[InstCombine] Canonicalize icmp instructions based on dominating conditions.

Summary:
    This patch canonicalizes conditions based on the constant range information
    of the dominating branch condition.
    For example:

      %cmp = icmp slt i64 %a, 0
      br i1 %cmp, label %land.lhs.true, label %lor.rhs
      lor.rhs:
        %cmp2 = icmp sgt i64 %a, 0

    Would now be canonicalized into:

      %cmp = icmp slt i64 %a, 0
      br i1 %cmp, label %land.lhs.true, label %lor.rhs
      lor.rhs:
        %cmp2 = icmp ne i64 %a, 0

Reviewers: mcrosier, gberry, t.p.northover, llvm-commits, reames, hfinkel, sanjoy, majnemer

Subscribers: MatzeB, majnemer, mcrosier

Differential Revision: http://reviews.llvm.org/D18841

llvm-svn: 268521

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index 417b752d1a2..a2eb1517f57 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -127,6 +127,18 @@ ConstantRange ConstantRange::makeSatisfyingICmpRegion(CmpInst::Predicate Pred,
       .inverse();
 }
 
+ConstantRange ConstantRange::makeExactICmpRegion(CmpInst::Predicate Pred,
+                                                 const APInt &C) {
+  // Computes the exact range that is equal to both the constant ranges returned
+  // by makeAllowedICmpRegion and makeSatisfyingICmpRegion. This is always true
+  // when RHS is a singleton such as an APInt and so the assert is valid.
+  // However for non-singleton RHS, for example ult [2,5) makeAllowedICmpRegion
+  // returns [0,4) but makeSatisfyICmpRegion returns [0,2).
+  //
+  assert(makeAllowedICmpRegion(Pred, C) == makeSatisfyingICmpRegion(Pred, C));
+  return makeAllowedICmpRegion(Pred, C);
+}
+
 ConstantRange
 ConstantRange::makeGuaranteedNoWrapRegion(Instruction::BinaryOps BinOp,
                                           const ConstantRange &Other,