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From: "richard.yao at alumni dot stonybrook.edu" <gcc-bugzilla@gcc.gnu.org>
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Date: Sat, 27 May 2023 17:54:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110007

            Bug ID: 110007
           Summary: Implement support for Clang=E2=80=99s
                    __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 encour=
age
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 C=
lang
does not support it and it does not always work as described in #110001.

Combining this with C=E2=80=99s ternary operator should make the proposed c=
mov 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=