Improvement to SsaLoadElimination

pkg/compiler/lib/src/ssa/optimize.dart

Index: pkg/compiler/lib/src/ssa/optimize.dart
diff --git a/pkg/compiler/lib/src/ssa/optimize.dart b/pkg/compiler/lib/src/ssa/optimize.dart
index 9aa2d98dd339645993dc3367293532c7c205f48a..6ff4e783c2628f33e7a24e2fe26721b75945ba10 100644
--- a/pkg/compiler/lib/src/ssa/optimize.dart
+++ b/pkg/compiler/lib/src/ssa/optimize.dart
@@ -2604,7 +2604,33 @@ class MemorySet {
   MemorySet intersectionFor(
       MemorySet other, HBasicBlock block, int predecessorIndex) {
     MemorySet result = new MemorySet(compiler);
-    if (other == null) return result;
+    if (other == null) {
+      // This is the first visit to a loop header ([other] is `null` because we
+      // have not visited the back edge). Copy the nonEscapingReceivers that are
+      // guaranteed to survive the loop because they are not escaped before
+      // method exit.
+      // TODO(sra): We should do a proper dataflow to find the maximal
+      // nonEscapingReceivers (a variant of Available-Expressions), which must
+      // converge before we edit the program in [findCommonInstruction].
+      for (HInstruction instruction in nonEscapingReceivers) {
+        bool isNonEscapingUse(HInstruction use) {
+          if (use is HReturn) return true; // Escapes, but so does control.
+          if (use is HFieldGet) return true;
+          if (use is HFieldSet &&
+              use.receiver.nonCheck() == instruction &&
+              use.value.nonCheck() != instruction) {
+            return true;
+          }
+          if (use is HTypeInfoReadVariable) return true;
+          return false;
+        }
+
+        if (instruction.usedBy.every(isNonEscapingUse)) {
+          result.nonEscapingReceivers.add(instruction);
+        }
+      }
+      return result;
+    }
 
     fieldValues.forEach((element, values) {
       var otherValues = other.fieldValues[element];