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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
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112844

            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 bef=
ore
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=20

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 br=
anch
layout of the output.=