VM: Improve phi-elimination to improve code inside of try-catch.

VM: Improve phi-elimination to improve code inside of try-catch.

Since we don't have precise liveness information inside try-catch blocks,
there are often redundant phis inside try-catch of the form

v <- phi(v, v, ..., R, v, v, ...)

This CL finds these redundant phis and replaces them with R.

BUG=
R=rmacnak@google.com

Committed: https://github.com/dart-lang/sdk/commit/6d537d7e61683611c6d0b42fb3f559313e47f0e8

[Florian Schneider, 2016-04-25 16:18:05]

fschneider@google.com changed reviewers:
+ rmacnak@google.com

[rmacnak, 2016-04-25 17:45:45]

lgtm

[Vyacheslav Egorov (Google), 2016-04-25 17:46:01]

vegorov@google.com changed reviewers:
+ vegorov@google.com

[Vyacheslav Egorov (Google), 2016-04-25 17:46:01]

what about phis like:

p0 <- phi (r, p1)
...
p1 <- phi (p0, r)

?

Seems too specific to limit ourselves to a specific form of phis even though we
have some more generic handling in some other places (e.g. I thought phi emit
code in load forwarding should be capable of handling phis like this).

[Florian Schneider, 2016-04-25 21:46:40]

On 2016/04/25 17:46:01, Vyacheslav Egorov (Google) wrote:
> what about phis like:
> 
> p0 <- phi (r, p1)
> ...
> p1 <- phi (p0, r)
> 
> ?
> 
> Seems too specific to limit ourselves to a specific form of phis even though
we
> have some more generic handling in some other places (e.g. I thought phi emit
> code in load forwarding should be capable of handling phis like this).

No, EmitPhi can replace one phi with another congruent phi, but not with an
arbitrary definition.

We could make it more general, but it seems that the case v <- phi(r, v) is by
far the most common - I looked at loops inside try-catch.

Ok, if I add a TODO for now?

[Florian Schneider, 2016-04-26 10:50:30]

Description was changed from

==========
VM: Improve phi-elimination to improve code inside of try-catch.

Since we don't have precise liveness information inside try-catch blocks,
there are often redundant phis inside try-catch of the form

v <- phi(v, v, ..., R, v, v, ...)

This CL finds these redundant phis and replaces them with R.

BUG=
==========

to

==========
VM: Improve phi-elimination to improve code inside of try-catch.

Since we don't have precise liveness information inside try-catch blocks,
there are often redundant phis inside try-catch of the form

v <- phi(v, v, ..., R, v, v, ...)

This CL finds these redundant phis and replaces them with R.

BUG=
R=rmacnak@google.com

Committed:
https://github.com/dart-lang/sdk/commit/6d537d7e61683611c6d0b42fb3f559313e47f0e8
==========

[Florian Schneider, 2016-04-26 10:50:31]

Committed patchset #2 (id:20001) manually as
6d537d7e61683611c6d0b42fb3f559313e47f0e8 (presubmit successful).

[Vyacheslav Egorov (Google), 2016-04-27 09:19:22]

LoadOptimizer::EliminateRedundantPhi(PhiInstr* phi) can replace phi with
arbitrary definition. Am I missing something?