[epilogue-release-matcher] Pass DenseSet by reference instead of by value.

This is particularly egrigious since we are only /reading/ from the DenseSet. So
we are basically mallocing/copying a DenseSet just to read from it... I don't
think I need to say more.

rdar://41146023

Author: gottesmm

include/swift/SILOptimizer/Analysis/ARCAnalysis.h

@@ -13,16 +13,17 @@
 #ifndef SWIFT_SILOPTIMIZER_ANALYSIS_ARCANALYSIS_H
 #define SWIFT_SILOPTIMIZER_ANALYSIS_ARCANALYSIS_H
 
+#include "swift/Basic/LLVM.h"
 #include "swift/SIL/SILArgument.h"
-#include "swift/SIL/SILValue.h"
 #include "swift/SIL/SILBasicBlock.h"
+#include "swift/SIL/SILValue.h"
 #include "swift/SILOptimizer/Analysis/AliasAnalysis.h"
 #include "swift/SILOptimizer/Analysis/PostOrderAnalysis.h"
 #include "swift/SILOptimizer/Analysis/RCIdentityAnalysis.h"
 #include "llvm/ADT/BitVector.h"
-#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/TinyPtrVector.h"
 
 namespace swift {
@@ -181,7 +182,8 @@ class ConsumedResultToEpilogueRetainMatcher {
 private:
   /// Return true if all the successors of the EpilogueRetainInsts do not have
   /// a retain.
-  bool isTransitiveSuccessorsRetainFree(llvm::DenseSet<SILBasicBlock *> BBs);
+  bool
+  isTransitiveSuccessorsRetainFree(const llvm::DenseSet<SILBasicBlock *> &BBs);
 
   /// Finds matching releases in the provided block \p BB.
   RetainKindValue findMatchingRetainsInBasicBlock(SILBasicBlock *BB,

lib/SILOptimizer/Analysis/ARCAnalysis.cpp

@@ -498,22 +498,24 @@ void ConsumedResultToEpilogueRetainMatcher::recompute() {
   findMatchingRetains(&*BB);
 }
 
-bool
-ConsumedResultToEpilogueRetainMatcher::
-isTransitiveSuccessorsRetainFree(llvm::DenseSet<SILBasicBlock *> BBs) {
+bool ConsumedResultToEpilogueRetainMatcher::isTransitiveSuccessorsRetainFree(
+    const llvm::DenseSet<SILBasicBlock *> &BBs) {
   // For every block with retain, we need to check the transitive
   // closure of its successors are retain-free.
   for (auto &I : EpilogueRetainInsts) {
-    auto *CBB = I->getParent();
-    for (auto &Succ : CBB->getSuccessors()) {
-      if (BBs.find(Succ) != BBs.end())
+    for (auto &Succ : I->getParent()->getSuccessors()) {
+      if (BBs.count(Succ))
         continue;
       return false;
     }
   }
+
+  // FIXME: We are iterating over a DenseSet. That can lead to non-determinism
+  // and is in general pretty inefficient since we are iterating over a hash
+  // table.
   for (auto CBB : BBs) {
     for (auto &Succ : CBB->getSuccessors()) {
-      if (BBs.find(Succ) != BBs.end())
+      if (BBs.count(Succ))
         continue;
       return false;
     }