Force GCC to unroll a loop?

["Jason Garrett-Glaser" <darkshikari at gmail dot com>]

>From what I've heard, GCC doesn't take into account the benefits of
constant propagation when deciding whether to unroll loops.  In some
cases, even with -funroll-loops, it doesn't unroll loops that would be
highly beneficial to unroll.  An example follows:

    for( i = 0; i < 3; i++ )
    {
        const int w = (i == 0 ? 16 : 8);
        const int i_stride = h->fdec->i_stride[i];
        const int i_stride2 = i_stride << h->mb.b_interlaced;
        const int i_pix_offset = h->mb.b_interlaced
                               ? w * (i_mb_x + (i_mb_y&~1) * i_stride)
+ (i_mb_y&1) * i_stride
                               : w * (i_mb_x + i_mb_y * i_stride);
        int ref_pix_offset[2] = { i_pix_offset, i_pix_offset };
        const uint8_t *intra_fdec = &h->mb.intra_border_backup[i_mb_y
& h->sh.b_mbaff][i][i_mb_x*16>>!!i];
        x264_frame_t **fref[2] = { h->fref0, h->fref1 };
        int j, k, l;

        if( h->mb.b_interlaced )
            ref_pix_offset[1] += (1-2*(i_mb_y&1)) * i_stride;

        h->mb.pic.i_stride[i] = i_stride2;

        h->mc.copy[i?PIXEL_8x8:PIXEL_16x16]( h->mb.pic.p_fenc[i], FENC_STRIDE,
            &h->fenc->plane[i][i_pix_offset], i_stride2, w );
        memcpy( &h->mb.pic.p_fdec[i][-1-FDEC_STRIDE], intra_fdec-1, w*3/2+1 );
        if( h->mb.b_interlaced )
        {
            const uint8_t *plane_fdec = &h->fdec->plane[i][i_pix_offset];
            for( j = 0; j < w; j++ )
                h->mb.pic.p_fdec[i][-1+j*FDEC_STRIDE] =
plane_fdec[-1+j*i_stride2];
        }

        for( l=0; l<2; l++ )
        {
            for( j=0; j<h->mb.pic.i_fref[l]; j++ )
            {
                h->mb.pic.p_fref[l][j][i==0 ? 0:i+3] = &fref[l][j >>
h->mb.b_interlaced]->plane[i][ref_pix_offset[j&1]];
                if( i == 0 )
                    for( k = 1; k < 4; k++ )
                        h->mb.pic.p_fref[l][j][k] = &fref[l][j >>
h->mb.b_interlaced]->filtered[k][ref_pix_offset[j&1]];
            }
        }
    }

Unrolling this loop saves over 100 clocks out of a total of about 620
on my system, as measured with rdtsc (using ffmpeg's bench.h).  Now,
I'd rather not mess up the code with an ugly preprocessor hack to
force unrolling (or copy-pasting code, even worse).  Is there an
intrinsic to force GCC to unroll a loop?  I'm using GCC 4.4, compiled
a few weeks ago from the latest tarball.

Dark Shikari, x264 project