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From: "pskocik at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/112844] New: Branches under -Os (unlike -O{1,2,3}) do not respect __builtin_expect hints Date: Mon, 04 Dec 2023 12:39:17 +0000 [thread overview] Message-ID: <bug-112844-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112844 Bug ID: 112844 Summary: Branches under -Os (unlike -O{1,2,3}) do not respect __builtin_expect hints Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: pskocik at gmail dot com Target Milestone: --- A simple example that demonstrates this is: int test(void); void yes(void); void expect_yes(void){ if (__builtin_expect(test(),1)) yes(); else {} } void expect_no(void){ if (__builtin_expect(test(),0)) yes(); else {} } For an optimized x86-64 output, one should expect: -a fall-through to a yes() tailcall for the expect_yes() case, preceded by a conditional jump to code doing a plain return -a fall-through to a plain return for the expect_no() case, preceded by a conditional jump to a yes() tailcall (or even more preferably: a conditional-taicall to yes() with the needed stack adjustment done once before the test instead of being duplicated in each branch after the test) Indeed, that's how gcc lays it out for -O{1,2,3} (https://godbolt.org/z/rG3P3d6f7) as does clang at -O{1,2,3,s} (https://godbolt.org/z/EcKbrn1b7) and icc at -O{1,2,3,s} (https://godbolt.org/z/Err73eGsb). But gcc at -Os seems to have a very strong preference to falling through to call yes() even in void expect_no(void){ if (__builtin_expect(test(),0)) yes(); else {} } and even in void expect_no2(void){ if (__builtin_expect(!test(),1)){} else yes(); } essentially completely disregarding any user attempts at controlling the branch layout of the output.
next reply other threads:[~2023-12-04 12:39 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-12-04 12:39 pskocik at gmail dot com [this message] 2023-12-04 12:52 ` [Bug c/112844] " jakub at gcc dot gnu.org 2024-03-30 14:10 ` [Bug middle-end/112844] " pskocik at gmail dot com
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