[PATCH] D83595: [Draft][MSAN] Optimize away poisoning allocas that are always written before load

[Gui Andrade via Phabricator via llvm-commits]

guiand added a comment.

It seems like `collectInitializers` leans heavily on the `isPointerOffset` function, which returns an offset if two pointers have a constant difference, `nullopt` if they don't. The problem here is that we can't distinguish `isPointerOffset == nullopt` happening because the offset is determined at runtime, or because the two pointers are completely unrelated.

It's a pretty big difference, because we don't want to poison a sequence like this:

  %x = alloca [ i32, i32 }
  %y = alloca i32
  %z = load i32, i32* %y ; isPointerOffset == false
  %x0 = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 0
  %x1 = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 1
  store i32 0, i32* %x0
  store i32 0, i32* %x1

But we want to poison a sequence like this:

  %x = alloca [ i32, i32 }
  %y = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 %dynamic_offs
  %z = load i32, i32* %y ; isPointerOffset == false
  %x0 = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 0
  %x1 = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 1
  store i32 0, i32* %x0
  store i32 0, i32* %x1


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D83595