Bump allocation for promotion

runtime/vm/bit_set.h

Index: runtime/vm/bit_set.h
===================================================================
--- runtime/vm/bit_set.h	(revision 39806)
+++ runtime/vm/bit_set.h	(working copy)
@@ -22,38 +22,49 @@
   void Set(intptr_t i, bool value) {
     ASSERT(i >= 0);
     ASSERT(i < N);
-    uword mask = (static_cast<uword>(1) << (i % kBitsPerWord));
+    uword mask = (static_cast<uword>(1) << (i & (kBitsPerWord - 1)));
     if (value) {
-      data_[i / kBitsPerWord] |= mask;
+      data_[i >> kBitsPerWordLog2] |= mask;
     } else {
-      data_[i / kBitsPerWord] &= ~mask;
+      data_[i >> kBitsPerWordLog2] &= ~mask;
     }
   }
 
-  bool Test(intptr_t i) {
+  bool Test(intptr_t i) const {
     ASSERT(i >= 0);
     ASSERT(i < N);
-    uword mask = (static_cast<uword>(1) << (i % kBitsPerWord));
-    return (data_[i / kBitsPerWord] & mask) != 0;
+    uword mask = (static_cast<uword>(1) << (i & (kBitsPerWord - 1)));
+    return (data_[i >> kBitsPerWordLog2] & mask) != 0;
   }
 
-  intptr_t Next(intptr_t i) {
+  intptr_t Next(intptr_t i) const {
     ASSERT(i >= 0);
     ASSERT(i < N);
-    intptr_t w = i / kBitsPerWord;
+    intptr_t w = i >> kBitsPerWordLog2;
     uword mask = ~static_cast<uword>(0) << i;
     if ((data_[w] & mask) != 0) {
       uword tz = Utils::CountTrailingZeros(data_[w] & mask);
-      return kBitsPerWord*w + tz;
+      return (w << kBitsPerWordLog2) + tz;
     }
-    while (++w < (1 + ((N - 1) / kBitsPerWord))) {
+    while (++w < kLengthInWords) {
       if (data_[w] != 0) {
-        return kBitsPerWord*w + Utils::CountTrailingZeros(data_[w]);
+        return (w << kBitsPerWordLog2) + Utils::CountTrailingZeros(data_[w]);
       }
     }
     return -1;
   }
 
+  intptr_t Last() const {
+    for (int w = kLengthInWords - 1; w >= 0; --w) {
+      uword d = data_[w];
+      if (d != 0) {
+        // TODO(koda): Define HighestBit(uword) or use uint64_t[] for data_.
+        return (w << kBitsPerWordLog2) + Utils::HighestBit(d);
+      }
+    }
+    return -1;
+  }
+
   void Reset() {
     memset(data_, 0, sizeof(data_));
   }
@@ -63,7 +74,8 @@
   }
 
  private:
-  uword data_[1 + ((N - 1) / kBitsPerWord)];
+  static const int kLengthInWords = 1 + ((N - 1) / kBitsPerWord);
+  uword data_[kLengthInWords];
 };
 
 }  // namespace dart