Infer the range of Math.abs

src/hydrogen-instructions.cc

 // Copyright 2012 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
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 //     * Redistributions in binary form must reproduce the above
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@@ skipping 1290 matching lines @@
     case kMathExp: return "exp";
     case kMathSqrt: return "sqrt";
     case kMathPowHalf: return "pow-half";
     default:
       UNREACHABLE();
       return NULL;
   }
 }
 
 
+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_lower = lower == kMinInt ? kMaxInt : abs(kMinInt);

[Jakob Kummerow, 2013/06/07 09:02:33]

I think you need to do this same trick for abs_upper, or is it guaranteed that
upper != kMinInt?

+      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);
   value()->PrintNameTo(stream);
 }
 
 
 void HUnaryOperation::PrintDataTo(StringStream* stream) {
   value()->PrintNameTo(stream);
 }
@@ skipping 2483 matching lines @@
     case kBackingStore:
       if (!name_.is_null()) stream->Add(*String::cast(*name_)->ToCString());
       stream->Add("[backing-store]");
       break;
   }
 
   stream->Add("@%d", offset());
 }
 
 } }  // namespace v8::internal