Rerun GVN if load elimination creates GVN candidates.

Rerun GVN if load elimination creates GVN candidates.

This does not fire often but is occasionally useful. e.g

      t3 = t2._collection$_length;
      if (t3 === 0)
        timer = null;
      else {
        if (t3 === 0)
          H.throwExpression(new P.StateError("No such element"));
        timer = t2._collection$_first;
      }

-->

      if (t2._collection$_length === 0)
        timer = null;
      else
        timer = t2._collection$_first;


After load-elimination creates t3 = t2._collections$_length, t3 == 0 becomes a GVN candidate. Then the inner "x" in "if (x) ... else { if (x) ... }" is replaced by "false".
The original code was something like "timer = t2.isEmpty ? null : t2.first".

BUG=
R=sigmund@google.com

Committed: https://github.com/dart-lang/sdk/commit/d6e324f0565b54d500987262821069dde498b584

[sra1, 2016-10-19 21:48:55]

Patchset #1 (id:1) has been deleted

[sra1, 2017-03-22 07:30:26]

Description was changed from

==========
Rerun GVN if load elimination creates GVN candidates.

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

to

==========
Rerun GVN if load elimination creates GVN candidates.

This does not fire often but is occasionally useful. e.g 

      t3 = t2._collection$_length;
      if (t3 === 0)
        timer = null;
      else {
        if (t3 === 0)
          H.throwExpression(new P.StateError("No such element"));
        timer = t2._collection$_first;
      }

-->

      if (t2._collection$_length === 0)
        timer = null;
      else
        timer = t2._collection$_first;


After load-elimination creates t3 = t2._collections$_length, t3 == 0 becomes a
GVN candidate. Then the inner "x" in "if (x) ... else { if (x) ... }" is
replaced by "false".
The original code was something like "timer = t2.isEmpty ? null : t2.first".

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

[sra1, 2017-03-22 07:30:27]

sra@google.com changed reviewers:
+ sigmund@google.com

[Siggi Cherem (dart-lang), 2017-03-22 16:20:10]

lgtm

[sra1, 2017-03-22 19:17:30]

Description was changed from

==========
Rerun GVN if load elimination creates GVN candidates.

This does not fire often but is occasionally useful. e.g 

      t3 = t2._collection$_length;
      if (t3 === 0)
        timer = null;
      else {
        if (t3 === 0)
          H.throwExpression(new P.StateError("No such element"));
        timer = t2._collection$_first;
      }

-->

      if (t2._collection$_length === 0)
        timer = null;
      else
        timer = t2._collection$_first;


After load-elimination creates t3 = t2._collections$_length, t3 == 0 becomes a
GVN candidate. Then the inner "x" in "if (x) ... else { if (x) ... }" is
replaced by "false".
The original code was something like "timer = t2.isEmpty ? null : t2.first".

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

to

==========
Rerun GVN if load elimination creates GVN candidates.

This does not fire often but is occasionally useful. e.g

      t3 = t2._collection$_length;
      if (t3 === 0)
        timer = null;
      else {
        if (t3 === 0)
          H.throwExpression(new P.StateError("No such element"));
        timer = t2._collection$_first;
      }

-->

      if (t2._collection$_length === 0)
        timer = null;
      else
        timer = t2._collection$_first;


After load-elimination creates t3 = t2._collections$_length, t3 == 0 becomes a
GVN candidate. Then the inner "x" in "if (x) ... else { if (x) ... }" is
replaced by "false".
The original code was something like "timer = t2.isEmpty ? null : t2.first".

BUG=
R=sigmund@google.com

Committed:
https://github.com/dart-lang/sdk/commit/d6e324f0565b54d500987262821069dde498b584
==========

[sra1, 2017-03-22 19:17:31]

Committed patchset #2 (id:40001) manually as
d6e324f0565b54d500987262821069dde498b584 (presubmit successful).