https://bugs.llvm.org/show_bug.cgi?id=41316
Bug ID: 41316
Summary: _mm_avg_epu16 preceded by a left shift results in poor
code
Product: libraries
Version: 8.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
Reproducer:
-
#include <xmmintrin.h>
__m128i f(__m128i a, __m128i b)
{
__m128i a_shifted = _mm_slli_epi16(a, 2);
__m128i b_shifted = _mm_slli_epi16(b, 2);
return _mm_avg_epu16(a_shifted, b_shifted);
}
-
with clang 5.0.0 or older, I get the expected:
-
f(long long __vector(2), long long __vector(2)):
psllw xmm0, 2
psllw xmm1, 2
pavgw xmm0, xmm1
ret
-
but looks like sometime between 5.0.0 and 6.0.0, things broke and now the same
code results in:
-
.LCPI0_0:
.short 1 # 0x1
.short 1 # 0x1
.short 1 # 0x1
.short 1 # 0x1
.short 1 # 0x1
.short 1 # 0x1
.short 1 # 0x1
.short 1 # 0x1
f(long long __vector(2), long long __vector(2)):
psllw xmm0, 2
psllw xmm1, 2
pxor xmm2, xmm2
movdqa xmm3, xmm1
punpckhwd xmm3, xmm2
punpcklwd xmm1, xmm2
por xmm0, xmmword ptr [rip + .LCPI0_0]
movdqa xmm4, xmm0
punpckhwd xmm4, xmm2
paddd xmm4, xmm3
punpcklwd xmm0, xmm2
paddd xmm0, xmm1
pslld xmm4, 15
psrad xmm4, 16
pslld xmm0, 15
psrad xmm0, 16
packssdw xmm0, xmm4
ret
-
everything after the "psllw xmm1, 2" appears to be a replacement expansion for
pavgw. This particular example was compiled with 8.0.0 but 6.x and 7.x are
similar.
If I just shift one of the inputs, I do get a pavgw; so the left shifts appear
to be important in some way. (Reduced from a more complex example.)
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