https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102494
Bug ID: 102494 Summary: Failure to optimize out vector reduction properly especially when using OpenMP Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: gabravier at gmail dot com Target Milestone: --- #include <stdint.h> #include <stddef.h> typedef int8_t simde_int8x8_t __attribute__((__vector_size__(8))); int16_t simde_vaddlv_s8(simde_int8x8_t a) { int16_t r = 0; #pragma omp simd reduction(+:r) for (size_t i = 0 ; i < (sizeof(a) / sizeof(a[0])) ; i++) { r += a[i]; } return r; } Compiled with -O3 -fopenmp-simd, this is the output on AMD64: simde_vaddlv_s8(signed char __vector(8)): pxor xmm1, xmm1 movdqa xmm2, xmm0 pcmpgtb xmm1, xmm0 punpcklbw xmm0, xmm1 punpcklbw xmm2, xmm1 pshufd xmm0, xmm0, 78 movq QWORD PTR [rsp-24], xmm2 movq QWORD PTR [rsp-16], xmm0 movdqa xmm0, XMMWORD PTR [rsp-24] psrldq xmm0, 8 paddw xmm0, XMMWORD PTR [rsp-24] movdqa xmm1, xmm0 psrldq xmm1, 4 paddw xmm0, xmm1 movdqa xmm1, xmm0 psrldq xmm1, 2 paddw xmm0, xmm1 pextrw eax, xmm0, 0 ret This is what Clang manages: simde_vaddlv_s8(signed char __vector(8)): punpcklbw xmm0, xmm0 # xmm0 = xmm0[0,0,1,1,2,2,3,3,4,4,5,5,6,6,7,7] psraw xmm0, 8 pshufd xmm1, xmm0, 238 # xmm1 = xmm0[2,3,2,3] paddw xmm1, xmm0 pshufd xmm0, xmm1, 85 # xmm0 = xmm1[1,1,1,1] paddw xmm0, xmm1 movdqa xmm1, xmm0 psrld xmm1, 16 paddw xmm1, xmm0 movd eax, xmm1 ret Weirdly enough, removing the `#pragma omp simd reduction(+r)` slightly improves GCC's output to this: simde_vaddlv_s8(signed char __vector(8)): pxor xmm1, xmm1 movdqa xmm2, xmm0 pcmpgtb xmm1, xmm0 punpcklbw xmm0, xmm1 punpcklbw xmm2, xmm1 pshufd xmm0, xmm0, 78 paddw xmm0, xmm2 pextrw edx, xmm0, 1 pextrw eax, xmm0, 0 add eax, edx pextrw edx, xmm0, 2 add eax, edx pextrw edx, xmm0, 3 add eax, edx ret