[Loop Peeling] Add support for peeling of loops with multiple exits

This patch modifies the loop peeling transformation so that
it does not expect that there is only one loop exit from latch.

It modifies only transformation. Update of branch weights remains
only for exit from latch.

The motivation is that in follow-up patch I plan to enable loop peeling for
loops with multiple exits but only if other exits then from latch one goes to
block with call to deopt.

For now this patch is NFC.

Reviewers: reames, mkuper, iajbar, fhahn	
Reviewed By: reames, fhahn
Subscribers: zzheng, llvm-commits
Differential Revision: https://reviews.llvm.org/D63921

llvm-svn: 365441

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -401,8 +401,11 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     // Successful peeling may result in a change in the loop preheader/trip
     // counts. If we later unroll the loop, we want these to be updated.
     if (Peeled) {
-      BasicBlock *ExitingBlock = L->getExitingBlock();
+      // According to our guards and profitability checks the only
+      // meaningful exit should be latch block.
+      BasicBlock *ExitingBlock = L->getLoopLatch();
       assert(ExitingBlock && "Loop without exiting block?");
+      assert(L->isLoopExiting(ExitingBlock) && "Latch is not exiting?");
       Preheader = L->getLoopPreheader();
       ULO.TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
       ULO.TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);

llvm/lib/Transforms/Utils/LoopUnrollPeel.cpp

@@ -432,18 +432,18 @@ static void fixupBranchWeights(BasicBlock *Header, BranchInst *LatchBR,
 /// InsertBot.
 /// \param IterNumber The serial number of the iteration currently being
 /// peeled off.
-/// \param Exit The exit block of the original loop.
+/// \param ExitEdges The exit edges of the original loop.
 /// \param[out] NewBlocks A list of the blocks in the newly created clone
 /// \param[out] VMap The value map between the loop and the new clone.
 /// \param LoopBlocks A helper for DFS-traversal of the loop.
 /// \param LVMap A value-map that maps instructions from the original loop to
 /// instructions in the last peeled-off iteration.
-static void cloneLoopBlocks(Loop *L, unsigned IterNumber, BasicBlock *InsertTop,
-                            BasicBlock *InsertBot, BasicBlock *Exit,
-                            SmallVectorImpl<BasicBlock *> &NewBlocks,
-                            LoopBlocksDFS &LoopBlocks, ValueToValueMapTy &VMap,
-                            ValueToValueMapTy &LVMap, DominatorTree *DT,
-                            LoopInfo *LI) {
+static void cloneLoopBlocks(
+    Loop *L, unsigned IterNumber, BasicBlock *InsertTop, BasicBlock *InsertBot,
+    SmallVectorImpl<std::pair<BasicBlock *, BasicBlock *> > &ExitEdges,
+    SmallVectorImpl<BasicBlock *> &NewBlocks, LoopBlocksDFS &LoopBlocks,
+    ValueToValueMapTy &VMap, ValueToValueMapTy &LVMap, DominatorTree *DT,
+    LoopInfo *LI) {
   BasicBlock *Header = L->getHeader();
   BasicBlock *Latch = L->getLoopLatch();
   BasicBlock *PreHeader = L->getLoopPreheader();
@@ -489,9 +489,11 @@ static void cloneLoopBlocks(Loop *L, unsigned IterNumber, BasicBlock *InsertTop,
   // iteration (for every other iteration)
   BasicBlock *NewLatch = cast<BasicBlock>(VMap[Latch]);
   BranchInst *LatchBR = cast<BranchInst>(NewLatch->getTerminator());
-  unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
-  LatchBR->setSuccessor(HeaderIdx, InsertBot);
-  LatchBR->setSuccessor(1 - HeaderIdx, Exit);
+  for (unsigned idx = 0, e = LatchBR->getNumSuccessors(); idx < e; ++idx)
+    if (LatchBR->getSuccessor(idx) == Header) {
+      LatchBR->setSuccessor(idx, InsertBot);
+      break;
+    }
   if (DT)
     DT->changeImmediateDominator(InsertBot, NewLatch);
 
@@ -522,14 +524,14 @@ static void cloneLoopBlocks(Loop *L, unsigned IterNumber, BasicBlock *InsertTop,
   // we've just created. Note that this must happen *after* the incoming
   // values are adjusted, since the value going out of the latch may also be
   // a value coming into the header.
-  for (BasicBlock::iterator I = Exit->begin(); isa<PHINode>(I); ++I) {
-    PHINode *PHI = cast<PHINode>(I);
-    Value *LatchVal = PHI->getIncomingValueForBlock(Latch);
-    Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
-    if (LatchInst && L->contains(LatchInst))
-      LatchVal = VMap[LatchVal];
-    PHI->addIncoming(LatchVal, cast<BasicBlock>(VMap[Latch]));
-  }
+  for (auto Edge : ExitEdges)
+    for (PHINode &PHI : Edge.second->phis()) {
+      Value *LatchVal = PHI.getIncomingValueForBlock(Edge.first);
+      Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
+      if (LatchInst && L->contains(LatchInst))
+        LatchVal = VMap[LatchVal];
+      PHI.addIncoming(LatchVal, cast<BasicBlock>(VMap[Edge.first]));
+    }
 
   // LastValueMap is updated with the values for the current loop
   // which are used the next time this function is called.
@@ -558,7 +560,8 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
   BasicBlock *Header = L->getHeader();
   BasicBlock *PreHeader = L->getLoopPreheader();
   BasicBlock *Latch = L->getLoopLatch();
-  BasicBlock *Exit = L->getUniqueExitBlock();
+  SmallVector<std::pair<BasicBlock *, BasicBlock *>, 4> ExitEdges;
+  L->getExitEdges(ExitEdges);
 
   Function *F = Header->getParent();
 
@@ -640,8 +643,8 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
     else
       CurHeaderWeight = 1;
 
-    cloneLoopBlocks(L, Iter, InsertTop, InsertBot, Exit,
-                    NewBlocks, LoopBlocks, VMap, LVMap, DT, LI);
+    cloneLoopBlocks(L, Iter, InsertTop, InsertBot, ExitEdges, NewBlocks,
+                    LoopBlocks, VMap, LVMap, DT, LI);
 
     // Remap to use values from the current iteration instead of the
     // previous one.
@@ -652,7 +655,9 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
       // latter is the first cloned loop body, as original PreHeader dominates
       // the original loop body.
       if (Iter == 0)
-        DT->changeImmediateDominator(Exit, cast<BasicBlock>(LVMap[Latch]));
+        for (auto Edge : ExitEdges)
+          DT->changeImmediateDominator(Edge.second,
+                                       cast<BasicBlock>(LVMap[Edge.first]));
 #ifdef EXPENSIVE_CHECKS
       assert(DT->verify(DominatorTree::VerificationLevel::Fast));
 #endif