Infer the range of Math.abs

R=jkummerow@chromium.org, svenpanne@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16268009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14996 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -1308,6 +1308,30 @@ const char* HUnaryMathOperation::OpName() const {
 }
 
 
+Range* HUnaryMathOperation::InferRange(Zone* zone) {
+  Representation r = representation();
+  if (r.IsSmiOrInteger32() && value()->HasRange()) {
+    if (op() == kMathAbs) {
+      int upper = value()->range()->upper();
+      int lower = value()->range()->lower();
+      bool spans_zero = value()->range()->CanBeZero();
+      // Math.abs(kMinInt) overflows its representation, on which the
+      // instruction deopts. Hence clamp it to kMaxInt.
+      int abs_upper = upper == kMinInt ? kMaxInt : abs(upper);
+      int abs_lower = lower == kMinInt ? kMaxInt : abs(lower);
+      Range* result =
+          new(zone) Range(spans_zero ? 0 : Min(abs_lower, abs_upper),
+                          Max(abs_lower, abs_upper));
+      // In case of Smi representation, clamp Math.abs(Smi::kMinValue) to
+      // Smi::kMaxValue.
+      if (r.IsSmi()) result->ClampToSmi();
+      return result;
+    }
+  }
+  return HValue::InferRange(zone);
+}
+
+
 void HUnaryMathOperation::PrintDataTo(StringStream* stream) {
   const char* name = OpName();
   stream->Add("%s ", name);

src/hydrogen-instructions.h

@@ -271,6 +271,10 @@ class Range: public ZoneObject {
   bool IsInSmiRange() const {
     return lower_ >= Smi::kMinValue && upper_ <= Smi::kMaxValue;
   }
+  void ClampToSmi() {
+    lower_ = Max(lower_, Smi::kMinValue);
+    upper_ = Min(upper_, Smi::kMaxValue);
+  }
   void KeepOrder();
 #ifdef DEBUG
   void Verify() const;
@@ -2644,6 +2648,8 @@ class HUnaryMathOperation: public HTemplateInstruction<2> {
     }
   }
 
+  virtual Range* InferRange(Zone* zone);
+
   virtual HValue* Canonicalize();
 
   BuiltinFunctionId op() const { return op_; }