Add pass to remove empty blocks and recompute fall-through targets.

Add pass to remove empty blocks and recompute fall-through targets.

This allows to cleanup the following code patterns:

1) chains of jumps:

  jxx block_a
...
block_a:
  jmp block_b


2) jumps over jumps

  jxx block_b
block_a:
  jmp block_c
block_b:
  ...

Committed: https://code.google.com/p/dart/source/detail?r=19711

[Vyacheslav Egorov (Google), 2013-03-08 17:47:45]

Please take a look.

[srdjan, 2013-03-08 17:50:42]

The common pattern (e.g., BubbleSort::sort) is:
  ..
  jmp C
B:
  jmp C
C:

Have you tested if that can be handled as well?

[srdjan, 2013-03-08 19:02:31]

LGTM

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compiler.h
File runtime/vm/flow_graph_compiler.h (right):

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compile...
runtime/vm/flow_graph_compiler.h:175: Label* fallthrough_label() const { return
fallthrough_label_; }
ASSERT that is not NULL?

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compile...
runtime/vm/flow_graph_compiler.h:520: 
Remove one line

[Vyacheslav Egorov (Google), 2013-03-08 19:04:23]

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compiler.h
File runtime/vm/flow_graph_compiler.h (right):

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compile...
runtime/vm/flow_graph_compiler.h:175: Label* fallthrough_label() const { return
fallthrough_label_; }
On 2013/03/08 19:02:31, srdjan wrote:
> ASSERT that is not NULL?

It can be NULL for the last block which has no fallthrough.

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compile...
runtime/vm/flow_graph_compiler.h:520: 
On 2013/03/08 19:02:31, srdjan wrote:
> Remove one line

Done.

[srdjan, 2013-03-08 19:08:08]

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compiler.h
File runtime/vm/flow_graph_compiler.h (right):

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compile...
runtime/vm/flow_graph_compiler.h:175: Label* fallthrough_label() const { return
fallthrough_label_; }
On 2013/03/08 19:04:23, Vyacheslav Egorov (Google) wrote:
> On 2013/03/08 19:02:31, srdjan wrote:
> > ASSERT that is not NULL?
> 
> It can be NULL for the last block which has no fallthrough.

Right. Maybe add a comment that it is NULL for blocks without fall-throughs..

[Vyacheslav Egorov (Google), 2013-03-08 19:10:04]

Committed patchset #1 manually as r19711 (presubmit successful).

[Vyacheslav Egorov (Google), 2013-03-08 19:10:11]

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compiler.h
File runtime/vm/flow_graph_compiler.h (right):

https://codereview.chromium.org/12412013/diff/1/runtime/vm/flow_graph_compile...
runtime/vm/flow_graph_compiler.h:175: Label* fallthrough_label() const { return
fallthrough_label_; }
On 2013/03/08 19:08:08, srdjan wrote:
> On 2013/03/08 19:04:23, Vyacheslav Egorov (Google) wrote:
> > On 2013/03/08 19:02:31, srdjan wrote:
> > > ASSERT that is not NULL?
> > 
> > It can be NULL for the last block which has no fallthrough.
> 
> Right. Maybe add a comment that it is NULL for blocks without fall-throughs..

Done.