Track users through phi nodes to eliminate '>>> 0' for more bitops

lib/compiler/implementation/ssa/codegen.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.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   final JavaScriptBackend backend;
   SsaCodeGeneratorTask(JavaScriptBackend backend)
       : this.backend = backend,
         super(backend.compiler);
   String get name() => 'SSA code generator';
@@ skipping 199 matching lines @@
   bool isNonNegativeInt32Constant(HInstruction instruction) {
     if (instruction.isConstantInteger()) {
       int value = instruction.constant.value;
       if (value >= 0 && value < (1 << 31)) {
         return true;
       }
     }
     return false;
   }
 
+  bool hasNonBitOpUser(HInstruction instruction, Set<HPhi> phiSet) {
+    for (HInstruction use in instruction.usedBy) {
+      if (use is HPhi) {
+        if (!phiSet.contains(use)) {
+          phiSet.add(use);
+          if (hasNonBitOpUser(use, phiSet)) return true;
+        }
+      } else if (use is! HBitNot && use is! HBinaryBitOp) {
+        return true;
+      }
+    }
+    return false;
+  }
+
   // We want the outcome of bit-operations to be positive. However, if
   // the result of a bit-operation is only used by other bit
   // operations we do not have to convert to an unsigned
   // integer. Also, if we are using & with a positive constant we know
   // that the result is positive already and need no conversion.
   bool requiresUintConversion(HInstruction instruction) {
     if (instruction is HBitAnd &&
         (isNonNegativeInt32Constant(instruction.left) ||
          isNonNegativeInt32Constant(instruction.right))) {
       return false;
     }
-    bool result = false;
-    for (HInstruction use in instruction.usedBy) {
-      if (use is! HBitNot && use is! HBinaryBitOp) {
-        result = true;
-        break;
-      }
-    }
-    return result;
+    return hasNonBitOpUser(instruction, new Set<HPhi>());
   }
 
   SsaCodeGenerator(this.backend,
                    this.work,
                    this.parameters,
                    this.parameterNames)
     : declaredVariables = new Set<String>(),
       delayedVariableDeclarations = new Set<String>(),
       buffer = new StringBuffer(),
       generateAtUseSite = new Set<HInstruction>(),
@@ skipping 2852 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endLabeledBlock(HLabeledBlockInformation labeledBlockInfo) {
     if (labeledBlockInfo.body.start.hasGuards()) {
       endBailoutSwitch();
     }
   }
 }