[InstCombine] canonicalize icmp predicate feeding select

This canonicalization was suggested in D33172 as a way to make InstCombine behavior more uniform. 
We have this transform for icmp+br, so unless there's some reason that icmp+select should be 
treated differently, we should do the same thing here.

The benefit comes from increasing the chances of creating identical instructions. This is shown in
the tests in logical-select.ll (PR32791). InstCombine doesn't fold those directly, but EarlyCSE 
can simplify the identical cmps, and then InstCombine can fold the selects together.

The possible regression for the tests in select.ll raises questions about poison/undef:
http://lists.llvm.org/pipermail/llvm-dev/2017-May/113261.html

...but that transform is just as likely to be triggered by this canonicalization as it is to be 
missed, so we're just pointing out a commutation deficiency in the pattern matching:
https://reviews.llvm.org/rL228409

Differential Revision: https://reviews.llvm.org/D34242

llvm-svn: 306435

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 33951e66497..80c6595904e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1167,6 +1167,23 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
   if (Instruction *I = canonicalizeSelectToShuffle(SI))
     return I;
 
+  // Canonicalize a one-use integer compare with a non-canonical predicate by
+  // inverting the predicate and swapping the select operands. This matches a
+  // compare canonicalization for conditional branches.
+  // TODO: Should we do the same for FP compares?
+  CmpInst::Predicate Pred;
+  if (match(CondVal, m_OneUse(m_ICmp(Pred, m_Value(), m_Value()))) &&
+      !isCanonicalPredicate(Pred)) {
+    // Swap true/false values and condition.
+    CmpInst *Cond = cast<CmpInst>(CondVal);
+    Cond->setPredicate(CmpInst::getInversePredicate(Pred));
+    SI.setOperand(1, FalseVal);
+    SI.setOperand(2, TrueVal);
+    SI.swapProfMetadata();
+    Worklist.Add(Cond);
+    return &SI;
+  }
+
   if (SelType->getScalarType()->isIntegerTy(1) &&
       TrueVal->getType() == CondVal->getType()) {
     if (match(TrueVal, m_One())) {