dart2js cps: Better fallthrough analysis and eliminate "return null".

dart2js cps: Better fallthrough analysis and eliminate "return null".

BUG=
R=kmillikin@google.com

Committed: https://github.com/dart-lang/sdk/commit/e7e3706c597e7043113381673a9bf601c00153a6

[asgerf, 2015-06-25 14:20:54]

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

[Kevin Millikin (Google), 2015-06-26 09:12:32]

lgtm

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/js...
File pkg/compiler/lib/src/js_backend/codegen/codegen.dart (right):

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/js...
pkg/compiler/lib/src/js_backend/codegen/codegen.dart:440: bool thenMayComplete =
(fallthrough.useCount > usesBefore);
thenMayComplete seems ambiguous (it also completes if it returns, doesn't it?). 
It doesn't matter a bunch because it's local, but what do you think of
thenHasFallthrough, thenFallsThrough or similar?

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/js...
pkg/compiler/lib/src/js_backend/codegen/codegen.dart:461: tree_ir.Label label,
Crazy indentation.

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/tr...
File pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart (right):

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/tr...
pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart:121: if
(isFallthrough(node.thenStatement) &&
I could figure out the why these are better in each case, but I had to scratch
my head.  I'd spell out the reasoning for when someone is tempted to improve the
code.

[asgerf, 2015-06-26 10:52:30]

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/js...
File pkg/compiler/lib/src/js_backend/codegen/codegen.dart (right):

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/js...
pkg/compiler/lib/src/js_backend/codegen/codegen.dart:440: bool thenMayComplete =
(fallthrough.useCount > usesBefore);
On 2015/06/26 09:12:32, Kevin Millikin (Google) wrote:
> thenMayComplete seems ambiguous (it also completes if it returns, doesn't
it?). 
> It doesn't matter a bunch because it's local, but what do you think of
> thenHasFallthrough, thenFallsThrough or similar?

Agreed, that's more clear.

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/js...
pkg/compiler/lib/src/js_backend/codegen/codegen.dart:461: tree_ir.Label label,
On 2015/06/26 09:12:32, Kevin Millikin (Google) wrote:
> Crazy indentation.

Done.

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/tr...
File pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart (right):

https://codereview.chromium.org/1203423003/diff/20001/pkg/compiler/lib/src/tr...
pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart:121: if
(isFallthrough(node.thenStatement) &&
On 2015/06/26 09:12:32, Kevin Millikin (Google) wrote:
> I could figure out the why these are better in each case, but I had to scratch
> my head.  I'd spell out the reasoning for when someone is tempted to improve
the
> code.

Done.

[asgerf, 2015-06-26 12:14:17]

Committed patchset #6 (id:100001) manually as
e7e3706c597e7043113381673a9bf601c00153a6 (presubmit successful).