SIL optimizer: propagate count and capacity from empty Array/Set/Dictionary singletons.

Constant-propagate the 0 value when loading "count" or "capacity" from the empty Array, Set or Dictionary storage.
On high-level SIL this optimization is also done by the ArrayCountPropagation pass, but only for Array.
And even for Array it's sometimes needed to propagate the empty-array count when high-level semantics function are already inlined.

Fixes an optimization deficiency for empty OptionSet literals.

https://bugs.swift.org/browse/SR-12046
rdar://problem/58861171

Author: eeckstein

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -594,6 +594,62 @@ SILInstruction *SILCombiner::optimizeLoadFromStringLiteral(LoadInst *LI) {
   return Builder.createIntegerLiteral(LI->getLoc(), LI->getType(), str[index]);
 }
 
+/// Returns true if \p LI loads a zero integer from the empty Array, Dictionary
+/// or Set singleton.
+static bool isZeroLoadFromEmptyCollection(LoadInst *LI) {
+  auto intTy = LI->getType().getAs<BuiltinIntegerType>();
+  if (!intTy)
+    return false;
+  
+  SILValue addr = LI->getOperand();
+
+  // Find the root object of the load-address.
+  for (;;) {
+    switch (addr->getKind()) {
+      case ValueKind::GlobalAddrInst: {
+        StringRef gName =
+          cast<GlobalAddrInst>(addr)->getReferencedGlobal()->getName();
+        return gName == "_swiftEmptyArrayStorage" ||
+               gName == "_swiftEmptyDictionarySingleton" ||
+               gName == "_swiftEmptySetSingleton";
+      }
+      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;
+      }
+      case ValueKind::RefElementAddrInst: {
+        auto *REA = cast<RefElementAddrInst>(addr);
+        Identifier className = REA->getClassDecl()->getName();
+        // For Dictionary and Set we support "count" and "capacity".
+        if (className.is("__RawDictionaryStorage") ||
+            className.is("__RawSetStorage")) {
+          Identifier fieldName = REA->getField()->getName();
+          if (!fieldName.is("_count") && !fieldName.is("_capacity"))
+            return false;
+        }
+        addr = REA->getOperand();
+        break;
+      }
+      case ValueKind::UncheckedRefCastInst:
+      case ValueKind::UpcastInst:
+      case ValueKind::RawPointerToRefInst:
+      case ValueKind::AddressToPointerInst:
+        addr = cast<SingleValueInstruction>(addr)->getOperand(0);
+        break;
+      default:
+        return false;
+    }
+  }
+}
+
 SILInstruction *SILCombiner::visitLoadInst(LoadInst *LI) {
   // (load (upcast-ptr %x)) -> (upcast-ref (load %x))
   Builder.setCurrentDebugScope(LI->getDebugScope());
@@ -606,6 +662,17 @@ SILInstruction *SILCombiner::visitLoadInst(LoadInst *LI) {
   if (SILInstruction *I = optimizeLoadFromStringLiteral(LI))
     return I;
 
+  // Constant-propagate the 0 value when loading "count" or "capacity" from the
+  // empty Array, Set or Dictionary storage.
+  // On high-level SIL this optimization is also done by the
+  // ArrayCountPropagation pass, but only for Array. And even for Array it's
+  // sometimes needed to propagate the empty-array count when high-level
+  // semantics function are already inlined.
+  // Note that for non-empty arrays/sets/dictionaries, the count can be
+  // propagated by redundant load elimination.
+  if (isZeroLoadFromEmptyCollection(LI))
+    return Builder.createIntegerLiteral(LI->getLoc(), LI->getType(), 0);
+
   return nullptr;
 }
 

test/SILOptimizer/empty_collection_count.swift

@@ -0,0 +1,38 @@
+// RUN: %target-swift-frontend -emit-sil -O %s | %FileCheck %s
+// RUN: %target-swift-frontend -emit-sil -Osize %s | %FileCheck %s
+
+// REQUIRES: swift_stdlib_no_asserts,optimized_stdlib
+
+// This is an end-to-end test if the count and/or capacity from empty
+// array/set/dictionary singletons can be propagated.
+
+// CHECK-LABEL: sil @{{.*}}testArray
+// CHECK-NOT: global_addr
+// CHECK: [[Z:%[0-9]+]] = integer_literal $Builtin.Int{{[0-9]*}}, 0 
+// CHECK: [[I:%[0-9]+]] = struct $Int ([[Z]] : $Builtin.Int{{[0-9]*}})
+// CHECK: return [[I]]
+public func testArray() -> Int {
+  let d = Array<Int>()
+  return d.count
+}
+
+// CHECK-LABEL: sil @{{.*}}testDictionary
+// CHECK-NOT: global_addr
+// CHECK: [[Z:%[0-9]+]] = integer_literal $Builtin.Int{{[0-9]*}}, 0 
+// CHECK: [[I:%[0-9]+]] = struct $Int ([[Z]] : $Builtin.Int{{[0-9]*}})
+// CHECK: return [[I]]
+public func testDictionary() -> Int {
+  let d = Dictionary<Int, Int>()
+  return d.count + d.capacity
+}
+
+// CHECK-LABEL: sil @{{.*}}testSet
+// CHECK-NOT: global_addr
+// CHECK: [[Z:%[0-9]+]] = integer_literal $Builtin.Int{{[0-9]*}}, 0 
+// CHECK: [[I:%[0-9]+]] = struct $Int ([[Z]] : $Builtin.Int{{[0-9]*}})
+// CHECK: return [[I]]
+public func testSet() -> Int {
+  let d = Set<Int>()
+  return d.count + d.capacity
+}
+

test/SILOptimizer/optionset.swift

@@ -22,6 +22,16 @@ public func returnTestOptions() -> TestOptions {
     return [.first, .second, .third, .fourth]
 }
 
+// CHECK:      sil @{{.*}}returnEmptyTestOptions{{.*}}
+// CHECK-NEXT: bb0:
+// CHECK-NEXT:   integer_literal {{.*}}, 0
+// CHECK-NEXT:   struct $Int
+// CHECK-NEXT:   struct $TestOptions
+// CHECK-NEXT:   return
+public func returnEmptyTestOptions() -> TestOptions {
+    return []
+}
+
 // CHECK:        alloc_global @{{.*}}globalTestOptions{{.*}}
 // CHECK-NEXT:   global_addr
 // CHECK-NEXT:   integer_literal {{.*}}, 15