[Ffmpeg-devel] SSE load and store doubts

[Roberto Pariset]

hello and thanks alot for your prompt answer!

Il giorno gio, 06/10/2005 alle 11.44 +0200, Balatoni Denes ha scritto:
> Hi!
> 
> If do_stuff doesn't use the MMX/SSE intrinsics, than SSE instructions usually 
> won't be used (the compiler itself however could decide to use SSE, but than 
> it is not because of you using the intrinsics). 

well, let's assume do_something() actually does SSE :)

> On another note I didn't actually find anything resembling your example in 
> current ffmpeg.

(source: fft_sse.c) 
try look at r and z: it actually uses no _mm_store_ps() function for
it... hence my doubts...

void ff_fft_calc_sse(FFTContext *s, FFTComplex *z)
{
    int ln = s->nbits;
    int j, np, np2;
    int nblocks, nloops;
    register FFTComplex *p, *q;
    FFTComplex *cptr, *cptr1;
    int k;

    np = 1 << ln;

    {
        __m128 *r, a, b, a1, c1, c2;

        r = (__m128 *)&z[0];
        c1 = *(__m128 *)p1p1m1m1;
        c2 = *(__m128 *)p1p1p1m1;
        if (s->inverse)
            c2 = *(__m128 *)p1p1m1p1;
        else
            c2 = *(__m128 *)p1p1p1m1;

        j = (np >> 2);
        do {
            a = r[0];
            b = _mm_shuffle_ps(a, a, _MM_SHUFFLE(1, 0, 3, 2));
            a = _mm_mul_ps(a, c1);
            /* do the pass 0 butterfly */
            a = _mm_add_ps(a, b);

            a1 = r[1];
            b = _mm_shuffle_ps(a1, a1, _MM_SHUFFLE(1, 0, 3, 2));
            a1 = _mm_mul_ps(a1, c1);
            /* do the pass 0 butterfly */
            b = _mm_add_ps(a1, b);

            /* multiply third by -i */
            b = _mm_shuffle_ps(b, b, _MM_SHUFFLE(2, 3, 1, 0));
            b = _mm_mul_ps(b, c2);

            /* do the pass 1 butterfly */
            r[0] = _mm_add_ps(a, b);
            r[1] = _mm_sub_ps(a, b);
            r += 2;
        } while (--j != 0);
    }
...

bye bye and thanks!
rob

> cs?t?rt?k 06 okt?ber 2005 11.10-kor Roberto Pariset ezeket a bolcs 
> gondolatokat fogalmazta meg:
> > hello everyone.
> >
> > please consider the following example:
> >
> > /* imagine long_array filled with floats here */
> > float long_array[MANY] __attribute__((aligned(16)));
> >
> > i often[1] see this kind of code:
> > __m128 *reg = (__m128 *) long_array;
> > for(i=0; i<MANY; i+=4)
> > {
> > 	do_stuff();
> > 	r++; /* skip to next 4 floats of long_array */
> > }
> >
> > while i'd expect the following:
> > __m128 reg;
> > for(i=0; i<MANY; i+=4)
> > {
> > 	reg = _mm_load_ps( &long_array[i] );
> > 	reg = do_stuff();
> > 	_mm_store_ps( &long_array[i], reg );
> > }
> >
> > if i compile and deassemble a simple example as the one before, i see
> > the first doesn't actually use XMMn registers, while the second does:
> >
> >     reg = _mm_load_ps( &long_array[i] );
> > 400548:    48 8d 7d 80             lea    0xffffffffffffff80(%rbp),%rdi
> > 40054c:    e8 67 00 00 00          callq  4005b8 <_mm_load_ps> 400551:
> > 0f 29 45 e0             movaps %xmm0,0xffffffffffffffe0(%rbp)
> >
> >     __m128 *reg = (__m128 *) long_array;
> > 400555:    48 8d 85 70 ff ff ff    lea    0xffffffffffffff70(%rbp),%rax
> > 40055c:    48 89 45 f0             mov    %rax,0xfffffffffffffff0(%rbp)
> >
> > so, basically, i am not sure if this is an error or not, as i am just a
> > n00b with SSE. to me, it seems that the first syntax is not taking
> > advantage of sse register, so it'd not make things faster. i might be
> > wrong, of course. i just wanted to point it out, and would appreciate
> > much if i could get some explanations, as i haven't found any on the web
> > (all the code i have found use either load/store or pointer with no
> > apparent difference, and none explains motivation of the choice). thanks
> > alot,
> > roberto
> >
> >
> >
> >
> > [1] as in ffmpeg-0.4.9-pre1/libavcodec/i386/
> >
> > _______________________________________________
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> > ffmpeg-devel at mplayerhq.hu
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> >
> >
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