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From: "barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/108953] New: inefficient codegen for trivial equality Date: Mon, 27 Feb 2023 16:29:01 +0000 [thread overview] Message-ID: <bug-108953-4@http.gcc.gnu.org/bugzilla/> (raw) 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.
next reply other threads:[~2023-02-27 16:29 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-02-27 16:29 barry.revzin at gmail dot com [this message] 2023-02-27 16:38 ` [Bug c++/108953] " pinskia at gcc dot gnu.org 2023-02-27 17:11 ` [Bug tree-optimization/108953] " jakub at gcc dot gnu.org 2023-02-28 10:44 ` rguenth at gcc dot gnu.org 2023-03-02 15:05 ` jakub at gcc dot gnu.org 2023-06-23 22:59 ` pinskia at gcc dot gnu.org 2023-07-13 18:15 ` [Bug c++/108953] " pinskia at gcc dot gnu.org
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