Don't strength reduce divisions by 1 or 2 as they can often be...

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 2635 matching lines @@
             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.
-        if (static_cast<double>(y_int) == y_val && IsPowerOf2(y_int)) {
+        // 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.  Negative integer powers of
+        // 2 work too, but for -2, -1, 1 and 2 we don't do the strength
+        // reduction because the inlined optimistic idiv has a reasonable
+        // chance of succeeding by producing a Smi answer with no remainder.
+        if (static_cast<double>(y_int) == y_val &&
+            (IsPowerOf2(y_int) || IsPowerOf2(-y_int)) &&
+            (y_int > 2 || y_int < -2)) {
           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.
@@ skipping 1970 matching lines @@
                        start_position,
                        is_expression);
   return result;
 }
 
 
 #undef NEW
 
 
 } }  // namespace v8::internal