https://bugs.llvm.org/show_bug.cgi?id=42674
Bug ID: 42674
Summary: [SIMD opt of checksum func] Smart, but could be
smarter
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: bisq...@iki.fi
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
#include <string.h>
unsigned char calculate_checksum(const void* ptr)
{
unsigned char bytes[16], result = 0;
memcpy(bytes, ptr, 16); // Endianess does not matter.
for(unsigned n=0; n<16; ++n) result += bytes[n];
return result;
}
For this code, Clang (generates the following SIMD code, which is not too bad.
vmovdqu xmm0, xmmword ptr [rdi]
vmovdqa xmmword ptr [rsp - 24], xmm0
vpshufd xmm1, xmm0, 78 # xmm1 = xmm0[2,3,0,1]
vpaddb xmm0, xmm0, xmm1
vpshufd xmm1, xmm0, 229 # xmm1 = xmm0[1,1,2,3]
vpaddb xmm0, xmm0, xmm1
vpsrld xmm1, xmm0, 16
vpaddb xmm0, xmm0, xmm1
vpsrlw xmm1, xmm0, 8
vpaddb xmm0, xmm0, xmm1
vpextrb eax, xmm0, 0
ret
But here is what ICC (version 18 or newer) generates for it.
vmovups xmm0, XMMWORD PTR [rdi] #5.9
vpxor xmm2, xmm2, xmm2 #4.41
vpaddb xmm0, xmm2, xmm0 #4.41
vpsrldq xmm1, xmm0, 8 #4.41
vpaddb xmm3, xmm0, xmm1 #4.41
vpsadbw xmm4, xmm2, xmm3 #4.41
vmovd eax, xmm4 #4.41
movsx rax, al #4.41
ret #7.16
Hence the title of this report: Smart, but could be smarter.
For the record, both GCC and MSVC generates almost identical code as Clang for
this testcase. They use VPSRLDQ instead of VPSHUFD rthough. Compiler Explorer
link: https://godbolt.org/z/hA4J94
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