[PATCH] D154051: [LangRef] Always allow getelementptr inbounds with zero offset

[Nuno Lopes via Phabricator via llvm-commits]

nlopes accepted this revision.
nlopes added a comment.
This revision is now accepted and ready to land.

I've implemented this semantics in Alive2 and run it over LLVM's test suite.
There are only 2 regressions, and they seem ok:

  +  LLVM :: Transforms/InstCombine/gep-inbounds-null.ll
  define i1 @test_eq(ptr %base, i64 %idx) {
    %gep = gep inbounds ptr %base, 1 x i64 %idx
    %cnd = icmp eq ptr %gep, null
    ret i1 %cnd
  }
  =>
  define i1 @test_eq(ptr %base, i64 %idx) {
    %cnd = icmp eq ptr %base, null                ; wrong since %base can be OOB
    ret i1 %cnd
  }
  
  
  +  LLVM :: Transforms/InstSimplify/gep.ll
  define ptr @undef_inbounds_var_idx(i64 %idx) {
    %el = gep inbounds ptr undef, 8 x i64 %idx
    ret ptr %el
  }
  =>
  define ptr @undef_inbounds_var_idx(i64 %idx) {
    ret ptr poison                                 ; only ok if %idx != 0
  }

Therefore, if this change helps Rust, let's do it. It's backwards compatible since it's removing some UB behavior from before.


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  https://reviews.llvm.org/D154051/new/

https://reviews.llvm.org/D154051