Fix PR10103: Less code for enum type translation.

In cases such as the attached test, where the case value for a switch
destination is used in a phi node that follows the destination, it
might be better to replace that value with the condition value of the
switch, so that more blocks can be folded away with
TryToSimplifyUncondBranchFromEmptyBlock because there are less
conflicts in the phi node.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133344 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2450,6 +2450,77 @@ static bool EliminateDeadSwitchCases(SwitchInst *SI) {
   return !DeadCases.empty();
 }
 
+/// FindPHIForConditionForwarding - If BB would be eligible for simplification
+/// by TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
+/// by an unconditional branch), look at the phi node for BB in the successor
+/// block and see if the incoming value is equal to CaseValue. If so, return
+/// the phi node, and set PhiIndex to BB's index in the phi node.
+static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue,
+                                              BasicBlock *BB,
+                                              int *PhiIndex) {
+  if (BB->getFirstNonPHIOrDbg() != BB->getTerminator())
+    return NULL; // BB must be empty to be a candidate for simplification.
+  if (!BB->getSinglePredecessor())
+    return NULL; // BB must be dominated by the switch.
+
+  BranchInst *Branch = dyn_cast<BranchInst>(BB->getTerminator());
+  if (!Branch || !Branch->isUnconditional())
+    return NULL; // Terminator must be unconditional branch.
+
+  BasicBlock *Succ = Branch->getSuccessor(0);
+
+  BasicBlock::iterator I = Succ->begin();
+  while (PHINode *PHI = dyn_cast<PHINode>(I++)) {
+    int Idx = PHI->getBasicBlockIndex(BB);
+    assert(Idx >= 0 && "PHI has no entry for predecessor?");
+
+    Value *InValue = PHI->getIncomingValue(Idx);
+    if (InValue != CaseValue) continue;
+
+    *PhiIndex = Idx;
+    return PHI;
+  }
+
+  return NULL;
+}
+
+/// ForwardSwitchConditionToPHI - Try to forward the condition of a switch
+/// instruction to a phi node dominated by the switch, if that would mean that
+/// some of the destination blocks of the switch can be folded away.
+/// Returns true if a change is made.
+static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
+  typedef DenseMap<PHINode*, SmallVector<int,4> > ForwardingNodesMap;
+  ForwardingNodesMap ForwardingNodes;
+
+  for (unsigned I = 1; I < SI->getNumCases(); ++I) { // 0 is the default case.
+    ConstantInt *CaseValue = SI->getCaseValue(I);
+    BasicBlock *CaseDest = SI->getSuccessor(I);
+
+    int PhiIndex;
+    PHINode *PHI = FindPHIForConditionForwarding(CaseValue, CaseDest,
+                                                 &PhiIndex);
+    if (!PHI) continue;
+
+    ForwardingNodes[PHI].push_back(PhiIndex);
+  }
+
+  bool Changed = false;
+
+  for (ForwardingNodesMap::iterator I = ForwardingNodes.begin(),
+       E = ForwardingNodes.end(); I != E; ++I) {
+    PHINode *Phi = I->first;
+    SmallVector<int,4> &Indexes = I->second;
+
+    if (Indexes.size() < 2) continue;
+
+    for (size_t I = 0, E = Indexes.size(); I != E; ++I)
+      Phi->setIncomingValue(Indexes[I], SI->getCondition());
+    Changed = true;
+  }
+
+  return Changed;
+}
+
 bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
   // If this switch is too complex to want to look at, ignore it.
   if (!isValueEqualityComparison(SI))
@@ -2486,6 +2557,9 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
   if (EliminateDeadSwitchCases(SI))
     return SimplifyCFG(BB) | true;
 
+  if (ForwardSwitchConditionToPHI(SI))
+    return SimplifyCFG(BB) | true;
+
   return false;
 }