Support conversion of clamped double values for pixel arrays in Crankshaft.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 03ff00751eb37929637ed757c18c10dae3fd136c..013f874b983349a208846af9b7aec47fa670a577 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -87,6 +87,7 @@ class LChunkBuilder;
   V(CheckNonSmi)                               \
   V(CheckPrototypeMaps)                        \
   V(CheckSmi)                                  \
+  V(ClampToUint8)                              \
   V(ClassOfTest)                               \
   V(Compare)                                   \
   V(CompareJSObjectEq)                         \
@@ -551,7 +552,6 @@ class HValue: public ZoneObject {
   Representation representation() const { return representation_; }
   void ChangeRepresentation(Representation r) {
     // Representation was already set and is allowed to be changed.
-    ASSERT(!representation_.IsNone());
     ASSERT(!r.IsNone());
     ASSERT(CheckFlag(kFlexibleRepresentation));
     RepresentationChanged(r);
@@ -1053,6 +1053,45 @@ class HChange: public HUnaryOperation {
 };
 
 
+class HClampToUint8: public HUnaryOperation {
+ public:
+  explicit HClampToUint8(HValue* value)
+      : HUnaryOperation(value),
+        input_rep_(Representation::None()) {
+    SetFlag(kFlexibleRepresentation);

[Kevin Millikin (Chromium), 2011/05/16 07:06:35]

Everywhere else we have kFlexibleRepresentation, we also have
set_representation(Representation::Tagged()) to establish a default
representation.  I don't know that the representation inference algorithm
depends on it, but maybe it should be here for safety should somebody later
assume it.

[danno, 2011/05/16 14:13:43]

Done.

+    SetFlag(kUseGVN);
+  }
+
+  virtual Representation RequiredInputRepresentation(int index) const {
+    return input_rep_;
+  }
+
+  virtual Representation InferredRepresentation() {
+    // TODO(danno): Inference on input types should happen separately from
+    // return representation.
+    Representation new_rep = value()->representation();
+    if (input_rep_.IsNone()) {
+      if (!new_rep.IsNone()) {
+        input_rep_ = new_rep;
+        return Representation::Integer32();
+      } else {
+        return Representation::None();
+      }
+    } else {
+      return Representation::Integer32();
+    }
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(ClampToUint8)
+
+ protected:
+  virtual bool DataEquals(HValue* other) { return true; }
+
+ private:
+  Representation input_rep_;
+};
+
+
 class HSimulate: public HInstruction {
  public:
   HSimulate(int ast_id, int pop_count)