https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105429
Bug ID: 105429
Summary: Unnecessary moves generated by the compiler.
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: mareksz1958 at wp dot pl
Target Milestone: ---
The following C code:
>>>
#include <nmmintrin.h>
#include <stdint.h>
uint32_t crc(uint32_t current, const uint8_t *buffer, size_t size) {
for(size_t i = 0; i < size; i++)
current = _mm_crc32_u64(current, buffer[i]);
return current;
}
<<<
Generates inefficient assembly on all optimisation presets due to the extra
`mov eax, eax' - Os and O3 below:
>>>
crc:
movl %edi, %eax
xorl %ecx, %ecx
.L2:
cmpq %rdx, %rcx
je .L5
movzbl (%rsi,%rcx), %edi
movl %eax, %eax
incq %rcx
crc32q %rdi, %rax
jmp .L2
.L5:
ret
crc:
movl %edi, %eax
testq %rdx, %rdx
je .L6
leaq (%rsi,%rdx), %rcx
.L3:
movzbl (%rsi), %edx
movl %eax, %eax
addq $1, %rsi
crc32q %rdx, %rax
cmpq %rsi, %rcx
jne .L3
.L6:
ret
<<<
The problem seems to be present in all GCC versions I have access to. The
redundant move greatly worsens the performance of the generated code. When
`_mm_crc32_u64' is replaced by any other function, the problem seems to
disappear.
