Add the first phi's input as possible generate-at-use-site.

lib/compiler/implementation/ssa/codegen_helpers.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.
 
 /**
  * Instead of emitting each SSA instruction with a temporary variable
  * mark instructions that can be emitted at their use-site.
  * For example, in:
  *   t0 = 4;
  *   t1 = 3;
  *   t2 = add(t0, t1);
  * t0 and t1 would be marked and the resulting code would then be:
  *   t2 = add(4, 3);
  */
 class SsaInstructionMerger extends HBaseVisitor {
   List<HInstruction> expectedInputs;
   Set<HInstruction> generateAtUseSite;
 
   SsaInstructionMerger(this.generateAtUseSite);
 
   void visitGraph(HGraph graph) {
     visitDominatorTree(graph);
   }
 
   bool usedOnlyByPhis(instruction) {
     for (HInstruction user in instruction.usedBy) {
-      if (user is !HPhi) return false;
+      if (user is! HPhi) return false;
     }
     return true;
   }
 
   void visitInstruction(HInstruction instruction) {
     // A code motion invariant instruction is dealt before visiting it.
     assert(!instruction.isCodeMotionInvariant());
     for (HInstruction input in instruction.inputs) {
       if (!generateAtUseSite.contains(input)
           && !input.isCodeMotionInvariant()
-          && input.usedBy.length == 1) {
+          && input.usedBy.length == 1
+          && input is! HPhi) {
         expectedInputs.add(input);
       }
     }
   }
 
   // The codegen might use the input multiple times, so it must not be
   // set generate at use site.
   void visitIs(HIs instruction) {}
 
   // A check method must not have its input generate at use site,
@@ skipping 40 matching lines @@
     // be generated at their use site, if they occur in the correct order.
     if (expectedInputs === null) expectedInputs = new List<HInstruction>();
 
     // Pop instructions from expectedInputs until instruction is found.
     // Return true if it is found, or false if not.
     bool findInInputs(HInstruction instruction) {
       while (!expectedInputs.isEmpty()) {
         HInstruction nextInput = expectedInputs.removeLast();
         assert(!generateAtUseSite.contains(nextInput));
         assert(nextInput.usedBy.length == 1);
-        if (nextInput == instruction) {
+        if (nextInput === instruction) {
           return true;
         }
       }
       return false;
     }
 
+    for (HBasicBlock successor in block.successors) {
+      // Only add the input of the first phi. Making inputs of
+      // later phis generate-at-use-site would make them move
+      // accross the assignment of the first phi, and we need
+      // more analysis before we can do that.
+      HPhi phi = successor.phis.first;
+      if (phi != null) {
+        int index = successor.predecessors.indexOf(block);
+        HInstruction input = phi.inputs[index];
+        if (!generateAtUseSite.contains(input)
+            && !input.isCodeMotionInvariant()
+            && input.usedBy.length == 1
+            && input is! HPhi) {
+          expectedInputs.add(input);
+        }
+        break;
+      }
+    }
+
     block.last.accept(this);
     for (HInstruction instruction = block.last.previous;
          instruction !== null;
          instruction = instruction.previous) {
       if (generateAtUseSite.contains(instruction)) {
         continue;
       }
       if (instruction.isCodeMotionInvariant()) {
         generateAtUseSite.add(instruction);
         continue;
       }
       bool foundInInputs = false;
       // See if the current instruction is the next non-trivial
       // expected input. If not, drop the expectedInputs and
       // start over.
       if (findInInputs(instruction)) {
         foundInInputs = true;
         tryGenerateAtUseSite(instruction);
       } else {
         assert(expectedInputs.isEmpty());
       }
       if (foundInInputs || usedOnlyByPhis(instruction)) {
         // Try merging all non-trivial inputs.
         instruction.accept(this);
       }
     }
 
     if (block.predecessors.length === 1
         && isBlockSinglePredecessor(block.predecessors[0])) {
+      assert(block.phis.isEmpty());
       tryMergingExpressions(block.predecessors[0]);
     } else {
       expectedInputs = null;
     }
   }
 }
 
 /**
  *  Detect control flow arising from short-circuit logical operators, and
  *  prepare the program to be generated using these operators instead of
@@ skipping 265 matching lines @@
     // improving the performance of future lookups.
     T root = getRepresentative(parent);
     if (root !== parent) representative[element] = root;
     return root;
   }
 
   bool areEquivalent(T a, T b) {
     return getRepresentative(a) === getRepresentative(b);
   }
 }