[clang] memcmp-like function with pointer parameters and written as a for loop never transformed into memcmp #167397
Description
Given the following code
auto equal( int const * const lhs, int const * const rhs ) -> bool { for (int n = 0; n != 10; ++n) { if (lhs[n] != rhs[n]) { return false; } } return true; }I would expect something like a call to memcmp for sufficiently large size. Instead, at -O3 it follows this pattern for 2 <= size <= 59:
equal(int const*, int const*): mov eax, dword ptr [rdi] cmp eax, dword ptr [rsi] jne .LBB0_10 mov eax, dword ptr [rdi + 4] cmp eax, dword ptr [rsi + 4] jne .LBB0_10 mov eax, dword ptr [rdi + 8] cmp eax, dword ptr [rsi + 8] jne .LBB0_10 mov eax, dword ptr [rdi + 12] cmp eax, dword ptr [rsi + 12] jne .LBB0_10 mov eax, dword ptr [rdi + 16] cmp eax, dword ptr [rsi + 16] jne .LBB0_10 mov eax, dword ptr [rdi + 20] cmp eax, dword ptr [rsi + 20] jne .LBB0_10 mov eax, dword ptr [rdi + 24] cmp eax, dword ptr [rsi + 24] jne .LBB0_10 mov eax, dword ptr [rdi + 28] cmp eax, dword ptr [rsi + 28] jne .LBB0_10 mov eax, dword ptr [rdi + 32] cmp eax, dword ptr [rsi + 32] jne .LBB0_10 mov eax, dword ptr [rdi + 36] cmp eax, dword ptr [rsi + 36] sete al ret .LBB0_10: xor eax, eax retAnd then for size >= 60 it does (with the constant adjusted appropriately)
equal(int const*, int const*): xor eax, eax .LBB0_1: mov ecx, dword ptr [rdi + 4*rax] mov edx, dword ptr [rsi + 4*rax] cmp ecx, edx jne .LBB0_3 cmp rax, 59 lea rax, [rax + 1] jne .LBB0_1 .LBB0_3: cmp ecx, edx sete al ret