Implement loop variable assignment analysis.

Implement loop variable assignment analysis.

This analysis computes the set of variables that are assigned in each loop. This is useful to avoid creating redundant loop phis when building an SSA graph, which just waste memory and require analysis to get rid of.

This CL implements an AST walk for the analysis and plugs the result into the TurboFan graph builder. I left this analysis under a flag for A/B testing and until sufficient unit tests can be developed.

R=danno@chromium.org, mstarzinger@chromium.org
BUG=

Committed: https://code.google.com/p/v8/source/detail?r=24957

[titzer, 2014-10-28 13:58:58]

On 2014/10/27 09:06:59, titzer wrote:

Added unit tests, PTAL.

[danno, 2014-10-28 14:23:29]

lgtm

https://codereview.chromium.org/656123005/diff/1/src/compiler/ast-graph-build...
File src/compiler/ast-graph-builder.cc (right):

https://codereview.chromium.org/656123005/diff/1/src/compiler/ast-graph-build...
src/compiler/ast-graph-builder.cc:592: if (loop_assignment_analysis_ == NULL)
return NULL;
This seems a bit weird. Why is it OK to call this at all when
loop_assignment_analysis_ == NULL?

[titzer, 2014-10-28 17:16:03]

https://codereview.chromium.org/656123005/diff/1/src/compiler/ast-graph-build...
File src/compiler/ast-graph-builder.cc (right):

https://codereview.chromium.org/656123005/diff/1/src/compiler/ast-graph-build...
src/compiler/ast-graph-builder.cc:592: if (loop_assignment_analysis_ == NULL)
return NULL;
On 2014/10/28 14:23:29, danno wrote:
> This seems a bit weird. Why is it OK to call this at all when
> loop_assignment_analysis_ == NULL?

It's so that there's one place where we make the decision about whether to
perform the analysis (i.e. in CreateGraph()), and later we just check if there
is an analysis result available.

[titzer, 2014-10-28 17:29:58]

Committed patchset #2 manually as 24957 (presubmit successful).