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From: "steffen.hirschmann at ipvs dot uni-stuttgart.de" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/95405] New: Unnecessary stores with std::optional Date: Fri, 29 May 2020 08:38:55 +0000 [thread overview] Message-ID: <bug-95405-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95405 Bug ID: 95405 Summary: Unnecessary stores with std::optional Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: steffen.hirschmann at ipvs dot uni-stuttgart.de Target Milestone: --- I posted this to the gcc-help mailing list a few days ago (https://gcc.gnu.org/pipermail/gcc-help/2020-May/138978.html). GCC produces stores that don't seem to be required for std::optional. Code: -------- #include <optional> std::optional<long> foo(); long bar() { auto r = foo(); if (r) return *r; else return 0L; } -------- What gcc 10.1 with -std=c++17 -O3 produces is: bar(): sub rsp, 24 call foo() mov QWORD PTR [rsp+8], rdx cmp BYTE PTR [rsp+8], 0 mov QWORD PTR [rsp], rax mov rax, QWORD PTR [rsp] jne .L1 xor eax, eax .L1: add rsp, 24 ret (see: https://godbolt.org/z/uHE6QB) I don't understand the stores (and loads) after the call to foo. They don't seem necessary to me. Marc Glisse pointed out (https://gcc.gnu.org/pipermail/gcc-help/2020-May/138982.html) that the first pair of store/load seems to be a tuning choice and can be removed with the correct tuning flags. What I expected is: mov QWORD PTR [rsp+8], rdx cmp BYTE PTR [rsp+8], 0 should be a compare/test directly of dl. And: mov QWORD PTR [rsp], rax mov rax, QWORD PTR [rsp] is not present at all. Can someone explain this behavior? Shouldn't the optimizer produce what I pointed out?
next reply other threads:[~2020-05-29 8:38 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-05-29 8:38 steffen.hirschmann at ipvs dot uni-stuttgart.de [this message] 2021-03-07 7:32 ` [Bug rtl-optimization/95405] " pinskia at gcc dot gnu.org 2021-06-06 15:30 ` gabravier at gmail dot com 2021-06-06 18:07 ` glisse at gcc dot gnu.org 2021-06-06 18:11 ` gabravier at gmail dot com 2021-06-06 18:58 ` glisse at gcc dot gnu.org 2023-07-12 21:34 ` pinskia at gcc dot gnu.org 2023-07-12 21:35 ` pinskia at gcc dot gnu.org 2023-07-12 21:35 ` pinskia at gcc dot gnu.org 2023-07-12 21:35 ` pinskia at gcc dot gnu.org
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