https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110007
Bug ID: 110007
Summary: Implement support for Clang’s
__builtin_unpredictable()
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: richard.yao at alumni dot stonybrook.edu
Target Milestone: ---
Having the ability to specify __builtin_unpredictable() as a hint to encourage
the compiler to use cmov would be useful for implementing algorithms like
binary search that have unpredictable branches.
__builtin_expect_with_probability() looks like a possible substitute, but Clang
does not support it and it does not always work as described in #110001.
Combining this with C’s ternary operator should make the proposed cmov builtin
from #98801 unnecessary.
Note that while clang implements __builtin_unpredictable(), it does not
currently pass that information to the backend:
https://github.com/llvm/llvm-project/issues/62790#issuecomment-1552671099
