https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108953
Bug ID: 108953 Summary: inefficient codegen for trivial equality Product: gcc Version: 12.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: barry.revzin at gmail dot com Target Milestone: --- Consider this example: #include <cstdint> #include <cstddef> #include <string.h> struct C { uint8_t a; uint8_t b; uint8_t c; uint8_t d; uint16_t e; uint16_t f; int32_t g; bool operator==(C const&) const = default; }; bool check(C const& lhs, C const& rhs) { #ifdef MEMCMP return memcmp(&lhs, &rhs, sizeof(lhs)) == 0; #else return lhs == rhs; #endif } There are two implementations of check here, but lead to suboptimal code. When using MEMCMP, gcc trunk -O3 emits: check(C const&, C const&): mov rax, QWORD PTR [rsi] cmp QWORD PTR [rdi], rax je .L5 .L2: mov eax, 1 test eax, eax sete al ret .L5: mov eax, DWORD PTR [rsi+8] cmp DWORD PTR [rdi+8], eax jne .L2 xor eax, eax test eax, eax sete al ret There's a few extra instructions here (mov eax, 1; test eax, eax; sete al;... do we need all three of those to return 0?) When using defaulted comparisons, gcc trunk -O3 doesn't collapse any of the comparisons, and instead emits 7 distinct checks: check(C const&, C const&): movzx ecx, BYTE PTR [rsi] xor eax, eax cmp BYTE PTR [rdi], cl jne .L1 movzx edx, BYTE PTR [rsi+1] cmp BYTE PTR [rdi+1], dl jne .L1 movzx edx, BYTE PTR [rsi+2] cmp BYTE PTR [rdi+2], dl jne .L1 movzx edx, BYTE PTR [rsi+3] cmp BYTE PTR [rdi+3], dl jne .L1 movzx edx, WORD PTR [rsi+4] cmp WORD PTR [rdi+4], dx jne .L1 movzx eax, WORD PTR [rsi+6] cmp WORD PTR [rdi+6], ax mov edx, DWORD PTR [rsi+8] sete al cmp DWORD PTR [rdi+8], edx sete dl and eax, edx .L1: ret Compare this to clang, which for both the memcmp and the default equality versions emits this: check(C const&, C const&): # @check(C const&, C const&) mov rax, qword ptr [rdi] xor rax, qword ptr [rsi] mov ecx, dword ptr [rdi + 8] xor ecx, dword ptr [rsi + 8] or rcx, rax sete al ret Looks like there are two missing optimizations here for gcc: (1) the memcmp does get optimized into an 8-byte and 4-byte comparison, but then the result of that optimization doesn't get optimized further and (2) multiple trivial comparisons don't get coalesced together.