[all-commits] [llvm/llvm-project] 597946: [ConstantFold] Don't convert getelementptr to ptrt...

[Nikita Popov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 597946a4dd2b3ba213f08a18285ab4362f873aa8
      https://github.com/llvm/llvm-project/commit/597946a4dd2b3ba213f08a18285ab4362f873aa8
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2022-05-02 (Mon, 02 May 2022)

  Changed paths:
    M llvm/lib/Analysis/ConstantFolding.cpp
    M llvm/test/Transforms/InstCombine/constant-fold-gep.ll

  Log Message:
  -----------
  [ConstantFold] Don't convert getelementptr to ptrtoint+inttoptr

ConstantFolding currently converts "getelementptr i8, Ptr, (sub 0, V)"
to "inttoptr (sub (ptrtoint Ptr), V)". This transform is, taken by
itself, correct, but does came with two issues:

1. It unnecessarily broadens provenance by introducing an inttoptr.
   We generally prefer not to introduce inttoptr during optimization.
2. For the case where V == ptrtoint Ptr, this folds to inttoptr 0,
   which further folds to null. In that case provenance becomes
   incorrect. This has been observed as a real-world miscompile with
   rustc.

We should probably address that incorrect inttoptr 0 fold at some
point, but in either case we should also drop this inttoptr-introducing
fold. Instead, replace it with a fold rooted at
ptrtoint(getelementptr), which seems to cover the original
motivation for this fold (test2 in the changed file).

Differential Revision: https://reviews.llvm.org/D124677