Add spill slot locations.

runtime/vm/locations.h

Index: runtime/vm/locations.h
diff --git a/runtime/vm/locations.h b/runtime/vm/locations.h
index fc65d01968db6c7dbbb1b83204cb44f84dfd85db..564b756dc1571360d90987eef3af0024c45b82a1 100644
--- a/runtime/vm/locations.h
+++ b/runtime/vm/locations.h
@@ -17,7 +17,7 @@ class BufferFormatter;
 // 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 {
@@ -26,17 +26,25 @@ class Location : public ValueObject {
   // 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;
@@ -116,7 +124,22 @@ class Location : public ValueObject {
     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 {
@@ -140,8 +163,8 @@ class Location : public ValueObject {
     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;