Infer the range of XOR as limited by the highest possible changed bit.

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
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2292 matching lines @@
 
 void HMathMinMax::InferRepresentation(HInferRepresentation* h_infer) {
   ASSERT(CheckFlag(kFlexibleRepresentation));
   Representation new_rep = RepresentationFromInputs();
   UpdateRepresentation(new_rep, h_infer, "inputs");
   // Do not care about uses.
 }
 
 
 Range* HBitwise::InferRange(Zone* zone) {
-  if (op() == Token::BIT_XOR) return HValue::InferRange(zone);
+  if (op() == Token::BIT_XOR) {
+    if (left()->range() != NULL && right()->range() != NULL) {

[Sven Panne, 2013/06/04 12:15:27]

We traditionally use HasRange() for stuff like this.

+      // The maximum value has the high bit, and all bits below, set:
+      // (1 << high) - 1.
+      // If the range can be negative, the minimum int is a negative number with
+      // the high bit, and all bits below, unset:
+      // -(1 << high).
+      // If it cannot be negative, conservatively choose 0 as minimum int.
+      int64_t left_upper = left()->range()->upper();
+      int64_t left_lower = left()->range()->lower();
+      int64_t right_upper = right()->range()->upper();
+      int64_t right_lower = right()->range()->lower();
+
+      // With the same number of bits, 1 lower value can be represented on the
+      // negative side. Lower the number of required negative bits of log2
+      // values by removing this value.
+      if (left_upper < 0) left_upper = llabs(left_upper + 1);
+      if (left_lower < 0) left_lower = llabs(left_lower + 1);
+      if (right_upper < 0) right_upper = llabs(right_upper + 1);
+      if (right_lower < 0) right_lower = llabs(right_lower + 1);
+
+      // Find the highest used bit.
+      int high = static_cast<int>(log2(left_upper));
+      high = Max(high, static_cast<int>(log2(left_lower)));
+      high = Max(high, static_cast<int>(log2(right_upper)));
+      high = Max(high, static_cast<int>(log2(right_lower)));
+
+      int64_t limit = 1;
+      limit <<= high + 1;
+      int32_t min = (left()->range()->CanBeNegative() ||
+                     right()->range()->CanBeNegative())
+          ? static_cast<int32_t>(-limit) : 0;
+      return new(zone) Range(min, static_cast<int32_t>(limit - 1));
+    }
+    return HValue::InferRange(zone);
+  }
   const int32_t kDefaultMask = static_cast<int32_t>(0xffffffff);
   int32_t left_mask = (left()->range() != NULL)
       ? left()->range()->Mask()
       : kDefaultMask;
   int32_t right_mask = (right()->range() != NULL)
       ? right()->range()->Mask()
       : kDefaultMask;
   int32_t result_mask = (op() == Token::BIT_AND)
       ? left_mask & right_mask
       : left_mask | right_mask;
@@ skipping 1436 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