[SILCombiner] Remove strong retains and strong releases of tagged strings. A retain/release of a string literal is a no-op.

Author: Joe Shajrawi

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -686,6 +686,20 @@ SILInstruction *SILCombiner::visitCondFailInst(CondFailInst *CFI) {
   return nullptr;
 }
 
+static bool isValueToBridgeObjectremovable(ValueToBridgeObjectInst *VTBOI) {
+  SILValue operand = VTBOI->getOperand();
+  // If operand is a struct $UInt (% : $Builtin.), fetch the real source
+  if (auto *SI = dyn_cast<StructInst>(operand)) {
+    assert(SI->SILInstruction::getAllOperands().size() == 1 &&
+           "Excted a single operand");
+    operand = SI->getOperand(0);
+  }
+  if (auto *BI = dyn_cast<BuiltinInst>(operand)) {
+    return (BI->getBuiltinInfo().ID == BuiltinValueKind::StringObjectOr);
+  }
+  return false;
+}
+
 SILInstruction *SILCombiner::visitStrongRetainInst(StrongRetainInst *SRI) {
   // Retain of ThinToThickFunction is a no-op.
   SILValue funcOper = SRI->getOperand();
@@ -699,6 +713,15 @@ SILInstruction *SILCombiner::visitStrongRetainInst(StrongRetainInst *SRI) {
       isa<ObjCMetatypeToObjectInst>(SRI->getOperand()))
     return eraseInstFromFunction(*SRI);
 
+  // Retain and Release of tagged strings is a no-op.
+  // The builtin code pattern to find tagged strings is:
+  // builtin "stringObjectOr_Int64" (or to tag the string)
+  // value_to_bridge_object (cast the UInt to bridge object)
+  if (auto *VTBOI = dyn_cast<ValueToBridgeObjectInst>(SRI->getOperand())) {
+    if (isValueToBridgeObjectremovable(VTBOI))
+      return eraseInstFromFunction(*SRI);
+  }
+
   // Sometimes in the stdlib due to hand offs, we will see code like:
   //
   // strong_release %0
@@ -1141,6 +1164,15 @@ SILInstruction *SILCombiner::visitStrongReleaseInst(StrongReleaseInst *SRI) {
       isa<ObjCMetatypeToObjectInst>(SRI->getOperand()))
     return eraseInstFromFunction(*SRI);
 
+  // Retain and Release of tagged strings is a no-op.
+  // The builtin code pattern to find tagged strings is:
+  // builtin "stringObjectOr_Int64" (or to tag the string)
+  // value_to_bridge_object (cast the UInt to bridge object)
+  if (auto *VTBOI = dyn_cast<ValueToBridgeObjectInst>(SRI->getOperand())) {
+    if (isValueToBridgeObjectremovable(VTBOI))
+      return eraseInstFromFunction(*SRI);
+  }
+
   // Release of a classbound existential converted from a class is just a
   // release of the class, squish the conversion.
   if (auto ier = dyn_cast<InitExistentialRefInst>(SRI->getOperand()))