https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101097
Bug ID: 101097
Summary: Vectorizer is too eager to use vec_unpack
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ubizjak at gmail dot com
Target Milestone: ---
Following two testcases:
void
foo (unsigned short* p1, unsigned short* p2, int* __restrict p3)
{
for (int i = 0 ; i != 8; i++)
p3[i] = p1[i] + p2[i];
return;
}
void
bar (unsigned short* p1, unsigned short* p2, int* __restrict p3)
{
for (int i = 0 ; i != 4; i++)
p3[i] = p1[i] + p2[i];
return;
}
compile with -O3 -mavx2 to:
foo:
vmovdqu (%rdi), %xmm1
vmovdqu (%rsi), %xmm0
vpmovzxwd %xmm1, %xmm3
vpsrldq $8, %xmm1, %xmm1
vpmovzxwd %xmm0, %xmm2
vpsrldq $8, %xmm0, %xmm0
vpmovzxwd %xmm1, %xmm1
vpaddd %xmm3, %xmm2, %xmm2
vpmovzxwd %xmm0, %xmm0
vmovdqu %xmm2, (%rdx)
vpaddd %xmm1, %xmm0, %xmm0
vmovdqu %xmm0, 16(%rdx)
ret
bar:
vpmovzxwd (%rsi), %xmm1
vpmovzxwd (%rdi), %xmm0
vpaddd %xmm1, %xmm0, %xmm0
vmovdqu %xmm0, (%rdx)
ret
However, with "foo" the vec_unpack* named patterns somehow interfere with the
compilation, preventing the compiler to generate code, similar to "bar", but
with %ymm registers.
Disabling vec_unpacku_hi_<mode> and vec_unpacku_lo_<mode> patterns in sse.md
results in the optimal code for foo:
foo:
vpmovzxwd (%rsi), %ymm0
vpmovzxwd (%rdi), %ymm1
vpaddd %ymm1, %ymm0, %ymm0
vmovdqu %ymm0, (%rdx)
vzeroupper
ret
