Miscompile with opt -passes="constraint-elimination"

[mikaelholmen]

llvm commit: 5842dfe
Reproduce with: opt -passes="constraint-elimination" bbi-90501.ll -S -o -
bbi-90501.ll.gz

There are two nested loops in the input that should finish and then we should end up at

bb.2:                                             ; preds = %bb.1
  ret i16 0

but with constraint-elimination the condition in bb.5 is changed to

  %i10 = or i1 true, %i9
  br i1 %i10, label %bb.7, label %bb.6

so we will go to

bb.7:                                             ; preds = %bb.5
  ret i16 1

instead.

[mikaelholmen]

This starts happening with e6a1657

[ConstraintElim] Add A < B if A is an increasing phi for A != B.

@fhahn : any idea what's up?

[fhahn]

Looks like an issue with sub decomposition with negative constants, preparing a fix.

[mikaelholmen]

Thanks for the quick fix! I've verified it solves the problem I saw.

[fhahn]

@mikaelholmen thanks for the reproducer!

[fhahn]

(appreciated as always, thanks for your extensive testing)