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From: "anthony.mikh at yandex dot ru" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/106804] New: Poor codegen for selecting and incrementing value behind a reference Date: Thu, 01 Sep 2022 17:11:58 +0000 [thread overview] Message-ID: <bug-106804-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106804 Bug ID: 106804 Summary: Poor codegen for selecting and incrementing value behind a reference Product: gcc Version: 12.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: anthony.mikh at yandex dot ru Target Milestone: --- Godbolt: https://godbolt.org/z/e9ePs7Ece For the following source code: void increment_largest(int& a, int& b) { ++(a > b ? a : b); } gcc 12 with -O2 produces the following asm: increment_largest(int&, int&): mov edx, DWORD PTR [rdi] mov eax, DWORD PTR [rsi] cmp edx, eax jle .L2 add edx, 1 mov DWORD PTR [rdi], edx ret .L2: add eax, 1 mov DWORD PTR [rsi], eax ret For equivalent code using pointers: void increment_largest(int* a, int* b) { ++*(*a > *b ? a : b); } gcc with -O2 gives something slightly different: increment_largest(int*, int*): mov edx, DWORD PTR [rdi] mov eax, DWORD PTR [rsi] cmp edx, eax jle .L2 mov eax, edx mov rsi, rdi .L2: add eax, 1 mov DWORD PTR [rsi], eax ret If one rewrites code with references to assign the selected reference to a variable: void increment_largest(int& a, int& b) { auto& tgt = (a > b ? a : b); ++tgt; } it gives exactly the same asm as the version with pointers. Anyway it is seemingly worse than what clang-14 -O2 produces for all three sources: increment_largest(int&, int&): mov eax, dword ptr [rdi] cmp eax, dword ptr [rsi] cmovg rsi, rdi add dword ptr [rsi], 1 ret Likely to be related to PR94006.
next reply other threads:[~2022-09-01 17:11 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-09-01 17:11 anthony.mikh at yandex dot ru [this message] 2022-09-01 17:17 ` [Bug middle-end/106804] " pinskia at gcc dot gnu.org 2022-09-01 17:17 ` pinskia at gcc dot gnu.org 2022-09-02 6:32 ` rguenth at gcc dot gnu.org 2022-09-02 7:18 ` amonakov at gcc dot gnu.org
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