[turbofan]: Add a context relaxation Reducer

[turbofan]: Add a context relaxation Reducer

Ensures that operations that only need to access the native context use
the outer-most context rather than the specific context given by the AST
graph builder. This makes it possible to use the operations with context
specialization (e.g. for generating stubs) without forcing inner
contexts to be embedded in generated code thus causing leaks.

R=mstarzinger@chromium.org

[danno, 2015-07-01 09:11:49]

I should add tests, but it's unclear if they will be anything other than a
trivial copy of the logic that is in the reducer itself.

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
File src/compiler/context-relaxation.cc (right):

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.cc:28: if (!context_is_wired_) {
This is a bit weird, any better ideas?

[Michael Starzinger, 2015-07-02 10:29:27]

LGTM.

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
File src/compiler/context-relaxation.cc (right):

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.cc:14: Reduction ContextRelaxation::Reduce(Node*
node) {
I think it would make sense to rename this class to JSContextRelaxation and the
file to js-context-relaxation.cc, WDYT?

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.cc:28: if (!context_is_wired_) {
On 2015/07/01 09:11:48, danno wrote:
> This is a bit weird, any better ideas?

As discussed offline: We could move the parameter into the JSGraph, thereby
graph trimming would not kill it. Beut we can do that in a follow-up CL.

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.cc:29: if (relaxed_context_->InputAt(0) ==
nullptr) {
Can we please use relaxed_context_->IsDead() instead?

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
File src/compiler/context-relaxation.h (right):

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.h:17: // builder. The makes it possible to use
they operations with context
nit: s/The/This/ and s/they/these/

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.h:18: // specialization (e.g. for generating
stubs) without embedding forcing inner
nit: s/embedding//

https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
src/compiler/context-relaxation.h:22: explicit ContextRelaxation(Node* start,
Node* relaxed_context)
nit: Doesn't need to be marked "explicit".

https://codereview.chromium.org/1220823004/diff/1/src/compiler/pipeline.cc
File src/compiler/pipeline.cc (right):

https://codereview.chromium.org/1220823004/diff/1/src/compiler/pipeline.cc#ne...
src/compiler/pipeline.cc:295: Node* relaxed_context_;
nit: Please reset this to {nullptr} in PipelineData::DeleteGraphZone.

[titzer, 2015-07-02 14:48:58]

On 2015/07/01 09:11:49, danno wrote:
> I should add tests, but it's unclear if they will be anything other than a
> trivial copy of the logic that is in the reducer itself.

You should add tests to verify that the reducer actually makes a change to the
graph. Otherwise, we might end up breaking it through refactoring.

> 
>
https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
> File src/compiler/context-relaxation.cc (right):
> 
>
https://codereview.chromium.org/1220823004/diff/1/src/compiler/context-relaxa...
> src/compiler/context-relaxation.cc:28: if (!context_is_wired_) {
> This is a bit weird, any better ideas?

[titzer, 2015-07-02 14:52:07]

titzer@chromium.org changed reviewers:
+ titzer@chromium.org

[titzer, 2015-07-02 14:52:07]

https://codereview.chromium.org/1220823004/diff/20001/src/compiler/ast-graph-...
File src/compiler/ast-graph-builder.h (right):

https://codereview.chromium.org/1220823004/diff/20001/src/compiler/ast-graph-...
src/compiler/ast-graph-builder.h:55: Node* relaxed_context() const { return
relaxed_context_; }
Can we just call this the outer_context_param() ?