New unified type representation

src/types.h

+// Copyright 2013 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
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+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef V8_TYPES_H_
+#define V8_TYPES_H_
+
+#include "v8.h"
+
+#include "objects.h"
+
+namespace v8 {
+namespace internal {
+
+
+// A simple type system for compiler-internal use. It is based entirely on
+// union types, and all subtyping hence amounts to set inclusion. Besides the
+// obvious primitive types and some predefined unions, the type language also
+// can express class types (a.k.a. specific maps) and singleton types (i.e.,
+// concrete constants).
+//
+// The following equations and inequations hold:
+//
+//   None <= T
+//   T <= Any
+//
+//   Oddball = Boolean \/ Null \/ Undefined
+//   Number = Smi \/ Double

[Jakob Kummerow, 2013/06/05 14:53:24]

As discussed offline, I think we'll need an Integer32 type before we can use
this to replace existing type description systems. But we can always add that
later.

[rossberg, 2013/06/05 15:37:02]

Yes, let's add stuff once we get there.

+//   Name = String \/ Symbol
+//   UniqueName = InternalizedString \/ Symbol
+//   InternalizedString < String
+//
+//   Receiver = Object \/ Proxy
+//   Array < Object
+//   Function < Object
+//
+//   Class(map) < T   iff instance_type(map) < T
+//   Constant(x) < T  iff instance_type(map(x)) < T
+//
+// Note that Constant(x) < Class(map(x)) does _not_ hold, since the x's map can
+// change! (Its instance type cannot, however.)
+// TODO(rossberg): the latter is not currently true for proxies, because of fix,
+// but will hold once we implement direct proxies.
+//
+// There are two main functions for testing types:
+//
+//   T1->Is(T2)     -- tests whether T1 is included in T2 (i.e., T1 <= T2)
+//   T1->Maybe(T2)  -- tests whether T1 and T2 overlap (i.e., T1 /\ T2 =/= 0)
+//
+// Typically, the latter should be used to check whether a specific case needs
+// handling (e.g., via T->Maybe(Number)).
+//
+// There is no functionality to discover whether a type is a leaf in the
+// lattice. That is intentional. It should always be possible to refine the
+// lattice (e.g., splitting up number types further) without invalidating any
+// existing assumptions or tests.
+//
+// Internally, all 'primitive' types, and their unions, are represented as
+// bitsets via smis. Class and Constant are heap pointers to the respective
+// argument. Only unions containing Class'es or Constant's require allocation.
+
+class Type : public Object {
+ public:
+  static Type* None() { return from_bitset(kNone); }
+  static Type* Any() { return from_bitset(kAny); }
+
+  static Type* Oddball() { return from_bitset(kOddball); }
+  static Type* Boolean() { return from_bitset(kBoolean); }
+  static Type* Null() { return from_bitset(kNull); }
+  static Type* Undefined() { return from_bitset(kUndefined); }
+
+  static Type* Number() { return from_bitset(kNumber); }
+  static Type* Smi() { return from_bitset(kSmi); }
+  static Type* Double() { return from_bitset(kDouble); }
+
+  static Type* Name() { return from_bitset(kName); }
+  static Type* UniqueName() { return from_bitset(kUniqueName); }
+  static Type* String() { return from_bitset(kString); }
+  static Type* InternalizedString() { return from_bitset(kInternalizedString); }
+  static Type* Symbol() { return from_bitset(kSymbol); }
+
+  static Type* Receiver() { return from_bitset(kReceiver); }
+  static Type* Object() { return from_bitset(kObject); }
+  static Type* Array() { return from_bitset(kArray); }
+  static Type* Function() { return from_bitset(kFunction); }
+  static Type* Proxy() { return from_bitset(kProxy); }
+
+  static Type* Class(Handle<Map> map) { return from_handle(map); }
+  static Type* Constant(Handle<HeapObject> value) {
+    ASSERT(!value->IsMap() && !value->IsFixedArray());
+    return from_handle(value);
+  }
+
+  static Type* Union(Handle<Type> type1, Handle<Type> type2);
+  static Type* Optional(Handle<Type> type);  // type \/ Undefined
+
+  bool Is(Handle<Type> that);
+  bool Maybe(Handle<Type> that);
+
+  // TODO(rossberg): method to iterate unions?
+
+ private:
+  // A union is a fixed array containing types. Invariants:
+  // - its length is at least 2
+  // - at most one field is a bitset, and it must go into index 0
+  // - no field is a union
+  typedef FixedArray Unioned;
+
+  enum {
+    kNull = 1<<0,

