Given this code:
#include <string.h>
int equal4(const char* a, const char* b)
{
return memcmp(a, b, 4) == 0;
}
int less4(const char* a, const char* b)
{
return memcmp(a, b, 4) < 0;
}
GCC 7 on x86_64 introduced an optimization for the first case (Clang has done it for a long time):
mov eax, DWORD PTR [rsi]
cmp DWORD PTR [rdi], eax
sete al
movzx eax, al
But the second case still calls memcmp():
sub rsp, 8
mov edx, 4
call memcmp
add rsp, 8
shr eax, 31
Could a similar optimization be applied to the second case? What's the best assembly for this, and is there any clear reason why it isn't being done (by GCC or Clang)?
See it on Godbolt's Compiler Explorer: https://godbolt.org/g/jv8fcf