[PATCH] D39835: [GVN PRE] Clear nsw/nuw for original values in LoadPRE

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

> That is just as illegal as the Scalar PRE case by the rules above, as replacing an add with an add nsw.

I agree.  And in fact, NewGVN has a bug here: it will transform the following.

  @H = common global i32 0
  @G = common global i32 0
  
  define i32 @test(i1 %cond, i32 %v) nounwind {
  entry:
    br i1 %cond, label %bb, label %bb1
  
  bb:
    %add.1 = add nuw nsw i32 %v, -1
    store i32 %add.1, i32* @G, align 4
    br label %merge
  
  bb1:
    %add.2 = add i32 %v, -1
    store i32 %add.2, i32* @G, align 4
    br label %merge
  
  merge:
    %foo = phi i32 [ %add.2, %bb1 ], [ %add.1, %bb ]
    %foo2 = add i32 %v, -1
    %cmp2 = icmp sgt i32 %foo2, 0
    br i1 %cmp2, label %action, label %return
  
  action:
    store i32 %foo, i32* @H, align 4
    br label %return
  
  return:
    %p = phi i32 [0, %merge], [1, %action]
    ret i32 %p
  }

But old GVN doesn't compute that equivalence, as far as I can tell.


https://reviews.llvm.org/D39835