From: Chris Lattner <sabre@nondot.org>
Date: Sat, 28 Oct 2006 19:22:10 +0000 (+0000)
Subject: split critical edges more carefully and intelligently.  In particular, critical
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=bad7f48c7070444eeb767c169f8bbed86b2c1ae1;p=oota-llvm.git

split critical edges more carefully and intelligently.  In particular, critical
edges whose destinations are not phi nodes don't bother us.  Also, share
split edges, since the split edge can't have a phi.  This significantly
reduces the complexity of generated code in some cases.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31274 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 8c57c580636..cf49143c30f 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -44,8 +44,6 @@
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Compiler.h"
-#include <map>
-#include <set>
 #include <iostream>
 #include <algorithm>
 using namespace llvm;
@@ -3490,6 +3488,63 @@ static bool OptimizeGEPExpression(GetElementPtrInst *GEPI,
   return true;
 }
 
+
+/// SplitEdgeNicely - Split the critical edge from TI to it's specified
+/// successor if it will improve codegen.  We only do this if the successor has
+/// phi nodes (otherwise critical edges are ok).  If there is already another
+/// predecessor of the succ that is empty (and thus has no phi nodes), use it
+/// instead of introducing a new block.
+static void SplitEdgeNicely(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
+  BasicBlock *TIBB = TI->getParent();
+  BasicBlock *Dest = TI->getSuccessor(SuccNum);
+  assert(isa<PHINode>(Dest->begin()) &&
+         "This should only be called if Dest has a PHI!");
+
+  /// TIPHIValues - This array is lazily computed to determine the values of
+  /// PHIs in Dest that TI would provide.
+  std::vector<Value*> TIPHIValues;
+  
+  // Check to see if Dest has any blocks that can be used as a split edge for
+  // this terminator.
+  for (pred_iterator PI = pred_begin(Dest), E = pred_end(Dest); PI != E; ++PI) {
+    BasicBlock *Pred = *PI;
+    // To be usable, the pred has to end with an uncond branch to the dest.
+    BranchInst *PredBr = dyn_cast<BranchInst>(Pred->getTerminator());
+    if (!PredBr || !PredBr->isUnconditional() ||
+        // Must be empty other than the branch.
+        &Pred->front() != PredBr)
+      continue;
+    
+    // Finally, since we know that Dest has phi nodes in it, we have to make
+    // sure that jumping to Pred will have the same affect as going to Dest in
+    // terms of PHI values.
+    PHINode *PN;
+    unsigned PHINo = 0;
+    bool FoundMatch = true;
+    for (BasicBlock::iterator I = Dest->begin();
+         (PN = dyn_cast<PHINode>(I)); ++I, ++PHINo) {
+      if (PHINo == TIPHIValues.size())
+        TIPHIValues.push_back(PN->getIncomingValueForBlock(TIBB));
+
+      // If the PHI entry doesn't work, we can't use this pred.
+      if (TIPHIValues[PHINo] != PN->getIncomingValueForBlock(Pred)) {
+        FoundMatch = false;
+        break;
+      }
+    }
+    
+    // If we found a workable predecessor, change TI to branch to Succ.
+    if (FoundMatch) {
+      Dest->removePredecessor(TIBB);
+      TI->setSuccessor(SuccNum, Pred);
+      return;
+    }
+  }
+  
+  SplitCriticalEdge(TI, SuccNum, P, true);  
+}
+
+
 bool SelectionDAGISel::runOnFunction(Function &Fn) {
   MachineFunction &MF = MachineFunction::construct(&Fn, TLI.getTargetMachine());
   RegMap = MF.getSSARegMap();
@@ -3505,11 +3560,13 @@ bool SelectionDAGISel::runOnFunction(Function &Fn) {
   while (MadeChange) {
     MadeChange = false;
   for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
-    // Split all critical edges.
+    // Split all critical edges where the dest block has a PHI.
     TerminatorInst *BBTI = BB->getTerminator();
     if (BBTI->getNumSuccessors() > 1) {
       for (unsigned i = 0, e = BBTI->getNumSuccessors(); i != e; ++i)
-        SplitCriticalEdge(BBTI, i, this, true);
+        if (isa<PHINode>(BBTI->getSuccessor(i)->begin()) &&
+            isCriticalEdge(BBTI, i, true))
+          SplitEdgeNicely(BBTI, i, this);
     }