Optimize smi multiply by 2 using shl by 1.

runtime/vm/intermediate_language_ia32.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_IA32.
 #if defined(TARGET_ARCH_IA32)
 
 #include "vm/intermediate_language.h"
 
 #include "lib/error.h"
@@ skipping 2191 matching lines @@
         if (deopt != NULL) __ j(OVERFLOW, deopt);
         break;
       case Token::kSUB: {
         __ subl(left, Immediate(imm));
         if (deopt != NULL) __ j(OVERFLOW, deopt);
         break;
       }
       case Token::kMUL: {
         // Keep left value tagged and untag right value.
         const intptr_t value = Smi::Cast(constant).Value();
-        __ imull(left, Immediate(value));
+        if (value == 2) {

[bakster, 2013/03/19 13:22:44]

What about 4, 8, 16 etc?

[Florian Schneider, 2013/03/19 13:24:33]

shl does not set the overflow flag for other than 1-bit shifts.

[bakster, 2013/03/19 13:31:25]

I guess you than can unroll into repeated shl(1) :-)

[ahe, 2013/03/19 14:13:11]

What about a compare?

+          __ shll(left, Immediate(1));
+        } else {
+          __ imull(left, Immediate(value));
+        }
         if (deopt != NULL) __ j(OVERFLOW, deopt);
         break;
       }
       case Token::kTRUNCDIV: {
         const intptr_t value = Smi::Cast(constant).Value();
         if (value == 1) {
           // Do nothing.
           break;
         } else if (value == -1) {
           // Check the corner case of dividing the 'MIN_SMI' with -1, in which
@@ skipping 1336 matching lines @@
                          PcDescriptors::kOther,
                          locs());
   __ Drop(2);  // Discard type arguments and receiver.
 }
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_IA32