Add spill slot locations.

runtime/vm/locations.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_LOCATIONS_H_
 #define VM_LOCATIONS_H_
 
 #include "vm/allocation.h"
 #include "vm/assembler.h"
 #include "vm/bitfield.h"
 
 namespace dart {
 
 class BufferFormatter;
 
 // Location objects are used to connect register allocator and code generator.
 // Instruction templates used by code generator have a corresponding
 // LocationSummary object which specifies expected location for every input
 // and output.
-// Each location is encoded as a single word: low 2 bits denote location kind,
+// Each location is encoded as a single word: low 3 bits denote location kind,

[Kevin Millikin (Google), 2012/07/23 12:52:30]

Not entirely true: tag is variable sized (alternatively, both 001 and 101 are
used as tags for constants and the payload steals the high bit of the tag).

[Vyacheslav Egorov (Google), 2012/07/23 14:06:10]

Rephrased to make it more clear.

 // rest is kind specific location payload e.g. for REGISTER kind payload is
 // register code (value of the Register enumeration).
 class Location : public ValueObject {
  public:
   // Constant payload can overlap with kind field so Kind values
   // have to be chosen in a way that their last 2 bits are never
   // the same as kConstant.
   enum Kind {
+    // This location is invalid.  Payload must be zero.
     kInvalid = 0,
 
+    // Constant value. This location contains a tagged Object handle.
     kConstant = 1,
 
     // Unallocated location represents a location that is not fixed and can be
     // allocated by a register allocator.  Each unallocated location has
-    // a policy that specifies what kind of location is suitable.
+    // a policy that specifies what kind of location is suitable. Payload
+    // contains register allocation policy.
     kUnallocated = 2,
 
-    // Register location represents a fixed register.
-    kRegister = 3
+    // Register location represents a fixed register.  Payload contains
+    // register code.
+    kRegister = 3,
+
+    // Spill slot allocated by the register allocator.  Payload contains
+    // a spill index.
+    kSpillSlot = 4
   };
 
   static const uword kInvalidLocation = 0;
   static const uword kConstantMask = 0x3;
 
   Location() : value_(kInvalidLocation) {
     ASSERT(IsInvalid());
   }
 
   bool IsInvalid() const {
     return value_ == kInvalidLocation;
   }
 
   // Constants.
   bool IsConstant() const {
     ASSERT((kConstant & kConstantMask) == kConstant);

[Kevin Millikin (Google), 2012/07/23 12:52:30]

I wonder why we don't have STATIC_ASSERT.  Here we really would want something
like:

STATIC_ASSERT((kConstant & kConstantMask) == kConstant);
ASSERT((kind() == kConstant) || ((value_ & kConstantMask) != kConstant));

[Vyacheslav Egorov (Google), 2012/07/23 14:06:10]

(kind() == kConstant) <- this is incorrect. 

     return (value_ & kConstantMask) == kConstant;
   }
 
   static Location Constant(const Object& obj) {
     Location loc(reinterpret_cast<uword>(&obj) | kConstant);
     ASSERT(&obj == &loc.constant());
     return loc;
   }
 
   const Object& constant() const {
@@ skipping 43 matching lines @@
 
   bool IsRegister() const {
     return kind() == kRegister;
   }
 
   Register reg() const {
     ASSERT(IsRegister());
     return static_cast<Register>(payload());
   }
 
+  // Spill slots.
+  static Location SpillSlot(intptr_t spill_index) {
+    return Location(kSpillSlot, static_cast<uword>(spill_index));
+  }
+
+  bool IsSpillSlot() const {
+    return kind() == kSpillSlot;
+  }
+
+  intptr_t spill_index() const {
+    ASSERT(IsSpillSlot());
+    return static_cast<uword>(payload());

[Kevin Millikin (Google), 2012/07/23 12:52:30]

payload() is a uword already.

[Vyacheslav Egorov (Google), 2012/07/23 14:06:10]

I guess I wanted cast to intptr_t

+  }
+
   const char* Name() const;
+  void PrintTo(BufferFormatter* f) const;
 
   // Compare two non-constant locations.
   bool Equals(Location other) const {
     ASSERT(!IsConstant() && !other.IsConstant());
     return value_ == other.value_;
   }
 
  private:
   explicit Location(uword value) : value_(value) { }
 
   Location(Kind kind, uword payload)
       : value_(KindField::encode(kind) | PayloadField::encode(payload)) { }
 
   uword payload() const {
     return PayloadField::decode(value_);
   }
 
   // If current location is constant might return something that
   // is not equal to any Kind.
   Kind kind() const {
     return KindField::decode(value_);
   }
 
-  typedef BitField<Kind, 0, 2> KindField;
-  typedef BitField<uword, 2, kWordSize * kBitsPerByte - 2> PayloadField;
+  typedef BitField<Kind, 0, 3> KindField;
+  typedef BitField<uword, 3, kWordSize * kBitsPerByte - 2> PayloadField;
 
   // Layout for kUnallocated locations payload.
   typedef BitField<Policy, 0, 1> PolicyField;
 
   // Location either contains kind and payload fields or a tagged handle for
   // a constant locations. Values of enumeration Kind are selected in such a
   // way that none of them can be interpreted as a kConstant tag.
   uword value_;
 };
 
@@ skipping 72 matching lines @@
   GrowableArray<Location> temp_locations_;
   Location output_location_;
 
   const bool is_call_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_LOCATIONS_H_