[IndVars] Sharpen context in eliminateIVComparison

When eliminating comparisons, we can use common dominator of
all its users as context. This gives better results when ICMP is not
computed right before the branch that uses it.

Differential Revision: https://reviews.llvm.org/D98924
Reviewed By: lebedev.ri

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index f0e4466..d4f325c 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -281,8 +281,11 @@ void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
 
   // If the condition is always true or always false in the given context,
   // replace it with a constant value.
-  // TODO: We can sharpen the context to common dominator of all ICmp's users.
-  if (auto Ev = SE->evaluatePredicateAt(Pred, S, X, ICmp)) {
+  SmallVector<Instruction *, 4> Users;
+  for (auto *U : ICmp->users())
+    Users.push_back(cast<Instruction>(U));
+  const Instruction *CtxI = findCommonDominator(Users, *DT);
+  if (auto Ev = SE->evaluatePredicateAt(Pred, S, X, CtxI)) {
     ICmp->replaceAllUsesWith(ConstantInt::getBool(ICmp->getContext(), *Ev));
     DeadInsts.emplace_back(ICmp);
     LLVM_DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');