Recognize pattern (a << b) & c with c being a positive Smi and allow left shift to truncate the res…

runtime/vm/compiler.cc

 // Copyright (c) 2012, 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/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/code_generator.h"
@@ skipping 160 matching lines @@
 
     if (optimized) {
       TimerScope timer(FLAG_compiler_stats,
                        &CompilerStats::graphoptimizer_timer,
                        isolate);
 
       FlowGraphOptimizer optimizer(flow_graph);
       optimizer.ApplyICData();
       DEBUG_ASSERT(flow_graph->VerifyUseLists());
 
+      // Optimize (a << b) & c patterns. Must occur before
+      // 'Selectrepresentations' which inserts conversion nodes.
+      // TODO(srdjan): Move de before inlinining until environmenr use list can

[Vyacheslav Egorov (Google), 2013/02/21 14:45:50]

"Moved", "environment" .

I find it very fragile. I could not come up with a way to break but it does not
mean there is none.

Florian & Kevin are sharing the same sentiment. I think we should discuss what
must be done to guarantee that we can use environment use list. 

[srdjan, 2013/02/21 21:55:31]

Fixed spelling.  As discussed, added a flag (truncating_left_shift) to quickly
turn it off and reasoned that it is probably safe to do it before inlining.

+      // be used to detect when shift-left is outside the scope of bit-and.
+      optimizer.TryOptimizeLeftShiftWithBitAndPattern();
+
       // Inlining (mutates the flow graph)
       if (FLAG_use_inlining) {
         TimerScope timer(FLAG_compiler_stats,
                          &CompilerStats::graphinliner_timer);
         FlowGraphInliner inliner(flow_graph);
         inliner.Inline();
         // Use lists are maintained and validated by the inliner.
         DEBUG_ASSERT(flow_graph->VerifyUseLists());
       }
 
@@ skipping 451 matching lines @@
       Object::Handle(isolate->object_store()->sticky_error());
     isolate->object_store()->clear_sticky_error();
     isolate->set_long_jump_base(base);
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart