Make LoopUnroll fold excessive BasicBlocks. This results in a significant speedup of

gccas on 252.eon


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29936 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopUnroll.cpp

@@ -25,6 +25,7 @@
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Support/CFG.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/ADT/Statistic.h"
@@ -48,6 +49,7 @@ namespace {
   public:
     virtual bool runOnFunction(Function &F);
     bool visitLoop(Loop *L);
+    BasicBlock* FoldBlockIntoPredecessor(BasicBlock* BB);
 
     /// This transformation requires natural loop information & requires that
     /// loop preheaders be inserted into the CFG...
@@ -118,6 +120,76 @@ static inline void RemapInstruction(Instruction *I,
   }
 }
 
+// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
+// only has one predecessor, and that predecessor only has one successor.
+// Returns the new combined block.
+BasicBlock* LoopUnroll::FoldBlockIntoPredecessor(BasicBlock* BB) {
+  // Merge basic blocks into their predecessor if there is only one distinct
+  // pred, and if there is only one distinct successor of the predecessor, and
+  // if there are no PHI nodes.
+  //
+  pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
+  BasicBlock *OnlyPred = *PI++;
+  for (; PI != PE; ++PI)  // Search all predecessors, see if they are all same
+    if (*PI != OnlyPred) {
+      OnlyPred = 0;       // There are multiple different predecessors...
+      break;
+    }
+
+  BasicBlock *OnlySucc = 0;
+  if (OnlyPred && OnlyPred != BB &&    // Don't break self loops
+      OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
+    // Check to see if there is only one distinct successor...
+    succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
+    OnlySucc = BB;
+    for (; SI != SE; ++SI)
+      if (*SI != OnlySucc) {
+        OnlySucc = 0;     // There are multiple distinct successors!
+        break;
+      }
+  }
+
+  if (OnlySucc) {
+    DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred);
+    TerminatorInst *Term = OnlyPred->getTerminator();
+
+    // Resolve any PHI nodes at the start of the block.  They are all
+    // guaranteed to have exactly one entry if they exist, unless there are
+    // multiple duplicate (but guaranteed to be equal) entries for the
+    // incoming edges.  This occurs when there are multiple edges from
+    // OnlyPred to OnlySucc.
+    //
+    while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
+      PN->replaceAllUsesWith(PN->getIncomingValue(0));
+      BB->getInstList().pop_front();  // Delete the phi node...
+    }
+
+    // Delete the unconditional branch from the predecessor...
+    OnlyPred->getInstList().pop_back();
+
+    // Move all definitions in the successor to the predecessor...
+    OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
+
+    // Make all PHI nodes that referred to BB now refer to Pred as their
+    // source...
+    BB->replaceAllUsesWith(OnlyPred);
+
+    std::string OldName = BB->getName();
+
+    // Erase basic block from the function...
+    LI->removeBlock(BB);
+    BB->eraseFromParent();
+
+    // Inherit predecessors name if it exists...
+    if (!OldName.empty() && !OnlyPred->hasName())
+      OnlyPred->setName(OldName);
+
+    return OnlyPred;
+  }
+  
+  return 0;
+}
+
 bool LoopUnroll::visitLoop(Loop *L) {
   bool Changed = false;
 
@@ -247,13 +319,7 @@ bool LoopUnroll::visitLoop(Loop *L) {
         RemapInstruction(I, LastValueMap);
   }
 
-  //  Insert the branches that link the different iterations together
-  for (unsigned i = 0; i < Latches.size()-1; ++i)
-    new BranchInst(Headers[i+1], Latches[i]);
   
-  // Finally, add an unconditional branch to the block to continue into the exit
-  // block.
-  new BranchInst(LoopExit, Latches[Latches.size()-1]);
  
   // Update PHI nodes that reference the final latch block
   if (TripCount > 1) {
@@ -281,14 +347,28 @@ bool LoopUnroll::visitLoop(Loop *L) {
     PHINode *PN = OrigPHINode[i];
     PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
     Header->getInstList().erase(PN);
-  }  
-
+  }
+  
+  //  Insert the branches that link the different iterations together
+  for (unsigned i = 0; i < Latches.size()-1; ++i) {
+    new BranchInst(Headers[i+1], Latches[i]);
+    if(BasicBlock* Fold = FoldBlockIntoPredecessor(Headers[i+1])) {
+      std::replace(Latches.begin(), Latches.end(), Headers[i+1], Fold);
+      std::replace(Headers.begin(), Headers.end(), Headers[i+1], Fold);
+    }
+  }
+  
+  // Finally, add an unconditional branch to the block to continue into the exit
+  // block.
+  new BranchInst(LoopExit, Latches[Latches.size()-1]);
+  FoldBlockIntoPredecessor(LoopExit);
+  
   // At this point, the code is well formed.  We now do a quick sweep over the
   // inserted code, doing constant propagation and dead code elimination as we
   // go.
   const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
   for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
-       E = NewLoopBlocks.end(); BB != E; ++BB)
+       BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
     for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
       Instruction *Inst = I++;