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* [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals @ 2023-04-19 12:39 i.nixman at autistici dot org 2023-04-19 12:42 ` [Bug middle-end/109555] " pinskia at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: i.nixman at autistici dot org @ 2023-04-19 12:39 UTC (permalink / raw) To: gcc-bugs 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! ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/109555] suboptimal code for comparing strings with string literals 2023-04-19 12:39 [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals i.nixman at autistici dot org @ 2023-04-19 12:42 ` pinskia at gcc dot gnu.org 2023-04-19 12:45 ` i.nixman at autistici dot org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-04-19 12:42 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109555 --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Note branchless might be worse. Anyways there is a duplicate of this bug already. I think I filed it. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/109555] suboptimal code for comparing strings with string literals 2023-04-19 12:39 [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals i.nixman at autistici dot org 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 3 siblings, 0 replies; 5+ messages in thread From: i.nixman at autistici dot org @ 2023-04-19 12:45 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109555 --- Comment #2 from niXman <i.nixman at autistici dot org> --- (In reply to Andrew Pinski from comment #1) > Note branchless might be worse. really? could you explain please? > Anyways there is a duplicate of this bug > already. I think I filed it. thank you! ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/109555] suboptimal code for comparing strings with string literals 2023-04-19 12:39 [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals i.nixman at autistici dot org 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 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-04-19 12:49 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109555 --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to niXman from comment #2) > (In reply to Andrew Pinski from comment #1) > > Note branchless might be worse. > > really? > could you explain please? Yes cache misses. That is the second load might miss the cache. Plus the non reduced case will most likely have a branch on the result so in the end, it would just do unnecessary things if it is different. > > > > > Anyways there is a duplicate of this bug > > already. I think I filed it. > > thank you! ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug middle-end/109555] memcmp with string literals cause branchy code maybe could use branchless? 2023-04-19 12:39 [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals i.nixman at autistici dot org ` (2 preceding siblings ...) 2023-04-19 12:49 ` pinskia at gcc dot gnu.org @ 2023-04-19 15:58 ` pinskia at gcc dot gnu.org 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-04-19 15:58 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109555 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Severity|normal |enhancement Summary|suboptimal code for |memcmp with string literals |comparing strings with |cause branchy code maybe |string literals |could use branchless? ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-04-19 15:58 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-04-19 12:39 [Bug middle-end/109555] New: suboptimal code for comparing strings with string literals i.nixman at autistici dot org 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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