https://bugs.llvm.org/show_bug.cgi?id=35016
Bug ID: 35016
Summary: suboptimal loop codegen?
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: tras...@gmail.com
CC: llvm-bugs@lists.llvm.org
#include <stdint.h>
uint8_t data[32] = { 1,2,3,4 };
int main(int argc, const char* argv[])
{
int sum = 0;
for (int i=0; i < argc; ++i)
#if 0
sum += ((uint32_t*)data)[2 + i];
#else
sum += *(uint32_t*)&data[8 + i * 4];
#endif
return sum;
}
https://godbolt.org/g/sJTGjz
$ clang -c -Oz
main: # @main
movsxd rcx, edi
xor edx, edx
push 8
pop rsi
xor eax, eax
jmp .LBB0_1
.LBB0_3: # in Loop: Header=BB0_1 Depth=1
movsxd rsi, esi
add eax, dword ptr [rsi + data]
inc rdx
add esi, 4
.LBB0_1: # =>This Inner Loop Header: Depth=1
cmp rdx, rcx
jl .LBB0_3
ret
data:
.asciz
"\001\002\003\004\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
$ g++ -c -Os
main:
xor edx, edx
xor eax, eax
.L3:
cmp edi, edx
jle .L1
add eax, DWORD PTR data[8+rdx*4]
inc rdx
jmp .L3
.L1:
ret
data:
.byte 1
.byte 2
.byte 3
.byte 4
.zero 28
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