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From: "i.nixman at autistici dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals Date: Wed, 19 Apr 2023 12:39:21 +0000 [thread overview] Message-ID: <bug-109555-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109555 Bug ID: 109555 Summary: suboptimal code for comparing strings with string literals Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: i.nixman at autistici dot org Target Milestone: --- hello! the example: #include <cstring> int main(int argc, char **argv) { return std::memcmp(argv[0], "MCRYF", 5) == 0; } this code compiled with GCC (-std=c++11 -O2) will be translated as the following assembler code (https://godbolt.org/z/zzjaGdWdT): main: mov rax, QWORD PTR [rsi] cmp DWORD PTR [rax], 1498563405 je .L5 .L2: mov eax, 1 .L3: xor eax, 1 ret .L5: cmp BYTE PTR [rax+4], 70 jne .L2 xor eax, eax jmp .L3 but if the code is compiled using CLang, we get a more optimal branch-less assembler code (https://godbolt.org/z/E9o1oMzra): main: # @main mov rax, qword ptr [rsi] mov ecx, 1498563405 xor ecx, dword ptr [rax] movzx edx, byte ptr [rax + 4] xor edx, 70 xor eax, eax or edx, ecx sete al ret maybe it's not a difficult fix, and maybe someone will see the point to improve this... thanks!
next reply other threads:[~2023-04-19 12:39 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-04-19 12:39 i.nixman at autistici dot org [this message] 2023-04-19 12:42 ` [Bug middle-end/109555] " pinskia at gcc dot gnu.org 2023-04-19 12:45 ` i.nixman at autistici dot org 2023-04-19 12:49 ` pinskia at gcc dot gnu.org 2023-04-19 15:58 ` [Bug middle-end/109555] memcmp with string literals cause branchy code maybe could use branchless? pinskia at gcc dot gnu.org
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