[IRGen][runtime] Reduce object header to 8 bytes on 32-bit platforms. (swiftlang#12790)

SR-4353, rdar://29765800

Author: Greg Parker

include/swift/Reflection/ReflectionContext.h

@@ -66,7 +66,7 @@ class ReflectionContext
 
   unsigned getSizeOfHeapObject() {
     // This must match sizeof(HeapObject) for the target.
-    return sizeof(StoredPointer) + 8;
+    return sizeof(StoredPointer) * 2;
   }
 
   void dumpAllSections(std::ostream &OS) {
@@ -483,9 +483,8 @@ class ReflectionContext
     case MetadataSourceKind::ClosureBinding: {
       unsigned Index = cast<ClosureBindingMetadataSource>(MS)->getIndex();
 
-      // Skip the context's isa pointer (4 or 8 bytes) and reference counts
-      // (4 bytes each regardless of platform word size). This is just
-      // sizeof(HeapObject) in the target.
+      // Skip the context's HeapObject header
+      // (one word each for isa pointer and reference counts).
       //
       // Metadata and conformance tables are stored consecutively after
       // the heap object header, in the 'necessary bindings' area.

lib/IRGen/GenDecl.cpp

@@ -3138,21 +3138,19 @@ IRGenModule::getAddrOfGlobalConstantString(StringRef utf8) {
   metaclass = llvm::ConstantExpr::getBitCast(metaclass, TypeMetadataPtrTy);
 
   // Get a reference count of two.
-  auto *strongRefCountInit = llvm::ConstantInt::get(
-      Int32Ty,
-      InlineRefCountBits(0 /*unowned ref count*/, 2 /*strong ref count*/)
+  auto *refCountInit = llvm::ConstantInt::get(
+      IntPtrTy,
+      InlineRefCountBits(1 /* "extra" strong ref count*/, 1 /* unowned count */)
           .getBitsValue());
-  auto *unownedRefCountInit = llvm::ConstantInt::get(Int32Ty, 0);
 
   auto *count = llvm::ConstantInt::get(Int32Ty, utf8.size());
   // Capacity is length plus one because of the implicitly added '\0'
   // character.
   auto *capacity = llvm::ConstantInt::get(Int32Ty, utf8.size() + 1);
   auto *flags = llvm::ConstantInt::get(Int8Ty, 0);
 
-  // FIXME: Big endian-ness.
   llvm::Constant *heapObjectHeaderFields[] = {
-      metaclass, strongRefCountInit, unownedRefCountInit
+      metaclass, refCountInit
   };
 
   auto *initRefCountStruct = llvm::ConstantStruct::get(
@@ -3216,19 +3214,18 @@ IRGenModule::getAddrOfGlobalUTF16ConstantString(StringRef utf8) {
   metaclass = llvm::ConstantExpr::getBitCast(metaclass, TypeMetadataPtrTy);
 
   // Get a reference count of two.
-  auto *strongRefCountInit = llvm::ConstantInt::get(
-      Int32Ty,
-      InlineRefCountBits(0 /*unowned ref count*/, 2 /*strong ref count*/)
+  auto *refCountInit = llvm::ConstantInt::get(
+      IntPtrTy,
+      InlineRefCountBits(1 /* "extra" strong ref count*/, 1 /* unowned count */)
           .getBitsValue());
-  auto *unownedRefCountInit = llvm::ConstantInt::get(Int32Ty, 0);
 
   auto *count = llvm::ConstantInt::get(Int32Ty, utf16Length);
   auto *capacity = llvm::ConstantInt::get(Int32Ty, utf16Length + 1);
   auto *flags = llvm::ConstantInt::get(Int8Ty, 0);
   auto *padding = llvm::ConstantInt::get(Int8Ty, 0);
 
   llvm::Constant *heapObjectHeaderFields[] = {
-      metaclass, strongRefCountInit, unownedRefCountInit
+      metaclass, refCountInit
   };
 
   auto *initRefCountStruct = llvm::ConstantStruct::get(

lib/IRGen/GenExistential.cpp

@@ -2128,7 +2128,7 @@ static llvm::Constant *getDeallocateBoxedOpaqueExistentialBufferFunction(
 
