Fix buggy computations in value ranges, spotted by floitsch@.

lib/compiler/implementation/ssa/value_range_analyzer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A [Value] represents both symbolic values like the value of a
  * parameter, or the length of an array, and concrete values, like
  * constants.
  */
 abstract class Value {
@@ skipping 268 matching lines @@
       else up = upper;
     }
     return new Range(low, up);
   }
 
   Range operator +(Range other) {
     return new Range.normalize(lower + other.lower, upper + other.upper);
   }
 
   Range operator -(Range other) {
-    return new Range.normalize(lower - other.lower, upper - other.upper);
+    return new Range.normalize(lower - other.upper, upper - other.lower);
   }
 
   Range operator &(Range other) {
-    return new Range.normalize(lower & other.lower, upper & other.upper);
+    // Only operate on simple operations, ie when one of the range is

[floitsch, 2012/10/02 09:32:28]

There is no need to make this restriction.
if this.isPositive && other.isPositive: [0 .. max(upper, other.upper)];
if (this.isPositive): [0 .. upper]
if (other.isPositive): [0 .. other.upper]
You could also get better ranges when the ranges are negative, but I'm not sure
it's worth it. For example:
if this.isNegative && other.isNegative: [-infinity, max(upper, other.upper)].
And there is probably a way to make the -infinity go away.

+    // constant.
+    if (lower == upper) {
+      if (isPositive()) return new Range(const IntValue(0), upper);
+      else if (isNegative()) return new Range(lower, const IntValue(0));

[floitsch, 2012/10/02 09:32:28]

This is wrong: -1 & 5 -> 5

+      else return const Range.unbound();
+    } else if (other.lower == other.upper) {
+      return other & this;
+    }
+    return const Range.unbound();
   }
 
   bool operator ==(other) {
     if (other is! Range) return false;
     return other.lower == lower && other.upper == upper;
   }
 
   bool isLessThan(Range other) {
     return upper != other.lower && upper.min(other.lower) == upper;
   }
@@ skipping 375 matching lines @@
     if (instruction is HPhi && !instruction.block.isLoopHeader()) {
       HInstruction result = unwrap(instruction.inputs[0]);
       for (int i = 1; i < instruction.inputs.length; i++) {
         if (result != unwrap(instruction.inputs[i])) return instruction;
       }
       return result;
     }
     return instruction;
   }
 }