[all-commits] [llvm/llvm-project] 89c4b2: [GuardWidening] Fix a nasty cast bug in c2eccc6

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 89c4b29e8d35ec352019d828e546bea3850403df
      https://github.com/llvm/llvm-project/commit/89c4b29e8d35ec352019d828e546bea3850403df
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2022-06-07 (Tue, 07 Jun 2022)

  Changed paths:
    M llvm/lib/Transforms/Scalar/GuardWidening.cpp

  Log Message:
  -----------
  [GuardWidening] Fix a nasty cast bug in c2eccc6

c2eccc6 introduced a call to etHasNoUnsignedWrap which implicitly assumes that Inst is a OverflowingBinaryOperator.  This is frequently untrue, but was not caught because cast<Ty>(X) has been broken, see https://discourse.llvm.org/t/cast-x-is-broken-implications-and-proposal-to-address/63033 for context.

I considered reverting this, but since doing so re-introduces a nasty miscompile of its own, I decided to fix forward instead.

I'll note that this is a particularly nasty form of the cast<Ty>(X) issue.  Because the cast was succeeding unexpected, we were writing data to instructions which weren't OBOs.  This could result in near arbitrary data or memory corruption.  I'm a bit shocked that the sanitizers didn't find this TBH.