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.
