With code like this (see attachement for a complete silly test case),
unsigned int quad = 0;
for (unsigned int dec=node.count/4; dec; --dec) {
_mm_prefetch(1024+(const char *)&base[quad], _MM_HINT_NTA);
sampler.countl[0] = _mm_add_epi32(sampler.countl[0],
_mm_cmpgt_epi32(sampler.pos[0], base[quad+0]));
sampler.countl[1] = _mm_add_epi32(sampler.countl[1],
_mm_cmpgt_epi32(sampler.pos[1], base[quad+0]));
sampler.countl[0] = _mm_add_epi32(sampler.countl[0],
_mm_cmpgt_epi32(sampler.pos[0], base[quad+1]));
sampler.countl[1] = _mm_add_epi32(sampler.countl[1],
_mm_cmpgt_epi32(sampler.pos[1], base[quad+1]));
sampler.countl[0] = _mm_add_epi32(sampler.countl[0],
_mm_cmpgt_epi32(sampler.pos[0], base[quad+2]));
sampler.countl[1] = _mm_add_epi32(sampler.countl[1],
_mm_cmpgt_epi32(sampler.pos[1], base[quad+2]));
sampler.countl[0] = _mm_add_epi32(sampler.countl[0],
_mm_cmpgt_epi32(sampler.pos[0], base[quad+3]));
sampler.countl[1] = _mm_add_epi32(sampler.countl[1],
_mm_cmpgt_epi32(sampler.pos[1], base[quad+3]));
quad += 4;
}
g++ 4.2 insists to use the same register for addressing 'base[quad]' and
prefetching 1k away from it. Horrible encoding ensues.
With gcc-4.2-20060311/gcc-4.2-20060826 -O3 -march=k8 i get something like
401080: 66 0f 6f c2 movdqa %xmm2,%xmm0
401084: 0f 18 00 prefetchnta (%eax)
401087: 66 0f 66 80 00 fc ff ff pcmpgtd 0xfffffc00(%eax),%xmm0
40108f: 66 0f fe 42 20 paddd 0x20(%edx),%xmm0
401094: 0f 29 42 20 movaps %xmm0,0x20(%edx)
401098: 66 0f 6f c1 movdqa %xmm1,%xmm0
40109c: 66 0f 66 80 00 fc ff ff pcmpgtd 0xfffffc00(%eax),%xmm0
4010a4: 66 0f fe 42 30 paddd 0x30(%edx),%xmm0
4010a9: 0f 29 42 30 movaps %xmm0,0x30(%edx)
4010ad: 66 0f 6f c2 movdqa %xmm2,%xmm0
4010b1: 66 0f 66 80 10 fc ff ff pcmpgtd 0xfffffc10(%eax),%xmm0
4010b9: 66 0f fe 42 20 paddd 0x20(%edx),%xmm0
4010be: 0f 29 42 20 movaps %xmm0,0x20(%edx)
4010c2: 66 0f 6f c1 movdqa %xmm1,%xmm0
etc...
There's other issues with the code produced, ie gcc writing back values instead
of just keeping them live, but i can kludge around them.
But i cannot fix that silly encoding.
msvc8, icc9.1 and g++ 3.4.4 do a much better job, here's g++ 3.4.4
401084: prefetchnta 0x400(%eax)
40108b: movdqa %xmm5,%xmm2
40108f: movdqa %xmm4,%xmm1
401093: movdqa %xmm3,%xmm0
401097: pcmpgtd (%eax),%xmm2
40109b: paddd %xmm2,%xmm6
40109f: movaps %xmm6,0x20(%edx)
4010a3: movdqa %xmm5,%xmm2
4010a7: pcmpgtd (%eax),%xmm1
4010ab: paddd %xmm1,%xmm0
4010af: movaps %xmm0,0x30(%edx)
4010b3: movdqa %xmm4,%xmm1
4010b7: pcmpgtd 0x10(%eax),%xmm2
4010bc: paddd %xmm2,%xmm6
4010c0: movaps %xmm6,0x20(%edx)
Note that -fprefetch-loop-arrays's heuristic is way off the mark and
counterproductive, even for that simplified testcase.
--
Summary: overzealous pointer coalescence leading to poor encoding
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: tbptbp at gmail dot com
GCC host triplet: x86*
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28919
