[clang] memcmp-like function written as a for loop not transformed to memcmp for large arrays #167389
Description
Given the following code:
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)
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 retbut when SIZE is defined to be 60 or larger, clang generates
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 retSee 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.