Insert type guards for loads from arrays in nested loops.

lib/compiler/implementation/ssa/bailout.dart

Index: lib/compiler/implementation/ssa/bailout.dart
diff --git a/lib/compiler/implementation/ssa/bailout.dart b/lib/compiler/implementation/ssa/bailout.dart
index 6cc69c59f8939a44d60830a62f54c98dec369b69..5c0b7a94559c742767354c1190a89fa1a22fceba 100644
--- a/lib/compiler/implementation/ssa/bailout.dart
+++ b/lib/compiler/implementation/ssa/bailout.dart
@@ -107,6 +107,8 @@ class SsaTypeGuardInserter extends HGraphVisitor implements OptimizationPhase {
     return type.isPrimitive() && !type.isNull();
   }
 
+  bool get hasTypeGuards() => work.guards.length != 0;
+
   bool typeGuardWouldBeValuable(HInstruction instruction,
                                 HType speculativeType) {
     // If the type itself is not valuable, do not generate a guard for it.
@@ -145,6 +147,20 @@ class SsaTypeGuardInserter extends HGraphVisitor implements OptimizationPhase {
       if (isNested(userLoopHeader, currentLoopHeader)) return true;
     }
 
+    // To speed up computations on values loaded from arrays, we
+    // insert type guards for builtin array indexing operations in
+    // nested loops. Since this can blow up code size quite
+    // significantly, we only do it if type guards have already been
+    // inserted for this method. The code size price for an additional
+    // type guard is much smaller than the first one that causes the
+    // generation of a bailout method.
+    if (instruction is HIndex && instruction.builtin && hasTypeGuards) {
+      HBasicBlock loopHeader = instruction.block.enclosingLoopHeader;
+      if (loopHeader != null && loopHeader.parentLoopHeader != null) {
+        return true;
+      }
+    }
+
     // Insert type guards if the method is likely to be called in a
     // loop.
     return calledInLoop || highTypeLikelyhood;