Optimize conditional branches that have same true/false targets.

Optimize conditional branches that have same true/false targets.

Branches that have the same true/false target, or where both
target blocks reach the same common join block via empty blocks
can be replaced by a goto to the common join block.

In code like this

var a = unknown();
var b = null;
if (a == null || b == null) {
    ...
}

it eliminates the test (a == null) if b is known to be null. Until now,
the compiler could only eliminate the test for b, if a was known.

R=vegorov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35669

[Vyacheslav Egorov (Google), 2014-05-01 22:57:05]

lgtm

https://codereview.chromium.org/261823005/diff/20001/runtime/vm/flow_graph_op...
File runtime/vm/flow_graph_optimizer.cc (right):

https://codereview.chromium.org/261823005/diff/20001/runtime/vm/flow_graph_op...
runtime/vm/flow_graph_optimizer.cc:8517: if (changed) {
Maybe we should update the dominator tree in place? All blocks we are removing
are dominated by a single block so it's just boils down to removing subtree.

[Florian Schneider, 2014-05-01 23:11:34]

https://codereview.chromium.org/261823005/diff/20001/runtime/vm/flow_graph_op...
File runtime/vm/flow_graph_optimizer.cc (right):

https://codereview.chromium.org/261823005/diff/20001/runtime/vm/flow_graph_op...
runtime/vm/flow_graph_optimizer.cc:8517: if (changed) {
On 2014/05/01 22:57:05, Vyacheslav Egorov (Google) wrote:
> Maybe we should update the dominator tree in place? All blocks we are removing
> are dominated by a single block so it's just boils down to removing subtree.

I'll add a TODO for now.

[Florian Schneider, 2014-05-01 23:17:30]

Committed patchset #3 manually as r35669 (presubmit successful).