[all-commits] [llvm/llvm-project] ea2e66: [LLVM][ConstantFold] Undefined values are not cons...

[Kees Cook via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ea2e66aa8b6e363b89df66dc44275a0d7ecd70ce
      https://github.com/llvm/llvm-project/commit/ea2e66aa8b6e363b89df66dc44275a0d7ecd70ce
  Author: Kees Cook <kees at kernel.org>
  Date:   2025-03-12 (Wed, 12 Mar 2025)

  Changed paths:
    M llvm/lib/IR/Constants.cpp
    M llvm/test/Transforms/LowerConstantIntrinsics/constant-intrinsics.ll

  Log Message:
  -----------
  [LLVM][ConstantFold] Undefined values are not constant (#130713)

llvm.is.constant (and therefore Clang's __builtin_constant_p()) need to
report undefined values as non-constant or future DCE choices end up
making no sense. This was encountered while building the Linux kernel
which uses __builtin_constant_p() while trying to evaluate if it is safe
to use a compile-time constant resolution for string lengths or if it
must kick over to a full runtime call to strlen(). Obviously an
undefined variable cannot be known at compile-time, so
__builtin_constant_p() needs to return false. This change will also mean
that Clang will match GCC's behavior under the same conditions.

Fixes #130649



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