[Jakob Kummerow, 2013/06/05 14:53:24]

nit: any reason not to have spaces around the operator '<<'?

[rossberg, 2013/06/05 15:37:02]

Done.

+    kUndefined = 1<<1,
+    kBoolean = 1<<2,
+    kSmi = 1<<3,
+    kDouble = 1<<4,
+    kSymbol = 1<<5,
+    kInternalizedString = 1<<6,
+    kOtherString = 1<<7,
+    kArray = 1<<8,
+    kFunction = 1<<9,
+    kOtherObject = 1<<10,
+    kProxy = 1<<11,
+
+    kOddball = kBoolean | kNull | kUndefined,
+    kNumber = kSmi | kDouble,
+    kString = kInternalizedString | kOtherString,
+    kUniqueName = kSymbol | kInternalizedString,
+    kName = kSymbol | kString,
+    kObject = kArray | kFunction | kOtherObject,
+    kReceiver = kObject | kProxy,
+    kAny = kOddball | kNumber | kName | kReceiver,
+    kNone = 0
+  };
+
+  bool is_bitset() { return this->IsSmi(); }
+  bool is_class() { return this->IsMap(); }
+  bool is_constant() { return !(is_bitset() || is_class() || is_union()); }
+  bool is_union() { return this->IsFixedArray(); }
+
+  int as_bitset() { return Smi::cast(this)->value(); }
+  Handle<Map> as_class() { return Handle<Map>::cast(handle()); }
+  Handle<HeapObject> as_constant() {
+    ASSERT(is_constant());
+    return Handle<HeapObject>::cast(handle());
+  }
+  Handle<Unioned> as_union() { return Handle<Unioned>::cast(handle()); }
+
+  Handle<Type> handle() { return handle_via(this); }
+  Handle<Type> handle_via(Type* type) {

[Jakob Kummerow, 2013/06/05 14:53:24]

I find it surprising that |type| is only used to retrieve its isolate. I'd
prefer making this more explicit: either change the signature to take an
Isolate* instead of a Type*, or rename to handle_with_isolate_from(), or
something like that.

[rossberg, 2013/06/05 15:37:02]

Done.

+    ASSERT(type->IsHeapObject());
+    return v8::internal::handle(this, HeapObject::cast(type)->GetIsolate());
+  }
+
+  static Type* from_bitset(int bitset) {
+    return static_cast<Type*>(Object::cast(Smi::FromInt(bitset)));
+  }
+  static Type* from_handle(Handle<HeapObject> handle) {
+    return static_cast<Type*>(Object::cast(*handle));
+  }
+
+  static Handle<Type> union_get(Handle<Unioned> unioned, int i) {
+    Type* type = static_cast<Type*>(unioned->get(i));
+    ASSERT(!type->is_union());
+    return type->handle_via(from_handle(unioned));
+  }
+
+  int LubBitset();

[Jakob Kummerow, 2013/06/05 14:53:24]

short comment "Least upper bound"? You can even add that on the same line.

[rossberg, 2013/06/05 15:37:02]

Done.

+  int GlbBitset();
+  bool InUnion(Handle<Unioned> unioned, int current_size);
+  int ExtendUnion(Handle<Unioned> unioned, int current_size);
+};
+
+} }  // namespace v8::internal
+
+#endif  // V8_TYPES_H_