Replace log2 with MostSignificantBit

R=ulan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15994015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14937 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -2328,14 +2328,11 @@ Range* HBitwise::InferRange(Zone* zone) {
       if (right_upper < 0) right_upper = ~right_upper;
       if (right_lower < 0) right_lower = ~right_lower;
 
-      // 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)));
+      int high = MostSignificantBit(
+          left_upper | left_lower | right_upper | right_lower);
 
       int64_t limit = 1;
-      limit <<= high + 1;
+      limit <<= high;
       int32_t min = (left()->range()->CanBeNegative() ||
                      right()->range()->CanBeNegative())
                     ? static_cast<int32_t>(-limit) : 0;

src/utils.h

@@ -86,6 +86,25 @@ inline int WhichPowerOf2(uint32_t x) {
 }
 
 
+inline int MostSignificantBit(uint32_t x) {
+  static const int msb4[] = {0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4};
+  int nibble = 0;
+  if (x & 0xffff0000) {
+    nibble += 16;
+    x >>= 16;
+  }
+  if (x & 0xff00) {
+    nibble += 8;
+    x >>= 8;
+  }
+  if (x & 0xf0) {
+    nibble += 4;
+    x >>= 4;
+  }
+  return nibble + msb4[x];
+}
+
+
 // Magic numbers for integer division.
 // These are kind of 2's complement reciprocal of the divisors.
 // Details and proofs can be found in: