[PATCH] D13717: [IndVars] Have `cloneArithmeticIVUser` guess better

[Sanjoy Das via llvm-commits]

sanjoy created this revision.
sanjoy added reviewers: atrick, hfinkel, reames.
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`cloneArithmeticIVUser` currently trips over expression like `add %iv,
-1` when `%iv` is being zero extended -- it tries to construct the
widened use as `add %iv.zext, zext(-1)` and (correctly) fails to prove
equivalence to `zext(add %iv, -1)` (here the SCEV for `%iv` is
`{1,+,1}`).

This change teaches `IndVars` to try sign extending the non-IV operand
if that makes the newly constructed IV use equivalent to the widened
narrow IV use.

http://reviews.llvm.org/D13717

Files:
  lib/Transforms/Scalar/IndVarSimplify.cpp
  test/Transforms/IndVarSimplify/iv-widen.ll

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