Optimize constant divisions by powers of 2.

src/parser.cc

 // Copyright 2006-2008 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 2629 matching lines @@
             uint32_t shift = DoubleToInt32(y_val) & 0x1f;
             int value = ArithmeticShiftRight(DoubleToInt32(x_val), shift);
             x = NewNumberLiteral(value);
             continue;
           }
           default:
             break;
         }
       }
 
+      // Convert constant divisions to multiplications for speed.
+      if (op == Token::DIV &&
+          y && y->AsLiteral() && y->AsLiteral()->handle()->IsNumber()) {
+        double y_val = y->AsLiteral()->handle()->Number();
+        int64_t y_int = static_cast<int64_t>(y_val);
+        // There are rounding issues with this optimization, but they don't
+        // apply if the number to be divided with has a reciprocal that can
+        // be precisely represented as a floating point number.  This is
+        // the case if the number is an integer power of 2.  You can test for
+        // an integer power of 2 by subtracting 1 and anding.
+        if (static_cast<double>(y_int) == y_val &&
+            ((y_int - 1) & y_int) == 0) {
+          y = NewNumberLiteral(1 / y_val);
+          op = Token::MUL;
+        }
+      }
+
       // For now we distinguish between comparisons and other binary
       // operations.  (We could combine the two and get rid of this
       // code an AST node eventually.)
       if (Token::IsCompareOp(op)) {
         // We have a comparison.
         Token::Value cmp = op;
         switch (op) {
           case Token::NE: cmp = Token::EQ; break;
           case Token::NE_STRICT: cmp = Token::EQ_STRICT; break;
           default: break;
@@ skipping 1965 matching lines @@
                        start_position,
                        is_expression);
   return result;
 }
 
 
 #undef NEW
 
 
 } }  // namespace v8::internal