https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80517
Bug ID: 80517
Summary: [missed optimization] constant propagation through
Intel intrinsics
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: kretz at kde dot org
Target Milestone: ---
Related: #55894
Testcase:
#include <x86intrin.h>
int f() {
__m128i x{};
x = _mm_cmpeq_epi16(x, x);
return _pext_u32(_mm_movemask_epi8(x), 0xaaaa);
}
(compile with `-mbmi2 -O3 -std=c++14`)
See also https://godbolt.org/g/n92wEc
This should compile to
f():
movl $0xff, %eax
ret
Clang already implements constant propagation for the testcase except for
`pext` (see godbolt link).
This is just a precursor to the following testcase:
#include <x86intrin.h>
auto g(__m128i x, __m128i y) {
__m128i mask0 = _mm_cmpeq_epi16(x, y);
auto bits = _pext_u32(_mm_movemask_epi8(mask0), 0xaaaa);
__m128i mask = _mm_set1_epi16(bits);
mask = _mm_and_si128(mask, _mm_setr_epi16(1, 2, 4, 8, 16, 32, 64, 128));
mask = _mm_cmpeq_epi16(mask, _mm_setzero_si128());
mask = _mm_xor_si128(mask, _mm_cmpeq_epi16(mask, mask));
return mask;
}
This should compile to
f():
vpcmpeqw %xmm0, %xmm1, %xmm0
ret
I.e. The xmm mask `mask0` is converted to a bitmask and back to an xmm mask.
Similar patterns exist for all arithmetic types for SSE and AVX. If you like, I
can produce a list of testcases for all vector element types for SSE and AVX.
Motivation: An ABI-stable mask type for x86 requires a storage format that is
independent of the ISA extensions available on the specific x86 CPU. In light
of AVX512, the most sensible choice for such a mask storage is std::bitset<N>.
This is a natural fit for AVX512 masks but requires frequent conversion to/from
xmm and ymm mask when AVX/SSE registers are modified. If the above is
optimized, it would go a long way to reducing the cost of using the ABI-stable
types.
Reference: https://wg21.link/p0214
