[llvm] r207409 - Fix rampant quadratic behavior in UpdatePHINodes. The operation of

[Chandler Carruth]

Author: chandlerc
Date: Mon Apr 28 05:37:30 2014
New Revision: 207409

URL: http://llvm.org/viewvc/llvm-project?rev=207409&view=rev
Log:
Fix rampant quadratic behavior in UpdatePHINodes. The operation of
mapping from a basic block to an incoming value, either for removal or
just lookup, is linear in the number of predecessors, and we were doing
this for every entry in the 'Preds' list which is in many cases almost
all of them!

Unfortunately, the fixes are quite ugly. PHI nodes just don't make this
operation easy. The efficient way to fix this is to have a clever
'remove_if' operation on PHI nodes that lets us do a single pass over
all the incoming values of the original PHI node, extracting the ones we
care about. Then we could quickly construct the new phi node from this
list. This would remove the remaining underlying quadratic movement of
unrelated incoming values and the need for silly backwards looping to
"minimize" how often we hit the quadratic case.

This is the last obvious fix for PR19499. It shaves another 20% off the
compile time for me, and while UpdatePHINodes remains in the profile,
most of the time is now stemming from the well known inefficiencies of
LVI and jump threading.

Modified:
    llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp

Modified: llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp?rev=207409&r1=207408&r2=207409&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp Mon Apr 28 05:37:30 2014
@@ -385,6 +385,7 @@ static void UpdatePHINodes(BasicBlock *O
                            Pass *P, bool HasLoopExit) {
   // Otherwise, create a new PHI node in NewBB for each PHI node in OrigBB.
   AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : nullptr;
+  SmallPtrSet<BasicBlock *, 16> PredSet(Preds.begin(), Preds.end());
   for (BasicBlock::iterator I = OrigBB->begin(); isa<PHINode>(I); ) {
     PHINode *PN = cast<PHINode>(I++);
 
@@ -393,42 +394,58 @@ static void UpdatePHINodes(BasicBlock *O
     Value *InVal = nullptr;
     if (!HasLoopExit) {
       InVal = PN->getIncomingValueForBlock(Preds[0]);
-      for (unsigned i = 1, e = Preds.size(); i != e; ++i)
-        if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
+      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
+        if (!PredSet.count(PN->getIncomingBlock(i)))
+          continue;
+        if (!InVal)
+          InVal = PN->getIncomingValue(i);
+        else if (InVal != PN->getIncomingValue(i)) {
           InVal = nullptr;
           break;
         }
+      }
     }
 
     if (InVal) {
       // If all incoming values for the new PHI would be the same, just don't
       // make a new PHI.  Instead, just remove the incoming values from the old
       // PHI.
-      for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
-        // Explicitly check the BB index here to handle duplicates in Preds.
-        int Idx = PN->getBasicBlockIndex(Preds[i]);
-        if (Idx >= 0)
-          PN->removeIncomingValue(Idx, false);
-      }
-    } else {
-      // If the values coming into the block are not the same, we need a PHI.
-      // Create the new PHI node, insert it into NewBB at the end of the block
-      PHINode *NewPHI =
+
+      // NOTE! This loop walks backwards for a reason! First off, this minimizes
+      // the cost of removal if we end up removing a large number of values, and
+      // second off, this ensures that the indices for the incoming values
+      // aren't invalidated when we remove one.
+      for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i)
+        if (PredSet.count(PN->getIncomingBlock(i)))
+          PN->removeIncomingValue(i, false);
+
+      // Add an incoming value to the PHI node in the loop for the preheader
+      // edge.
+      PN->addIncoming(InVal, NewBB);
+      continue;
+    }
+
+    // If the values coming into the block are not the same, we need a new
+    // PHI.
+    // Create the new PHI node, insert it into NewBB at the end of the block
+    PHINode *NewPHI =
         PHINode::Create(PN->getType(), Preds.size(), PN->getName() + ".ph", BI);
-      if (AA) AA->copyValue(PN, NewPHI);
+    if (AA)
+      AA->copyValue(PN, NewPHI);
 
-      // Move all of the PHI values for 'Preds' to the new PHI.
-      for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
-        Value *V = PN->removeIncomingValue(Preds[i], false);
-        NewPHI->addIncoming(V, Preds[i]);
+    // NOTE! This loop walks backwards for a reason! First off, this minimizes
+    // the cost of removal if we end up removing a large number of values, and
+    // second off, this ensures that the indices for the incoming values aren't
+    // invalidated when we remove one.
+    for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i) {
+      BasicBlock *IncomingBB = PN->getIncomingBlock(i);
+      if (PredSet.count(IncomingBB)) {
+        Value *V = PN->removeIncomingValue(i, false);
+        NewPHI->addIncoming(V, IncomingBB);
       }
-
-      InVal = NewPHI;
     }
 
-    // Add an incoming value to the PHI node in the loop for the preheader
-    // edge.
-    PN->addIncoming(InVal, NewBB);
+    PN->addIncoming(NewPHI, NewBB);
   }
 }