Fix GVN for loops.

lib/compiler/implementation/ssa/optimize.dart

 // 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.
 
 interface OptimizationPhase {
   String get name();
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 885 matching lines @@
     }
   }
 
   bool isInputDefinedAfterDominator(HInstruction input,
                                     HBasicBlock dominator) {
     return input.block.id > dominator.id;
   }
 
   void visitBasicBlock(HBasicBlock block, ValueSet values) {
     HInstruction instruction = block.first;
+    if (block.isLoopHeader()) {
+      int flags = loopChangesFlags[block.id];
+      if (flags != 0) {
+        values.kill(flags);
+      }
+    }
     while (instruction !== null) {
       HInstruction next = instruction.next;
       int flags = instruction.getChangesFlags();
       if (flags != 0) {
         assert(!instruction.useGvn());
         values.kill(flags);
       } else if (instruction.useGvn()) {
         HInstruction other = values.lookup(instruction);
         if (other !== null) {
           assert(other.gvnEquals(instruction) && instruction.gvnEquals(other));
@@ skipping 13 matching lines @@
       ValueSet successorValues = (i == length - 1) ? values : values.copy();
       // If we have no values in our set, we do not have to kill
       // anything. Also, if the range of block ids from the current
       // block to the dominated block is empty, there is no blocks on
       // any path from the current block to the dominated block so we
       // don't have to do anything either.
       assert(block.id < dominated.id);
       if (!successorValues.isEmpty() && block.id + 1 < dominated.id) {
         visited.clear();
         int changesFlags = getChangesFlagsForDominatedBlock(block, dominated);
         successorValues.kill(changesFlags);

[kasperl, 2012/06/18 05:51:33]

Would it make sense to do a fast check (changesFlags == 0) here too? Or should
we move the check to ValueSet.kill instead (early bailout)?

[Mads Ager (google), 2012/06/18 08:01:13]

Added early bailout to ValueSet.kill and called it unconditionally in all three
places. 

       }
       visitBasicBlock(dominated, successorValues);
     }
   }
 
   void computeChangesFlags(HGraph graph) {
     // Create the changes flags lists. Make sure to initialize the
     // loop changes flags list to zero so we can use bitwise or when
     // propagating loop changes upwards.
     final int length = graph.blocks.length;
@@ skipping 211 matching lines @@
         // the if block terminates. So any use of the instruction
         // after the join block should be changed to the new
         // instruction.
         changeUsesDominatedBy(ifUser.joinBlock, input, convertedType);
       }
       // TODO(ngeoffray): Also change uses for the then block on a HType
       // that knows it is not of a specific Type.
     }
   }
 }