indvars: insert truncate at loop boundary to avoid redundant IVs.

When widening an IV to remove s/zext, we generally try to eliminate
the original narrow IV. However, LCSSA phi nodes outside the loop were
still using the original IV. Clean this up more aggressively to avoid
redundancy in generated code.

llvm-svn: 198338

1 files changed, 12 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index c291f68bd63..c992f8c12a6 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -897,15 +897,24 @@ const SCEVAddRecExpr *WidenIV::GetWideRecurrence(Instruction *NarrowUse) {
   return AddRec;
 }
 
+/// This IV user cannot be widen. Replace this use of the original narrow IV
+/// with a truncation of the new wide IV to isolate and eliminate the narrow IV.
+static void truncateIVUse(NarrowIVDefUse DU, DominatorTree *DT) {
+  IRBuilder<> Builder(getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT));
+  Value *Trunc = Builder.CreateTrunc(DU.WideDef, DU.NarrowDef->getType());
+  DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, Trunc);
+}
+
 /// WidenIVUse - Determine whether an individual user of the narrow IV can be
 /// widened. If so, return the wide clone of the user.
 Instruction *WidenIV::WidenIVUse(NarrowIVDefUse DU, SCEVExpander &Rewriter) {
 
   // Stop traversing the def-use chain at inner-loop phis or post-loop phis.
   if (isa<PHINode>(DU.NarrowUse) &&
-      LI->getLoopFor(DU.NarrowUse->getParent()) != L)
+      LI->getLoopFor(DU.NarrowUse->getParent()) != L) {
+    truncateIVUse(DU, DT);
     return 0;
-
+  }
   // Our raison d'etre! Eliminate sign and zero extension.
   if (IsSigned ? isa<SExtInst>(DU.NarrowUse) : isa<ZExtInst>(DU.NarrowUse)) {
     Value *NewDef = DU.WideDef;
@@ -953,9 +962,7 @@ Instruction *WidenIV::WidenIVUse(NarrowIVDefUse DU, SCEVExpander &Rewriter) {
     // This user does not evaluate to a recurence after widening, so don't
     // follow it. Instead insert a Trunc to kill off the original use,
     // eventually isolating the original narrow IV so it can be removed.
-    IRBuilder<> Builder(getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT));
-    Value *Trunc = Builder.CreateTrunc(DU.WideDef, DU.NarrowDef->getType());
-    DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, Trunc);
+    truncateIVUse(DU, DT);
     return 0;
   }
   // Assume block terminators cannot evaluate to a recurrence. We can't to