         //  Size = ((sizeof(HeapObject) + align) & ~align) + size
         auto *heapHeaderSize = llvm::ConstantInt::get(
-            IGF.IGM.SizeTy, getHeapHeaderSize(IGM).getValue());
+            IGF.IGM.SizeTy, IGM.RefCountedStructSize.getValue());
         size = Builder.CreateAdd(
             Builder.CreateAnd(Builder.CreateAdd(heapHeaderSize, alignmentMask),
                               Builder.CreateNot(alignmentMask)),
@@ -2222,7 +2222,7 @@ getProjectBoxedOpaqueExistentialFunction(IRGenFunction &IGF,
 
           //  StartOffset = ((sizeof(HeapObject) + align) & ~align)
           auto *heapHeaderSize = llvm::ConstantInt::get(
-              IGF.IGM.SizeTy, getHeapHeaderSize(IGM).getValue());
+              IGF.IGM.SizeTy, IGM.RefCountedStructSize.getValue());
           auto *startOffset = Builder.CreateAnd(
               Builder.CreateAdd(heapHeaderSize, alignmentMask),
               Builder.CreateNot(alignmentMask));

lib/IRGen/GenKeyPath.cpp

@@ -700,11 +700,6 @@ IRGenModule::getAddrOfKeyPathPattern(KeyPathPattern *pattern,
   fields.add(emitMetadataGenerator(rootTy));
   fields.add(emitMetadataGenerator(valueTy));
 
-  // TODO: 32-bit heap object header still has an extra word
-  if (SizeTy == Int32Ty) {
-    fields.addInt32(0);
-  }
-  
 #ifndef NDEBUG
   auto endOfObjectHeader = fields.getNextOffsetFromGlobal();
   unsigned expectedObjectHeaderSize;

lib/IRGen/GenValueWitness.cpp

@@ -242,8 +242,8 @@ static Address emitDefaultProjectBuffer(IRGenFunction &IGF, Address buffer,
         buffer.getAlignment());
     auto *boxStart = IGF.Builder.CreateLoad(boxAddress);
     auto *alignmentMask = type.getAlignmentMask(IGF, T);
-    auto *heapHeaderSize =
-        llvm::ConstantInt::get(IGM.SizeTy, getHeapHeaderSize(IGM).getValue());
+    auto *heapHeaderSize = llvm::ConstantInt::get(
+        IGM.SizeTy, IGM.RefCountedStructSize.getValue());
     auto *startOffset =
         Builder.CreateAnd(Builder.CreateAdd(heapHeaderSize, alignmentMask),
                           Builder.CreateNot(alignmentMask));
@@ -820,7 +820,7 @@ getCopyOutOfLineBoxPointerFunction(IRGenModule &IGM,
         IGF.Builder.CreateStore(ptr, dest);
         auto *alignmentMask = fixedTI.getStaticAlignmentMask(IGM);
         auto *heapHeaderSize = llvm::ConstantInt::get(
-            IGM.SizeTy, getHeapHeaderSize(IGM).getValue());
+            IGM.SizeTy, IGM.RefCountedStructSize.getValue());
         auto *startOffset = IGF.Builder.CreateAnd(
             IGF.Builder.CreateAdd(heapHeaderSize, alignmentMask),
             IGF.Builder.CreateNot(alignmentMask));

lib/IRGen/IRGenModule.cpp

@@ -264,8 +264,11 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   });
   FullBoxMetadataPtrTy = FullBoxMetadataStructTy->getPointerTo(DefaultAS);
 
-  llvm::Type *refCountedElts[] = {TypeMetadataPtrTy, Int32Ty, Int32Ty};
+  // This must match struct HeapObject in the runtime.
+  llvm::Type *refCountedElts[] = {TypeMetadataPtrTy, IntPtrTy};
   RefCountedStructTy->setBody(refCountedElts);
+  RefCountedStructSize =
+    Size(DataLayout.getStructLayout(RefCountedStructTy)->getSizeInBytes());
 
