Simple redundant load elimination.

runtime/vm/bit_vector.h

Index: runtime/vm/bit_vector.h
===================================================================
--- runtime/vm/bit_vector.h	(revision 12420)
+++ runtime/vm/bit_vector.h	(working copy)
@@ -66,6 +66,8 @@
     data_[i / kBitsPerWord] &= ~(static_cast<uword>(1) << (i % kBitsPerWord));
   }
 
+  bool Equals(const BitVector& other) const;
+
   // Add all elements that are in the bitvector from.
   bool AddAll(BitVector* from);
 
@@ -73,6 +75,8 @@
   // bitvector kill.
   bool KillAndAdd(BitVector* kill, BitVector* gen);
 
+  bool Intersect(const BitVector& other);
+
   bool Contains(int i) const {
     ASSERT(i >= 0 && i < length());
     uword block = data_[i / kBitsPerWord];
@@ -85,8 +89,16 @@
     }
   }
 
+  void SetAll() {

[Kevin Millikin (Google), 2012/09/17 12:09:47]

There's a subtle interaction between Clear(), SetAll(), and Equals().  The first
two clear and set bits past length_.  The last one compares bits past length_. 
Bit vectors that have all bits in range 0..length_-1 equal might not compare
Equals.

It might be better to choose either:

1. Never set a bit past length_ (requires a mask for the last word of the data
in SetAll), or

2. Ignore bits past length_ for Equals (requires a mask for the last word
comparison in Equals).

[Florian Schneider, 2012/09/17 14:20:59]

Thanks. I opted for (2) and changed Equals accordingly.

+    for (intptr_t i = 0; i < data_length_; i++) {
+      data_[i] = static_cast<uword>(-1);
+    }
+  }
+
   intptr_t length() const { return length_; }
 
+  void Print() const;
+
  private:
   intptr_t length_;
   intptr_t data_length_;