[LV] Minor savings to Sink casts to unravel first order recurrence

Two minor savings: avoid copying the SinkAfter map and avoid moving a cast if it
is not needed.

Differential Revision: https://reviews.llvm.org/D36408


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310910 91177308-0d34-0410-b5e6-96231b3b80d8

Author: azaks

lib/Transforms/Utils/LoopUtils.cpp

@@ -565,7 +565,8 @@ bool RecurrenceDescriptor::isFirstOrderRecurrence(
     auto *I = Phi->user_back();
     if (I->isCast() && (I->getParent() == Phi->getParent()) && I->hasOneUse() &&
         DT->dominates(Previous, I->user_back())) {
-      SinkAfter[I] = Previous;
+      if (!DT->dominates(Previous, I)) // Otherwise we're good w/o sinking.
+        SinkAfter[I] = Previous;
       return true;
     }
   }

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7655,7 +7655,7 @@ void LoopVectorizationPlanner::executePlan(InnerLoopVectorizer &ILV) {
   // 2. Copy and widen instructions from the old loop into the new loop.
 
   // Move instructions to handle first-order recurrences.
-  DenseMap<Instruction *, Instruction *> SinkAfter = Legal->getSinkAfter();
+  DenseMap<Instruction *, Instruction *> &SinkAfter = Legal->getSinkAfter();
   for (auto &Entry : SinkAfter) {
     Entry.first->removeFromParent();
     Entry.first->insertAfter(Entry.second);

test/Transforms/LoopVectorize/first-order-recurrence.ll

@@ -140,7 +140,10 @@ scalar.body:
 ; CHECK:       vector.body:
 ; CHECK:         %vector.recur = phi <4 x i16> [ %vector.recur.init, %vector.ph ], [ [[L1:%[a-zA-Z0-9.]+]], %vector.body ]
 ; CHECK:         [[L1]] = load <4 x i16>
-; CHECK:         {{.*}} = shufflevector <4 x i16> %vector.recur, <4 x i16> [[L1]], <4 x i32> <i32 3, i32 4, i32 5, i32 6>
+; CHECK:         [[SHUF:%[a-zA-Z0-9.]+]] = shufflevector <4 x i16> %vector.recur, <4 x i16> [[L1]], <4 x i32> <i32 3, i32 4, i32 5, i32 6>
+; Check also that the casts were not moved needlessly.
+; CHECK:         sitofp <4 x i16> [[L1]] to <4 x double>
+; CHECK:         sitofp <4 x i16> [[SHUF]] to <4 x double> 
 ; CHECK:       middle.block:
 ; CHECK:         %vector.recur.extract = extractelement <4 x i16> [[L1]], i32 3
 ; CHECK:       scalar.ph: