[semantic-arc-opts] Change semantic arc opts to use a SmallBlotSetVector instead of a SmallSetVector.

Blotting is a trick with Map/Set vectors that involves one assigning None to the
vector at the index mapped to an entry instead of deleting the entry itself from
the vector. This ensures that we do not need to move elements of the vector down
when erasing elements from the worklist.

Author: gottesmm

include/swift/Basic/BlotSetVector.h

@@ -125,6 +125,15 @@ class BlotSetVector {
     return true;
   }
 
+  /// Return the last element of the blot map vector. Will be None if blotted.
+  Optional<ValueT> pop_back_val() {
+    auto result = Vector.pop_back_val();
+    if (!result)
+      return result;
+    Map.erase(*result);
+    return result;
+  }
+
   /// Attempt to lookup the index of \p V. Returns None upon failure and the
   /// value on success.
   Optional<unsigned> getIndex(const ValueT &V) {

lib/SILOptimizer/Mandatory/SemanticARCOpts.cpp

@@ -11,19 +11,20 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "sil-semantic-arc-opts"
+#include "swift/Basic/BlotSetVector.h"
 #include "swift/Basic/STLExtras.h"
 #include "swift/SIL/BasicBlockUtils.h"
 #include "swift/SIL/BranchPropagatedUser.h"
 #include "swift/SIL/MemAccessUtils.h"
 #include "swift/SIL/OwnershipUtils.h"
-#include "swift/SILOptimizer/Utils/Local.h"
 #include "swift/SIL/SILArgument.h"
 #include "swift/SIL/SILBuilder.h"
 #include "swift/SIL/SILInstruction.h"
 #include "swift/SIL/SILVisitor.h"
 #include "swift/SILOptimizer/Analysis/PostOrderAnalysis.h"
 #include "swift/SILOptimizer/PassManager/Passes.h"
 #include "swift/SILOptimizer/PassManager/Transforms.h"
+#include "swift/SILOptimizer/Utils/Local.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
@@ -138,7 +139,7 @@ namespace {
 /// the worklist before we delete them.
 struct SemanticARCOptVisitor
     : SILInstructionVisitor<SemanticARCOptVisitor, bool> {
-  SmallSetVector<SILValue, 32> worklist;
+  SmallBlotSetVector<SILValue, 32> worklist;
   SILFunction &F;
   Optional<DeadEndBlocks> TheDeadEndBlocks;
 
@@ -175,7 +176,7 @@ struct SemanticARCOptVisitor
     // Remove all SILValues of the instruction from the worklist and then erase
     // the instruction.
     for (SILValue result : i->getResults()) {
-      worklist.remove(result);
+      worklist.erase(result);
     }
     i->eraseFromParent();
   }
@@ -205,7 +206,11 @@ bool SemanticARCOptVisitor::processWorklist() {
   bool madeChange = false;
 
   while (!worklist.empty()) {
-    SILValue next = worklist.pop_back_val();
+    // Pop the last element off the list. If we were returned None, we blotted
+    // this element, so skip it.
+    SILValue next = worklist.pop_back_val().getValueOr(SILValue());
+    if (!next)
+      continue;
 
     // First check if this is an instruction that is trivially dead. This can
     // occur if we eliminate rr traffic resulting in dead projections and the
@@ -224,7 +229,7 @@ bool SemanticARCOptVisitor::processWorklist() {
                 worklist.insert(operand);
               }
               for (SILValue result : i->getResults()) {
-                worklist.remove(result);
+                worklist.erase(result);
               }
               ++NumEliminatedInsts;
             });