| Issue |
167389
|
| Summary |
[clang] memcmp written as a for loop not transformed to `memcmp` for large arrays
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
davidstone
|
Given the following code:
```c++
constexpr auto size = SIZE;
struct array {
int m[size];
};
auto equal(
array const lhs,
array const rhs
) -> bool {
for (int n = 0; n != size; ++n) {
if (lhs.m[n] != rhs.m[n]) {
return false;
}
}
return true;
}
```
With `-O3` and `SIZE` defined as any number between 9 and 59 inclusive, clang generates (with different constants)
```asm
equal(array, array):
push rax
lea rdi, [rsp + 16]
lea rsi, [rsp + 256]
mov edx, 236
call memcmp@PLT
test eax, eax
sete al
pop rcx
ret
```
but when `SIZE` is defined to be 60 or larger, clang generates
```asm
equal(array, array):
lea rax, [rsp + 248]
lea rcx, [rsp + 8]
xor edx, edx
.LBB0_1:
mov esi, dword ptr [rcx + 4*rdx]
mov edi, dword ptr [rax + 4*rdx]
cmp esi, edi
jne .LBB0_3
cmp rdx, 59
lea rdx, [rdx + 1]
jne .LBB0_1
.LBB0_3:
cmp esi, edi
sete al
ret
```
See it live: https://godbolt.org/z/xfsdd7Ks8
I would expect that if it ever makes sense to generate a call to `memcmp`, it would continue to make sense for larger sizes.
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