SILCombine: replace Array.capacity with 0 if it's the empty array singleton

We already do that for Array.count. Now add Array.capacity.

Author: eeckstein

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -804,6 +804,18 @@ SILInstruction *SILCombiner::optimizeLoadFromStringLiteral(LoadInst *LI) {
   return Builder.createIntegerLiteral(LI->getLoc(), LI->getType(), str[index]);
 }
 
+static bool isShiftRightByAtLeastOne(SILInstruction *inst) {
+  auto *bi = dyn_cast<BuiltinInst>(inst);
+  if (!bi)
+    return false;
+  if (bi->getBuiltinInfo().ID != BuiltinValueKind::LShr)
+    return false;
+  auto *shiftVal = dyn_cast<IntegerLiteralInst>(bi->getArguments()[1]);
+  if (!shiftVal)
+    return false;
+  return shiftVal->getValue().isStrictlyPositive();
+}
+
 /// Returns true if \p LI loads a zero integer from the empty Array, Dictionary
 /// or Set singleton.
 static bool isZeroLoadFromEmptyCollection(SingleValueInstruction *LI) {
@@ -826,15 +838,23 @@ static bool isZeroLoadFromEmptyCollection(SingleValueInstruction *LI) {
       }
       case ValueKind::StructElementAddrInst: {
         auto *SEA = cast<StructElementAddrInst>(addr);
-        // For Array, we only support "count". The value of "capacityAndFlags"
-        // is not defined in the ABI and could change in another version of the
-        // runtime (the capacity must be 0, but the flags may be not 0).
-        if (SEA->getStructDecl()->getName().is("_SwiftArrayBodyStorage") &&
-            !SEA->getField()->getName().is("count")) {
-          return false;
-        }
         addr = SEA->getOperand();
-        break;
+        if (!SEA->getStructDecl()->getName().is("_SwiftArrayBodyStorage"))
+          break;
+        if (SEA->getField()->getName().is("count"))
+          break;
+        // For Array, the value of `capacityAndFlags` has only a zero capacity
+        // but not necessarily a zero flag (in fact, the flag is 1).
+        // Therefore only replace `capacityAndFlags` with zero if the flag is
+        // masked out by a right-shift of 1.
+        if (SEA->getField()->getName().is("_capacityAndFlags")) {
+          for (Operand *loadUse : LI->getUses()) {
+            if (!isShiftRightByAtLeastOne(loadUse->getUser()))
+              return false;
+          }
+          break;
+        }
+        return false;
       }
       case ValueKind::RefElementAddrInst: {
         auto *REA = cast<RefElementAddrInst>(addr);
@@ -858,6 +878,13 @@ static bool isZeroLoadFromEmptyCollection(SingleValueInstruction *LI) {
       case ValueKind::EndCOWMutationInst:
         addr = cast<SingleValueInstruction>(addr)->getOperand(0);
         break;
+      case ValueKind::MultipleValueInstructionResult:
+        if (auto *bci = dyn_cast<BeginCOWMutationInst>(
+                                              addr->getDefiningInstruction())) {
+          addr = bci->getOperand();
+          break;
+        }
+        return false;
       default:
         return false;
     }

test/SILOptimizer/empty_collection_count.swift

@@ -13,7 +13,7 @@
 // CHECK: return [[I]]
 public func testArray() -> Int {
   let d = Array<Int>()
-  return d.count
+  return d.count + d.capacity
 }
 
 // CHECK-LABEL: sil @{{.*}}testDictionary