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

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 18774)
+++ runtime/vm/compiler.cc	(working copy)
@@ -178,6 +178,12 @@
       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,