Eliminate unnecessary deoptimization environments.

Eliminate unnecessary deoptimization environments.

This CL adds a CanOptimize predicate to every instruction. Before register
allocation there is a simple pass over the IR to remove environments from
instruction that never deoptmize. This reduces the number of uses and shortens
live ranges of values resulting in better code.
Committed: https://code.google.com/p/dart/source/detail?r=10115

[srdjan, 2012-07-31 15:53:15]

Good idea & LGTM with comments.

Please add ASSERTS in EmitNatives wherever we deoptimizen that CanDeoptimize is
set true.

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/flow_graph_compiler.cc
File vm/flow_graph_compiler.cc (right):

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/flow_graph_compiler...
vm/flow_graph_compiler.cc:204: ASSERT((pending_deoptimization_env_ != NULL) ||
!is_ssa_);
Alternative would be to split this into two asserts and move them inside the
if-s

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
File vm/intermediate_language.h (right):

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
vm/intermediate_language.h:150: // we assume every computation can deoptimize.
The comment (by default we return true) does not match the implementation
(abstract method). Change method to return true or change comment.

[srdjan, 2012-07-31 19:49:28]

We plan to use CHA for optimization. That means that we will generate code that
may become invalid if another class is loaded dynamically, that means that we
need to be able to deoptimize at call sites as well.

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
File vm/intermediate_language.h (right):

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
vm/intermediate_language.h:521: virtual bool CanDeoptimize() const { return
false; }
All calls have to return true (future work).

[Florian Schneider, 2012-08-01 11:46:35]

Yes, the predicate has to be changed in the future once we can deoptimize at
calls.

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/flow_graph_compiler.cc
File vm/flow_graph_compiler.cc (right):

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/flow_graph_compiler...
vm/flow_graph_compiler.cc:204: ASSERT((pending_deoptimization_env_ != NULL) ||
!is_ssa_);
On 2012/07/31 15:53:15, srdjan wrote:
> Alternative would be to split this into two asserts and move them inside the
> if-s

Done.

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
File vm/intermediate_language.h (right):

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
vm/intermediate_language.h:150: // we assume every computation can deoptimize.
On 2012/07/31 15:53:15, srdjan wrote:
> The comment (by default we return true) does not match the implementation
> (abstract method). Change method to return true or change comment.

Done.

https://chromiumcodereview.appspot.com/10829098/diff/1/vm/intermediate_langua...
vm/intermediate_language.h:521: virtual bool CanDeoptimize() const { return
false; }
On 2012/07/31 19:49:28, srdjan wrote:
> All calls have to return true (future work).

Yes. It will have to change once we have something like that. If we accidentally
remove an environment that is needed the ASSERT in the code generator will
trigger.