   PtrSize = Size(DataLayout.getPointerSize(DefaultAS));
 

lib/IRGen/IRGenModule.h

@@ -446,6 +446,7 @@ class IRGenModule {
     llvm::PointerType *WitnessTablePtrTy;
   };
   llvm::StructType *RefCountedStructTy;/// %swift.refcounted = type { ... }
+  Size RefCountedStructSize;     /// sizeof(%swift.refcounted)
   llvm::PointerType *RefCountedPtrTy;  /// %swift.refcounted*
   llvm::PointerType *WeakReferencePtrTy;/// %swift.weak_reference*
   llvm::PointerType *UnownedReferencePtrTy;/// %swift.unowned_reference*

lib/IRGen/StructLayout.cpp

@@ -38,11 +38,6 @@ static bool requiresHeapHeader(LayoutKind kind) {
   llvm_unreachable("bad layout kind!");
 }
 
-/// Return the size of the standard heap header.
-Size irgen::getHeapHeaderSize(IRGenModule &IGM) {
-  return IGM.getPointerSize() + Size(8);
-}
-
 /// Perform structure layout on the given types.
 StructLayout::StructLayout(IRGenModule &IGM, CanType astTy,
                            LayoutKind layoutKind,
@@ -184,7 +179,7 @@ Address ElementLayout::project(IRGenFunction &IGF, Address baseAddr,
 
 void StructLayoutBuilder::addHeapHeader() {
   assert(StructFields.empty() && "adding heap header at a non-zero offset");
-  CurSize = getHeapHeaderSize(IGM);
+  CurSize = IGM.RefCountedStructSize;
   CurAlignment = IGM.getPointerAlignment();
   StructFields.push_back(IGM.RefCountedStructTy);
 }

lib/IRGen/StructLayout.h

@@ -395,8 +395,6 @@ class StructLayout {
                      const llvm::Twine &name = "") const;
 };
 
-Size getHeapHeaderSize(IRGenModule &IGM);
-
 /// Different policies for accessing a physical field.
 enum class FieldAccess : uint8_t {
   /// Instance variable offsets are constant.

stdlib/public/SwiftShims/HeapObject.h

@@ -16,8 +16,7 @@
 #include "System.h"
 
 #define SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_64 16
-// TODO: Should be 8
-#define SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_32 12
+#define SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_32 8
 
 #ifdef __cplusplus
 #include <type_traits>
@@ -39,12 +38,12 @@ typedef struct HeapMetadata HeapMetadata;
   InlineRefCounts refCounts
 
 /// The Swift heap-object header.
+/// This must match RefCountedStructTy in IRGen.
 struct HeapObject {
   /// This is always a valid pointer to a metadata object.
   HeapMetadata const *metadata;
 
   SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS;
-  // FIXME: allocate two words of metadata on 32-bit platforms
 
 #ifdef __cplusplus
   HeapObject() = default;
@@ -75,15 +74,8 @@ static_assert(swift::IsTriviallyConstructible<HeapObject>::value,
 static_assert(std::is_trivially_destructible<HeapObject>::value,
               "HeapObject must be trivially destructible");
 
-// FIXME: small header for 32-bit
-//static_assert(sizeof(HeapObject) == 2*sizeof(void*),
-//              "HeapObject must be two pointers long");
-//
-static_assert(sizeof(HeapObject) ==
-  (sizeof(void*) == 8 ? SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_64 :
-   sizeof(void*) == 4 ? SWIFT_ABI_HEAP_OBJECT_HEADER_SIZE_32 :
-   0 && "unexpected pointer size"),
-  "HeapObject must match ABI heap object header size");
+static_assert(sizeof(HeapObject) == 2*sizeof(void*),
+              "HeapObject must be two pointers long");
 
 static_assert(alignof(HeapObject) == alignof(void*),
               "HeapObject must be pointer-aligned");