Don't allow phi equivalences along back-edges.

lib/compiler/implementation/ssa/codegen_helpers.dart

Index: lib/compiler/implementation/ssa/codegen_helpers.dart
diff --git a/lib/compiler/implementation/ssa/codegen_helpers.dart b/lib/compiler/implementation/ssa/codegen_helpers.dart
index 64453d782273bd701cbabf7e79d867a67e8296ff..44f071cb9c8f6a6d7376292f3549e4a7eb648892 100644
--- a/lib/compiler/implementation/ssa/codegen_helpers.dart
+++ b/lib/compiler/implementation/ssa/codegen_helpers.dart
@@ -440,14 +440,22 @@ class PhiEquivalator {
   PhiEquivalator(this.equivalence, this.logicalOperations);
 
   void analyzeGraph(HGraph graph) {
-    graph.blocks.forEach((HBasicBlock block) => analyzeBlock(block));
+    graph.blocks.forEach(analyzeBlock);
+  }
+
+  static bool isPhiBeforeUse(HPhi phi1) {
+    assert(phi1.usedBy.length == 1);
+    HPhi phi2 = phi1.usedBy[0];
+    // A phi can never be used in another phi in the same block.

[ngeoffray, 2012/05/08 09:13:01]

That you could assert here.

[Lasse Reichstein Nielsen, 2012/05/08 09:17:14]

I'll just inline it all in the test that already tested that.

+    return phi1.block.id < phi2.block.id;

[ngeoffray, 2012/05/08 09:13:01]

If that is true, maybe also assert that phi2 is a loop phi?

[Lasse Reichstein Nielsen, 2012/05/08 09:17:14]

Not sure it's necessary. If it's an invariant, it's probably already checked by
the validator, and if not, it should not be asserted.

   }
 
   void analyzeBlock(HBasicBlock block) {
     for (HPhi phi = block.phis.first; phi !== null; phi = phi.next) {
       if (!logicalOperations.containsKey(phi) &&
           phi.usedBy.length == 1 &&
-          phi.usedBy[0] is HPhi) {
+          phi.usedBy[0] is HPhi &&
+          isPhiBeforeUse(phi)) {
         equivalence.makeEquivalent(phi, phi.usedBy[0]);
       }
     }