, q;
> p = 5;
> asm ("movl %a1, %0" : "=r" (q) : "p" (&p));
> return q;
> }
> gcc might very well eliminate the p = 5; store.
OK, thanks for the example.
Thanks,
Martin
>
> Jakub
>
