From: Chandler Carruth <chandlerc@gmail.com>
Date: Thu, 24 Jan 2013 10:40:51 +0000 (+0000)
Subject: Simplify the PHI node operand rewriting.
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=0afa33115c5f0bce263ef370886f53bc845ab7c1;p=oota-llvm.git

Simplify the PHI node operand rewriting.

Previously, the code would scan the PHI nodes and build up a small
setvector of candidate value pairs in phi nodes to go and rewrite. Once
certain the rewrite could be performed, the code walks the set, and for
each one re-scans the entire PHI node list looking for nodes to rewrite
operands.

Instead, scan the PHI nodes once to check for hazards, and then scan it
a second time to rewrite the operands to selects. No set vector, and
a max of two scans.

The only downside is that we might form identical selects, but
instcombine or anything else should fold those easily, and it seems
unlikely to happen often.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173337 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 29e118035e7..5492b600c85 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1424,8 +1424,8 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
     }
   }
 
-  // Collect interesting PHIs, and scan for hazards.
-  SmallSetVector<std::pair<Value *, Value *>, 4> PHIs;
+  // Check that the PHI nodes can be converted to selects.
+  bool HaveRewritablePHIs = false;
   for (BasicBlock::iterator I = EndBB->begin();
        PHINode *PN = dyn_cast<PHINode>(I); ++I) {
     Value *OrigV = PN->getIncomingValueForBlock(BB);
@@ -1435,26 +1435,27 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
     if (ThenV == OrigV)
       continue;
 
+    HaveRewritablePHIs = true;
+
     // Check for safety.
-    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(ThenV)) {
-      // An unfolded ConstantExpr could end up getting expanded into
-      // Instructions. Don't speculate this and another instruction at
-      // the same time.
-      if (HInst)
-        return false;
-      if (!isSafeToSpeculativelyExecute(CE))
-        return false;
-      if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold)
-        return false;
-    }
+    ConstantExpr *CE = dyn_cast<ConstantExpr>(ThenV);
+    if (!CE)
+      continue; // Known safe.
 
-    // Ok, we may insert a select for this PHI.
-    PHIs.insert(std::make_pair(ThenV, OrigV));
+    // An unfolded ConstantExpr could end up getting expanded into
+    // Instructions. Don't speculate this and another instruction at
+    // the same time.
+    if (HInst)
+      return false;
+    if (!isSafeToSpeculativelyExecute(CE))
+      return false;
+    if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold)
+      return false;
   }
 
   // If there are no PHIs to process, bail early. This helps ensure idempotence
   // as well.
-  if (PHIs.empty())
+  if (!HaveRewritablePHIs)
     return false;
 
   // If we get here, we can hoist the instruction and if-convert.
@@ -1466,35 +1467,27 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
 
   // Insert selects and rewrite the PHI operands.
   IRBuilder<true, NoFolder> Builder(BI);
-  for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
-    Value *TrueV = PHIs[i].first;
-    Value *FalseV = PHIs[i].second;
+  for (BasicBlock::iterator I = EndBB->begin();
+       PHINode *PN = dyn_cast<PHINode>(I); ++I) {
+    unsigned OrigI = PN->getBasicBlockIndex(BB);
+    unsigned ThenI = PN->getBasicBlockIndex(ThenBB);
+    Value *OrigV = PN->getIncomingValue(OrigI);
+    Value *ThenV = PN->getIncomingValue(ThenI);
+
+    // Skip PHIs which are trivial.
+    if (OrigV == ThenV)
+      continue;
 
     // Create a select whose true value is the speculatively executed value and
-    // false value is the previously determined FalseV.
-    SelectInst *SI;
+    // false value is the preexisting value. Swap them if the branch
+    // destinations were inverted.
+    Value *TrueV = ThenV, *FalseV = OrigV;
     if (Invert)
-      SI = cast<SelectInst>
-        (Builder.CreateSelect(BrCond, FalseV, TrueV,
-                              FalseV->getName() + "." + TrueV->getName()));
-    else
-      SI = cast<SelectInst>
-        (Builder.CreateSelect(BrCond, TrueV, FalseV,
-                              TrueV->getName() + "." + FalseV->getName()));
-
-    // Make the PHI node use the select for all incoming values for "then" and
-    // "if" blocks.
-    for (BasicBlock::iterator I = EndBB->begin();
-         PHINode *PN = dyn_cast<PHINode>(I); ++I) {
-      unsigned ThenI = PN->getBasicBlockIndex(ThenBB);
-      unsigned OrigI = PN->getBasicBlockIndex(BB);
-      Value *ThenV = PN->getIncomingValue(ThenI);
-      Value *OrigV = PN->getIncomingValue(OrigI);
-      if (TrueV == ThenV && FalseV == OrigV) {
-        PN->setIncomingValue(ThenI, SI);
-        PN->setIncomingValue(OrigI, SI);
-      }
-    }
+      std::swap(TrueV, FalseV);
+    Value *V = Builder.CreateSelect(BrCond, TrueV, FalseV,
+                                    TrueV->getName() + "." + FalseV->getName());
+    PN->setIncomingValue(OrigI, V);
+    PN->setIncomingValue(ThenI, V);
   }
 
   ++NumSpeculations;