Reapply: Use GVN for length loads from arrays and strings.

lib/compiler/implementation/ssa/variable_allocator.dart

Index: lib/compiler/implementation/ssa/variable_allocator.dart
diff --git a/lib/compiler/implementation/ssa/variable_allocator.dart b/lib/compiler/implementation/ssa/variable_allocator.dart
index 927448e5551895b180b20ddee0f036b797befd2c..a3dfc47a3b4d09e151ec92a42da3cda5c91f40ec 100644
--- a/lib/compiler/implementation/ssa/variable_allocator.dart
+++ b/lib/compiler/implementation/ssa/variable_allocator.dart
@@ -22,7 +22,7 @@ class LiveRange {
  */
 class LiveInterval {
   /**
-   * The id where there instruction is defined.
+   * The id where the instruction is defined.
    */
   int start;
   final List<LiveRange> ranges;
@@ -138,7 +138,7 @@ class LiveEnvironment {
    * already in the set, we save the id where it dies.
    */
   void add(HInstruction instruction, int userId) {
-    // Note that we are visiting the grap in post-dominator order, so
+    // Note that we are visiting the graph in post-dominator order, so
     // the first time we see a variable is when it dies.
     liveInstructions.putIfAbsent(instruction, () => userId);
     if (instruction is HCheck) {
@@ -589,12 +589,22 @@ class SsaVariableAllocator extends HBaseVisitor {
   }
 
   void handleInstruction(HInstruction instruction, VariableNamer namer) {
-    for (int i = 0, len = instruction.inputs.length; i < len; i++) {
-      HInstruction input = instruction.inputs[i];
-      // If [input] has a name, and its use here is the last use, free
-      // its name.
-      if (needsName(input) && diesAt(input, instruction)) {
-        namer.freeName(input);
+    // TODO(ager): We cannot perform this check to free names for
+    // HCheck instructions because they are special cased to have the
+    // same live intervals as the instruction they are checking. This
+    // includes sharing the start id with the checked
+    // input. Therefore, for HCheck(checkedInput, otherInput) we would
+    // end up checking that otherInput dies not here, but at the
+    // location of checkedInput. We should preserve the start id for
+    // the check instruction.
+    if (instruction is! HCheck) {
+      for (int i = 0, len = instruction.inputs.length; i < len; i++) {
+        HInstruction input = instruction.inputs[i];
+        // If [input] has a name, and its use here is the last use, free
+        // its name.
+        if (needsName(input) && diesAt(input, instruction)) {
+          namer.freeName(input);
+        }
       }
     }