[LSR] Create fewer redundant instructions.

Summary:
Fix LSRInstance::HoistInsertPosition() to check the original insert
position block first for a canonical insertion point that is dominated
by all inputs.  This leads to SCEV being able to reuse more instructions
since it currently tracks the instructions it creates for reuse by
keeping a table of <Value, insert point> pairs.

Reviewers: atrick

Subscribers: mcrosier, mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D18001

llvm-svn: 263644

1 files changed, 22 insertions, 20 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index acfdec43d21..3b0fccca83e 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -4331,28 +4331,10 @@ BasicBlock::iterator
 LSRInstance::HoistInsertPosition(BasicBlock::iterator IP,
                                  const SmallVectorImpl<Instruction *> &Inputs)
                                                                          const {
+  Instruction *Tentative = &*IP;
   for (;;) {
-    const Loop *IPLoop = LI.getLoopFor(IP->getParent());
-    unsigned IPLoopDepth = IPLoop ? IPLoop->getLoopDepth() : 0;
-
-    BasicBlock *IDom;
-    for (DomTreeNode *Rung = DT.getNode(IP->getParent()); ; ) {
-      if (!Rung) return IP;
-      Rung = Rung->getIDom();
-      if (!Rung) return IP;
-      IDom = Rung->getBlock();
-
-      // Don't climb into a loop though.
-      const Loop *IDomLoop = LI.getLoopFor(IDom);
-      unsigned IDomDepth = IDomLoop ? IDomLoop->getLoopDepth() : 0;
-      if (IDomDepth <= IPLoopDepth &&
-          (IDomDepth != IPLoopDepth || IDomLoop == IPLoop))
-        break;
-    }
-
     bool AllDominate = true;
     Instruction *BetterPos = nullptr;
-    Instruction *Tentative = IDom->getTerminator();
     for (Instruction *Inst : Inputs) {
       if (Inst == Tentative || !DT.dominates(Inst, Tentative)) {
         AllDominate = false;
@@ -4360,7 +4342,7 @@ LSRInstance::HoistInsertPosition(BasicBlock::iterator IP,
       }
       // Attempt to find an insert position in the middle of the block,
       // instead of at the end, so that it can be used for other expansions.
-      if (IDom == Inst->getParent() &&
+      if (Tentative->getParent() == Inst->getParent() &&
           (!BetterPos || !DT.dominates(Inst, BetterPos)))
         BetterPos = &*std::next(BasicBlock::iterator(Inst));
     }
@@ -4370,6 +4352,26 @@ LSRInstance::HoistInsertPosition(BasicBlock::iterator IP,
       IP = BetterPos->getIterator();
     else
       IP = Tentative->getIterator();
+
+    const Loop *IPLoop = LI.getLoopFor(IP->getParent());
+    unsigned IPLoopDepth = IPLoop ? IPLoop->getLoopDepth() : 0;
+
+    BasicBlock *IDom;
+    for (DomTreeNode *Rung = DT.getNode(IP->getParent()); ; ) {
+      if (!Rung) return IP;
+      Rung = Rung->getIDom();
+      if (!Rung) return IP;
+      IDom = Rung->getBlock();
+
+      // Don't climb into a loop though.
+      const Loop *IDomLoop = LI.getLoopFor(IDom);
+      unsigned IDomDepth = IDomLoop ? IDomLoop->getLoopDepth() : 0;
+      if (IDomDepth <= IPLoopDepth &&
+          (IDomDepth != IPLoopDepth || IDomLoop == IPLoop))
+        break;
+    }
+
+    Tentative = IDom->getTerminator();
   }
 
   return IP;