Optimize LocalVarDescriptor objects for functions that have no local variables

Optimize LocalVarDescriptor objects for functions that have no local variables

Allocate a canonical “empty” variable descriptor object. Saves about 10% of
variable descriptor space when running dart2js.

R=asiva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40158

[hausner, 2014-09-10 23:40:58]

hausner@google.com changed reviewers:
+ asiva@google.com

[siva, 2014-09-11 00:04:29]

lgtm

https://codereview.chromium.org/565513002/diff/1/runtime/vm/object.cc
File runtime/vm/object.cc (right):

https://codereview.chromium.org/565513002/diff/1/runtime/vm/object.cc#newcode680
runtime/vm/object.cc:680: empty_var_descriptors_->raw_ptr()->names_ =
empty_array_->raw_ptr();
to be safe:

StorePointer(&empty_var_descriptors_->raw_ptr()->names_,
empty_array_->raw_ptr());

[hausner, 2014-09-11 16:08:12]

https://codereview.chromium.org/565513002/diff/1/runtime/vm/object.cc
File runtime/vm/object.cc (right):

https://codereview.chromium.org/565513002/diff/1/runtime/vm/object.cc#newcode680
runtime/vm/object.cc:680: empty_var_descriptors_->raw_ptr()->names_ =
empty_array_->raw_ptr();
StorePointer() is an instance method, can't use it here. I added a comment
remarking that the pointer assignment is safe here.

On 2014/09/11 00:04:29, siva wrote:
> to be safe:
> 
> StorePointer(&empty_var_descriptors_->raw_ptr()->names_,
> empty_array_->raw_ptr());

[hausner, 2014-09-11 16:09:04]

Committed patchset #2 (id:20001) manually as r40158 (presubmit successful).