Two small improvements on epilogue retain/release matcher.

1. Make sure to abort the data flow as soon as we know we cant find the epilogue retain/release.
2. Ignore retain in the throw block, because we do not use the result or insert retain for it
in the throw block on caller side. This is a bug really, we have a test case for it in the
functionsigopts.sil. It will be tested once this new epilogue retain matcher is wired up.

Author: trentxintong

include/swift/SILOptimizer/Analysis/ARCAnalysis.h

@@ -455,6 +455,16 @@ class EpilogueARCContext {
   /// The exit blocks of the function.
   llvm::SmallPtrSet<SILBasicBlock *, 2> ExitBlocks;
 
+  /// Return true if this is a function exitting block this epilogue ARC
+  /// matcher is interested in. 
+  bool isInterestedFunctionExitingBlock(SILBasicBlock *BB) {
+    if (EpilogueARCKind::Release == Kind)  
+      return BB->getTerminator()->isFunctionExiting();
+
+    return BB->getTerminator()->isFunctionExiting() &&
+           BB->getTerminator()->getTermKind() != TermKind::ThrowInst;
+  }
+
   /// Return true if this is a function exit block.
   bool isExitBlock(SILBasicBlock *BB) {
     return ExitBlocks.count(BB);
@@ -489,7 +499,8 @@ class EpilogueARCContext {
     // Initialize the epilogue arc data flow context.
     initializeDataflow();
     // Converge the data flow.
-    convergeDataflow();
+    if (!convergeDataflow())
+      return false;
     // Lastly, find the epilogue ARC instructions.
     return computeEpilogueARC();
   }
@@ -504,7 +515,7 @@ class EpilogueARCContext {
   void initializeDataflow();
 
   /// Keep iterating until the data flow is converged.
-  void convergeDataflow();
+  bool convergeDataflow();
 
   /// Find the epilogue ARC instructions.
   bool computeEpilogueARC();

lib/SILOptimizer/Analysis/ARCAnalysis.cpp

@@ -1173,7 +1173,7 @@ SILInstruction *swift::findReleaseToMatchUnsafeGuaranteedValue(
 void EpilogueARCContext::initializeDataflow() {
   for (auto &B : *F) {
     // Find the exit blocks.
-    if (B.getTerminator()->isFunctionExiting()) {
+    if (isInterestedFunctionExitingBlock(&B)) {
       ExitBlocks.insert(&B);
     }
     // Allocate the storage.
@@ -1186,11 +1186,13 @@ void EpilogueARCContext::initializeDataflow() {
   llvm::DenseSet<SILValue> Processed;
   ToProcess.push_back(Arg);
   while (!ToProcess.empty()) {
-    SILValue Arg = ToProcess.pop_back_val();
-    if (Processed.find(Arg) != Processed.end())
+    SILValue CArg = ToProcess.pop_back_val();
+    if (!CArg)
+      continue;
+    if (Processed.find(CArg) != Processed.end())
        continue;
-    Processed.insert(Arg);
-    SILArgument *A = dyn_cast<SILArgument>(Arg);
+    Processed.insert(CArg);
+    SILArgument *A = dyn_cast<SILArgument>(CArg);
     if (A && !A->isFunctionArg()) {
       // Find predecessor and break the SILArgument to predecessors.
       for (auto X : A->getParent()->getPreds()) {
@@ -1204,7 +1206,7 @@ void EpilogueARCContext::initializeDataflow() {
   }
 }
 
-void EpilogueARCContext::convergeDataflow() {
+bool EpilogueARCContext::convergeDataflow() {
   // Keep iterating until Changed is false.
   bool Changed = false;
   do {
@@ -1238,9 +1240,10 @@ void EpilogueARCContext::convergeDataflow() {
             break;
           }
           // This is a transition from 1 to 0 due to a blocking instruction.
+          // at this point, its OK to abort the data flow as we have one path
+          // which we did not find an epilogue retain before getting blocked.
           if (mayBlockEpilogueARC(&*I, RCFI->getRCIdentityRoot(Arg))) {
-            BBSetOut = false;
-            break;
+            return false;
           }
         }
       }
@@ -1250,6 +1253,7 @@ void EpilogueARCContext::convergeDataflow() {
       BS->BBSetIn = BBSetOut;
     }
   } while(Changed);
+  return true;
 }
 
 bool EpilogueARCContext::computeEpilogueARC() {