[GVN] Split critical-edge on the fly, instead of postpone edge-splitting to next

iteration.
  
  This on step toward non-iterative GVN. My local hack suggests that getting rid
of iteration will speedup GVN by 30%+ on a medium sized input (2k LOC, C++).
I cannot explain why not 2x or more at this moment.

llvm-svn: 181532

lib/Transforms/Scalar/GVN.cpp

@@ -45,6 +45,7 @@
 #include "llvm/Target/TargetLibraryInfo.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/SSAUpdater.h"
+#include <vector>
 using namespace llvm;
 using namespace PatternMatch;
 
@@ -692,6 +693,7 @@ namespace {
     void cleanupGlobalSets();
     void verifyRemoved(const Instruction *I) const;
     bool splitCriticalEdges();
+    BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);
     unsigned replaceAllDominatedUsesWith(Value *From, Value *To,
                                          const BasicBlockEdge &Root);
     bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root);
@@ -1513,7 +1515,7 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
   for (unsigned i = 0, e = UnavailableBlocks.size(); i != e; ++i)
     FullyAvailableBlocks[UnavailableBlocks[i]] = false;
 
-  SmallVector<std::pair<TerminatorInst*, unsigned>, 4> NeedToSplit;
+  SmallVector<BasicBlock *, 4> CriticalEdgePred;
   for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB);
        PI != E; ++PI) {
     BasicBlock *Pred = *PI;
@@ -1536,20 +1538,14 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
         return false;
       }
 
-      unsigned SuccNum = GetSuccessorNumber(Pred, LoadBB);
-      NeedToSplit.push_back(std::make_pair(Pred->getTerminator(), SuccNum));
+      CriticalEdgePred.push_back(Pred);
     }
   }
 
-  if (!NeedToSplit.empty()) {
-    toSplit.append(NeedToSplit.begin(), NeedToSplit.end());
-    return false;
-  }
-
   // Decide whether PRE is profitable for this load.
   unsigned NumUnavailablePreds = PredLoads.size();
   assert(NumUnavailablePreds != 0 &&
-         "Fully available value should be eliminated above!");
+         "Fully available value should already be eliminated!");
 
   // If this load is unavailable in multiple predecessors, reject it.
   // FIXME: If we could restructure the CFG, we could make a common pred with
@@ -1558,6 +1554,17 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
   if (NumUnavailablePreds != 1)
       return false;
 
+  // Split critical edges, and update the unavailable predecessors accordingly.
+  for (SmallVector<BasicBlock *, 4>::iterator I = CriticalEdgePred.begin(), 
+         E = CriticalEdgePred.end(); I != E; I++) {
+    BasicBlock *OrigPred = *I;
+    BasicBlock *NewPred = splitCriticalEdges(OrigPred, LoadBB);
+    PredLoads.erase(OrigPred);
+    PredLoads[NewPred] = 0;
+    DEBUG(dbgs() << "Split critical edge " << OrigPred->getName() << "->"
+                 << LoadBB->getName() << '\n');
+  }
+
   // Check if the load can safely be moved to all the unavailable predecessors.
   bool CanDoPRE = true;
   SmallVector<Instruction*, 8> NewInsts;
@@ -1594,7 +1601,9 @@ bool GVN::PerformLoadPRE(LoadInst *LI, AvailValInBlkVect &ValuesPerBlock,
       if (MD) MD->removeInstruction(I);
       I->eraseFromParent();
     }
-    return false;
+    // HINT:Don't revert the edge-splitting as following transformation may 
+    // also need to split these critial edges.
+    return !CriticalEdgePred.empty();
   }
 
   // Okay, we can eliminate this load by inserting a reload in the predecessor
@@ -2297,8 +2306,6 @@ bool GVN::runOnFunction(Function& F) {
   while (ShouldContinue) {
     DEBUG(dbgs() << "GVN iteration: " << Iteration << "\n");
     ShouldContinue = iterateOnFunction(F);
-    if (splitCriticalEdges())
-      ShouldContinue = true;
     Changed |= ShouldContinue;
     ++Iteration;
   }
@@ -2310,6 +2317,7 @@ bool GVN::runOnFunction(Function& F) {
       Changed |= PREChanged;
     }
   }
+
   // FIXME: Should perform GVN again after PRE does something.  PRE can move
   // computations into blocks where they become fully redundant.  Note that
   // we can't do this until PRE's critical edge splitting updates memdep.
@@ -2543,6 +2551,15 @@ bool GVN::performPRE(Function &F) {
   return Changed;
 }
 
+/// Split the critical edge connecting the given two blocks, and return
+/// the block inserted to the critical edge.
+BasicBlock *GVN::splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ) {
+  BasicBlock *BB = SplitCriticalEdge(Pred, Succ, this);
+  if (MD)
+    MD->invalidateCachedPredecessors();
+  return BB;
+}
+
 /// splitCriticalEdges - Split critical edges found during the previous
 /// iteration that may enable further optimization.
 bool GVN::splitCriticalEdges() {
@@ -2569,9 +2586,18 @@ bool GVN::iterateOnFunction(Function &F) {
        RE = RPOT.end(); RI != RE; ++RI)
     Changed |= processBlock(*RI);
 #else
+  // Save the blocks this function have before transformation begins. GVN may
+  // split critical edge, and hence may invalidate the RPO/DT iterator.
+  //
+  std::vector<BasicBlock *> BBVect;
+  BBVect.reserve(256);
   for (df_iterator<DomTreeNode*> DI = df_begin(DT->getRootNode()),
        DE = df_end(DT->getRootNode()); DI != DE; ++DI)
-    Changed |= processBlock(DI->getBlock());
+    BBVect.push_back(DI->getBlock());
+
+  for (std::vector<BasicBlock *>::iterator I = BBVect.begin(), E = BBVect.end();
+       I != E; I++)
+    Changed |= processBlock(*I);
 #endif
 
   return Changed;