Propagate pseudo-constants expressions separately in tree rewriter.

Propagate pseudo-constants expressions separately in tree rewriter.

Two constants would fail to swap places even though there clearly is no
danger of swapping side effects. Moreover, a constant would prevent other
expressions from propagating.

Example:
var z = foo();
var y = 1;
bar('x', y, z); // neither foo() or 1 propagates past 'x'

This change may be made redundant in the future by a more sophisticated
side-effect analysis.

But for now, it helps debugging since the code is more readable and
it's easier to see if other rewritings are working when they are not
blocked by redundant variable assignments.

BUG=
R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=43953

[asgerf, 2015-02-11 17:29:49]

asgerf@google.com changed reviewers:
+ kmillikin@google.com

[Kevin Millikin (Google), 2015-02-12 11:52:51]

Yay!  LGTM.

https://codereview.chromium.org/918653002/diff/1/pkg/compiler/lib/src/tree_ir...
File pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart (right):

https://codereview.chromium.org/918653002/diff/1/pkg/compiler/lib/src/tree_ir...
pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart:203: // the
previous assignment might still propagate.
Maybe "previous non-constant assignment" or "immediately previous non-constant
assignment" just to make it more clear.

[asgerf, 2015-02-12 12:27:18]

New patchsets have been uploaded after l-g-t-m from kmillikin@google.com

[asgerf, 2015-02-12 12:31:14]

Bugfix: I had to strengthen the requirement for an assignment to be inlinable as
a constant.

https://codereview.chromium.org/918653002/diff/1/pkg/compiler/lib/src/tree_ir...
File pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart (right):

https://codereview.chromium.org/918653002/diff/1/pkg/compiler/lib/src/tree_ir...
pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart:203: // the
previous assignment might still propagate.
On 2015/02/12 11:52:51, kmillikin wrote:
> Maybe "previous non-constant assignment" or "immediately previous non-constant
> assignment" just to make it more clear.

Rephrased a bit.

https://codereview.chromium.org/918653002/diff/20001/pkg/compiler/lib/src/tre...
File pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart (right):

https://codereview.chromium.org/918653002/diff/20001/pkg/compiler/lib/src/tre...
pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart:193:
node.variable.writeCount == 1) {
Unfortunately, other expressions in [environment] might have references to
node.variable. And these must be blocked because the assignment itself has a
side-effect.

Added a test case to capture this.

[Kevin Millikin (Google), 2015-02-12 14:25:43]

Good catch.  LGTM.

[asgerf, 2015-02-23 12:51:35]

New patchsets have been uploaded after l-g-t-m from kmillikin@google.com

[asgerf, 2015-02-23 12:57:50]

Committed patchset #4 (id:60001) manually as 43953 (presubmit successful).