dart2js cps: Global value numbering and loop-invariant code motion.

pkg/compiler/lib/src/cps_ir/loop_effects.dart

Index: pkg/compiler/lib/src/cps_ir/loop_effects.dart
diff --git a/pkg/compiler/lib/src/cps_ir/loop_effects.dart b/pkg/compiler/lib/src/cps_ir/loop_effects.dart
index 9dd39ff78518b5cfe9fc6e2d725f2c49fe7695f2..8552786e241d21f8b61b3ae4d9da5799ab7b3234 100644
--- a/pkg/compiler/lib/src/cps_ir/loop_effects.dart
+++ b/pkg/compiler/lib/src/cps_ir/loop_effects.dart
@@ -46,9 +46,7 @@ class LoopSideEffects extends TrampolineRecursiveVisitor {
   Expression traverseContinuation(Continuation cont) {
     if (cont.isRecursive) {
       SideEffects oldEffects = currentLoopSideEffects;
-      Continuation oldLoopHeader = currentLoopHeader;
       bool oldChangesLength = currentLoopChangesLength;
-      currentLoopHeader = cont;
       loopSideEffects[cont] = currentLoopSideEffects = new SideEffects.empty();
       exitContinuations[cont] = <Continuation>[];
       pushAction(() {
@@ -57,11 +55,15 @@ class LoopSideEffects extends TrampolineRecursiveVisitor {
           loopsChangingLength.add(cont);
         }
         currentLoopChangesLength = currentLoopChangesLength || oldChangesLength;
-        currentLoopHeader = oldLoopHeader;
         currentLoopSideEffects = oldEffects;
         exitContinuations[cont].forEach(push);
       });
     }
+    Continuation oldLoopHeader = currentLoopHeader;
+    currentLoopHeader = loopHierarchy.getLoopHeader(cont);
+    pushAction(() {
+      currentLoopHeader = oldLoopHeader;
+    });
     return cont.body;
   }
 
@@ -87,6 +89,12 @@ class LoopSideEffects extends TrampolineRecursiveVisitor {
       Continuation inner = currentLoopHeader;
       Continuation outer = loopHierarchy.getEnclosingLoop(currentLoopHeader);
       while (outer != loop) {
+        if (inner == null) {
+          // The shrinking reductions pass must run before any pass that relies
+          // on computing loop side effects.
+          throw 'Unreachable continuations must be removed before computing '
+                'loop side effects.';

[asgerf, 2015/11/16 15:42:39]

This is similar to the problem of putting unreachable blocks into a dominator
tree. There is no correct place to put it, and putting it in an arbitrary place
can break some assumptions in the client analysis.

[sra1, 2015/11/17 05:41:14]

Use the proper internal error (possibly as separate CL)

[asgerf, 2015/11/17 12:43:43]

Done.

+        }
         inner = outer;
         outer = loopHierarchy.getEnclosingLoop(outer);
       }
@@ -171,6 +179,7 @@ class LoopSideEffects extends TrampolineRecursiveVisitor {
   }
 
   void visitApplyBuiltinMethod(ApplyBuiltinMethod node) {
+    currentLoopSideEffects.setChangesIndex();
     currentLoopChangesLength = true; // Push and pop.
   }
 }