Add support for calculating a scopes maximum nested context chain.

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 26c092fe3e4d16f8dfbd86eaf172710c645e2123..16581d776f09355a099645170a69ce53bb0c18e1 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -341,7 +341,12 @@ class Scope: public ZoneObject {
   bool is_nonlinear() const { return scope_nonlinear_; }
 
   // Whether this needs to be represented by a runtime context.
-  bool NeedsContext() const { return num_heap_slots() > 0; }
+  bool NeedsContext() const {
+    // Catch and module scopes always have heap slots.
+    DCHECK(!is_catch_scope() || num_heap_slots() > 0);
+    DCHECK(!is_module_scope() || num_heap_slots() > 0);
+    return is_with_scope() || num_heap_slots() > 0;

[adamk, 2015/10/15 07:38:45]

The other callers of NeedsContext() only call it on a block scope, so this is
sort of weird. Maybe change them to do the num_heap_slots() check directly, and
make this method private on Scope?

[rmcilroy, 2015/10/15 09:23:07]

I could do this, however personally I think it is a bit weird that NeedsContext
is only valid for block scopes - is there any particular reason for this that
I'm missing? As for making this private - it is called by Fullcodegen,
Crankshaft and TF, so I would need to change a bunch of other places if I made
this private.

[adamk, 2015/10/15 09:34:32]

What I'm saying is that testing "is_with_scope()" for cases where the code knows
it has a block scope (or in the case of your other caller, a function scope)
seems a bit odd. But I guess it doesn't cause any problems, so I'll take your
perspective as someone new to this code as advise here, and since this
additional check won't cause any problems, it's fine.

[rmcilroy, 2015/10/15 14:06:16]

Acknowledged.

+  }
 
   bool NeedsHomeObject() const {
     return scope_uses_super_property_ ||
@@ -519,6 +524,10 @@ class Scope: public ZoneObject {
   // The number of contexts between this and scope; zero if this == scope.
   int ContextChainLength(Scope* scope);
 
+  // The maximum number of nested contexts required for this scope and any inner
+  // scopes.
+  int MaxNestedContextChainLength();
+
   // Find the first function, script, eval or (declaration) block scope. This is
   // the scope where var declarations will be hoisted to in the implementation.
   Scope* DeclarationScope();