https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105751
Bug ID: 105751
Summary: std::array comparision does not inline memcmp
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc at maxbachmann dot de
Target Milestone: ---
On trunk and gcc 12.1, with -O2/-O3 the following array comparison is only
optimized to a memcmp, but not to a direct comparision:
```
bool test(std::array<int8_t, 8>& a, std::array<int8_t, 8>& b)
{
return a == b;
}
```
which is optimized to:
```
test(std::array<signed char, 8ul>&, std::array<signed char, 8ul>&):
sub rsp, 8
mov edx, 8
call memcmp
test eax, eax
sete al
add rsp, 8
ret
```
However for a direct memcmp gcc is able to optimize the memcmp to a single 64
bit comparision:
```
bool test(int8_t* a, int8_t* b)
{
return memcmp(a, b, sizeof(int8_t) * 8) == 0;
}
```
which is optimized to
```
test(signed char*, signed char*):
mov rax, QWORD PTR [rsi]
cmp QWORD PTR [rdi], rax
sete al
ret
```
This is optimized to a single comparision both in clang and msvc:
https://godbolt.org/z/99reqf3Yz
