VM: Fix illegal code motion.

VM: Fix illegal code motion.

Without tracking dependencies more explicitly we can't propagate the
cid of values whose cid is guarded by a CheckClassId:

The CheckClassId c may not be hoisted by LICM because it is guarded
by a Redefinition r. However, after constant propagation and branch simplification
code that was not previously dominated by c (and r) may become dominated by c and r.

This code won't be using the redefined value r, but the original value and therefore
LICM can illegally eliminated checks / hoist dependent loads.

BUG=
R=vegorov@google.com

Committed: https://github.com/dart-lang/sdk/commit/b043314bce0e18e4b87f736d672f6b8dd60ccd6e

[Florian Schneider, 2016-04-27 16:46:37]

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

[Vyacheslav Egorov (Google), 2016-04-27 16:50:26]

LGTM as a stop gap. 

Seems like a serious pessimization for polymorphic code. Could you open a bug
for this?

[Florian Schneider, 2016-04-27 17:03:24]

On 2016/04/27 16:50:26, Vyacheslav Egorov (Google) wrote:
> LGTM as a stop gap. 
> 
> Seems like a serious pessimization for polymorphic code. Could you open a bug
> for this?

Opened https://github.com/dart-lang/sdk/issues/26347.

I'd like to fix this in a nicer way, too! Let's chat next week.

[Florian Schneider, 2016-04-27 17:05:24]

Description was changed from

==========
VM: Fix illegal code motion.

Without tracking dependencies more explicitly we can't propagate the
cid of values whose cid is guarded by a CheckClassId:

The CheckClassId c may not be hoisted by LICM because it is guarded
by a Redefinition r. However, after constant propagation and branch
simplification
code that was not previously dominated by c (and r) may become dominated by c
and r.

This code won't be using the redefined value r, but the original value and
therefore
LICM can illegally eliminated checks / hoist dependent loads.

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

to

==========
VM: Fix illegal code motion.

Without tracking dependencies more explicitly we can't propagate the
cid of values whose cid is guarded by a CheckClassId:

The CheckClassId c may not be hoisted by LICM because it is guarded
by a Redefinition r. However, after constant propagation and branch
simplification
code that was not previously dominated by c (and r) may become dominated by c
and r.

This code won't be using the redefined value r, but the original value and
therefore
LICM can illegally eliminated checks / hoist dependent loads.

BUG=
R=vegorov@google.com

Committed:
https://github.com/dart-lang/sdk/commit/b043314bce0e18e4b87f736d672f6b8dd60ccd6e
==========

[Florian Schneider, 2016-04-27 17:05:25]

Committed patchset #1 (id:1) manually as
b043314bce0e18e4b87f736d672f6b8dd60ccd6e (presubmit successful).