[LoopInterchange] Move PHI handling to adjustLoopBranches.

This patch moves the logic to handle reduction PHI nodes to the end of
adjustLoopBranches. Reduction PHI nodes in the outer loop header can be
moved to the inner loop header and reduction PHI nodes from the inner loop
header can be moved to the outer loop header. In the latter situation,
we have to deal with 1 kind of PHI nodes:

    PHI nodes that are part of inner loop-only reductions.

We can replace the PHI node with the value coming from outside
the inner loop.

Reviewers: mcrosier, efriedma, karthikthecool

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D46198

llvm-svn: 335027

1 files changed, 40 insertions, 30 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 9addb04e81b..2978165ed8a 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1278,8 +1278,7 @@ bool LoopInterchangeTransform::transform() {
     else
       InnerIndexVar = dyn_cast<Instruction>(InductionPHI->getIncomingValue(0));
 
-    // Ensure that InductionPHI is the first Phi node as required by
-    // splitInnerLoopHeader
+    // Ensure that InductionPHI is the first Phi node.
     if (&InductionPHI->getParent()->front() != InductionPHI)
       InductionPHI->moveBefore(&InductionPHI->getParent()->front());
 
@@ -1290,8 +1289,9 @@ bool LoopInterchangeTransform::transform() {
     LLVM_DEBUG(dbgs() << "splitInnerLoopLatch done\n");
 
     // Splits the inner loops phi nodes out into a separate basic block.
-    splitInnerLoopHeader();
-    LLVM_DEBUG(dbgs() << "splitInnerLoopHeader done\n");
+    BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
+    SplitBlock(InnerLoopHeader, InnerLoopHeader->getFirstNonPHI(), DT, LI);
+    LLVM_DEBUG(dbgs() << "splitting InnerLoopHeader done\n");
   }
 
   Transformed |= adjustLoopLinks();
@@ -1309,32 +1309,7 @@ void LoopInterchangeTransform::splitInnerLoopLatch(Instruction *Inc) {
   InnerLoopLatch = SplitBlock(InnerLoopLatchPred, Inc, DT, LI);
 }
 
-void LoopInterchangeTransform::splitInnerLoopHeader() {
-  // Split the inner loop header out. Here make sure that the reduction PHI's
-  // stay in the innerloop body.
-  BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
-  BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
-  SplitBlock(InnerLoopHeader, InnerLoopHeader->getFirstNonPHI(), DT, LI);
-  if (InnerLoopHasReduction) {
-    // Adjust Reduction PHI's in the block. The induction PHI must be the first
-    // PHI in InnerLoopHeader for this to work.
-    SmallVector<PHINode *, 8> PHIVec;
-    for (auto I = std::next(InnerLoopHeader->begin()); isa<PHINode>(I); ++I) {
-      PHINode *PHI = dyn_cast<PHINode>(I);
-      Value *V = PHI->getIncomingValueForBlock(InnerLoopPreHeader);
-      PHI->replaceAllUsesWith(V);
-      PHIVec.push_back((PHI));
-    }
-    for (PHINode *P : PHIVec) {
-      P->eraseFromParent();
-    }
-  }
-
-  LLVM_DEBUG(dbgs() << "Output of splitInnerLoopHeader InnerLoopHeaderSucc & "
-                       "InnerLoopHeader\n");
-}
-
-/// Move all instructions except the terminator from FromBB right before
+/// \brief Move all instructions except the terminator from FromBB right before
 /// InsertBefore
 static void moveBBContents(BasicBlock *FromBB, Instruction *InsertBefore) {
   auto &ToList = InsertBefore->getParent()->getInstList();
@@ -1470,6 +1445,41 @@ bool LoopInterchangeTransform::adjustLoopBranches() {
   restructureLoops(OuterLoop, InnerLoop, InnerLoopPreHeader,
                    OuterLoopPreHeader);
 
+  // Now update the reduction PHIs in the inner and outer loop headers.
+  SmallVector<PHINode *, 4> InnerLoopPHIs, OuterLoopPHIs;
+  for (PHINode &PHI : drop_begin(InnerLoopHeader->phis(), 1))
+    InnerLoopPHIs.push_back(cast<PHINode>(&PHI));
+  for (PHINode &PHI : drop_begin(OuterLoopHeader->phis(), 1))
+    OuterLoopPHIs.push_back(cast<PHINode>(&PHI));
+
+  for (PHINode *PHI : OuterLoopPHIs)
+    PHI->moveBefore(InnerLoopHeader->getFirstNonPHI());
+
+  // Move the PHI nodes from the inner loop header to the outer loop header.
+  // We have to deal with one kind of PHI nodes:
+  //  1) PHI nodes that are part of inner loop-only reductions.
+  // We only have to move the PHI node and update the incoming blocks.
+  for (PHINode *PHI : InnerLoopPHIs) {
+    PHI->moveBefore(OuterLoopHeader->getFirstNonPHI());
+    for (BasicBlock *InBB : PHI->blocks()) {
+      if (InnerLoop->contains(InBB))
+        continue;
+
+      assert(!isa<PHINode>(PHI->getIncomingValueForBlock(InBB)) &&
+             "Unexpected incoming PHI node, reductions in outer loop are not "
+             "supported yet");
+      PHI->replaceAllUsesWith(PHI->getIncomingValueForBlock(InBB));
+      PHI->eraseFromParent();
+      break;
+    }
+  }
+
+  // Update the incoming blocks for moved PHI nodes.
+  updateIncomingBlock(OuterLoopHeader, InnerLoopPreHeader, OuterLoopPreHeader);
+  updateIncomingBlock(OuterLoopHeader, InnerLoopLatch, OuterLoopLatch);
+  updateIncomingBlock(InnerLoopHeader, OuterLoopPreHeader, InnerLoopPreHeader);
+  updateIncomingBlock(InnerLoopHeader, OuterLoopLatch, InnerLoopLatch);
+
   return true;
 }