From 9c797fdf1d0a710e99793454123594fa0a898bec Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Sat, 13 Feb 2010 05:01:14 +0000
Subject: [PATCH] Speed up codegen prepare from 3.58s to 0.488s.

llvm-svn: 96081
---
 lib/Transforms/Utils/BreakCriticalEdges.cpp | 71 +++++++++++++++------
 1 file changed, 52 insertions(+), 19 deletions(-)

diff --git a/lib/Transforms/Utils/BreakCriticalEdges.cpp b/lib/Transforms/Utils/BreakCriticalEdges.cpp
index dac60131c3f..36573906224 100644
--- a/lib/Transforms/Utils/BreakCriticalEdges.cpp
+++ b/lib/Transforms/Utils/BreakCriticalEdges.cpp
@@ -192,23 +192,47 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
   
   // If there are any PHI nodes in DestBB, we need to update them so that they
   // merge incoming values from NewBB instead of from TIBB.
-  //
-  unsigned BBIdx = 0;
-  for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
-    // We no longer enter through TIBB, now we come in through NewBB.  Revector
-    // exactly one entry in the PHI node that used to come from TIBB to come
-    // from NewBB.
-    PHINode *PN = cast<PHINode>(I);
+  if (PHINode *APHI = dyn_cast<PHINode>(DestBB->begin())) {
+    // This conceptually does:
+    //  foreach (PHINode *PN in DestBB)
+    //    PN->setIncomingBlock(PN->getIncomingBlock(TIBB), NewBB);
+    // but is optimized for two cases.
     
-    // Reuse the previous value of BBIdx if it lines up.  In cases where we have
-    // multiple phi nodes with *lots* of predecessors, this is a speed win
-    // because we don't have to scan the PHI looking for TIBB.  This happens
-    // because the BB list of PHI nodes are usually in the same order.
-    if (PN->getIncomingBlock(BBIdx) != TIBB)
-      BBIdx = PN->getBasicBlockIndex(TIBB);
-    PN->setIncomingBlock(BBIdx, NewBB);
+    if (APHI->getNumIncomingValues() <= 8) {  // Small # preds case.
+      unsigned BBIdx = 0;
+      for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
+        // We no longer enter through TIBB, now we come in through NewBB.
+        // Revector exactly one entry in the PHI node that used to come from
+        // TIBB to come from NewBB.
+        PHINode *PN = cast<PHINode>(I);
+        
+        // Reuse the previous value of BBIdx if it lines up.  In cases where we
+        // have multiple phi nodes with *lots* of predecessors, this is a speed
+        // win because we don't have to scan the PHI looking for TIBB.  This
+        // happens because the BB list of PHI nodes are usually in the same
+        // order.
+        if (PN->getIncomingBlock(BBIdx) != TIBB)
+          BBIdx = PN->getBasicBlockIndex(TIBB);
+        PN->setIncomingBlock(BBIdx, NewBB);
+      }
+    } else {
+      // However, the foreach loop is slow for blocks with lots of predecessors
+      // because PHINode::getIncomingBlock is O(n) in # preds.  Instead, walk
+      // the user list of TIBB to find the PHI nodes.
+      SmallPtrSet<PHINode*, 16> UpdatedPHIs;
+    
+      for (Value::use_iterator UI = TIBB->use_begin(), E = TIBB->use_end();
+           UI != E; ) {
+        Value::use_iterator Use = UI++;
+        if (PHINode *PN = dyn_cast<PHINode>(Use)) {
+          // Remove one entry from each PHI.
+          if (PN->getParent() == DestBB && UpdatedPHIs.insert(PN))
+            PN->setOperand(Use.getOperandNo(), NewBB);
+        }
+      }
+    }
   }
-  
+   
   // If there are any other edges from TIBB to DestBB, update those to go
   // through the split block, making those edges non-critical as well (and
   // reducing the number of phi entries in the DestBB if relevant).
@@ -228,6 +252,15 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
 
   // If we don't have a pass object, we can't update anything...
   if (P == 0) return NewBB;
+  
+  DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
+  DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>();
+  LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
+  ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
+  
+  // If we have nothing to update, just return.
+  if (DT == 0 && DF == 0 && LI == 0 && PI == 0)
+    return NewBB;
 
   // Now update analysis information.  Since the only predecessor of NewBB is
   // the TIBB, TIBB clearly dominates NewBB.  TIBB usually doesn't dominate
@@ -252,7 +285,7 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
   bool NewBBDominatesDestBB = true;
   
   // Should we update DominatorTree information?
-  if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
+  if (DT) {
     DomTreeNode *TINode = DT->getNode(TIBB);
 
     // The new block is not the immediate dominator for any other nodes, but
@@ -283,7 +316,7 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
   }
 
   // Should we update DominanceFrontier information?
-  if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
+  if (DF) {
     // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
     if (!OtherPreds.empty()) {
       // FIXME: IMPLEMENT THIS!
@@ -317,7 +350,7 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
   }
   
   // Update LoopInfo if it is around.
-  if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
+  if (LI) {
     if (Loop *TIL = LI->getLoopFor(TIBB)) {
       // If one or the other blocks were not in a loop, the new block is not
       // either, and thus LI doesn't need to be updated.
@@ -398,7 +431,7 @@ BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
   }
 
   // Update ProfileInfo if it is around.
-  if (ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>())
+  if (PI)
     PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges);
 
   return NewBB;