Hi All,

Here is implementation of Richard proposal:

< For general infrastructure it would be nice to expose a (post-)dominator
< compute for MESE (post-dominators) / SEME (dominators) regions.  I believe
< what makes if-conversion expensive is the post-dom compute which happens
< for each loop for the whole function.  It shouldn't be very difficult
< to write this,
< sharing as much as possible code with the current DOM code might need
< quite some refactoring though.

I implemented this proposal by adding calculation of dominance info
for SESE regions and incorporate this change to if conversion pass.
SESE region is built by adding loop pre-header and possibly fake
post-header blocks to loop body. Fake post-header is deleted after
predication completion.

Bootstrapping and regression testing did not show any new failures.

Is it OK for trunk?

ChangeLog:
2016-10-05  Yuri Rumyantsev  <ysrumyan@gmail.com>

* dominance.c : Include cfgloop.h for loop recognition.
(dom_info): Add new functions and add boolean argument to recognize
computation for loop region.
(dom_info::dom_info): New function.
(dom_info::calc_dfs_tree): Add boolean argument IN_REGION to not
handle unvisited blocks.
(dom_info::calc_idoms): Likewise.
(compute_dom_fast_query_in_region): New function.
(calculate_dominance_info): Invoke calc_dfs_tree and calc_idoms with
false argument.
(calculate_dominance_info_for_region): New function.
(free_dominance_info_for_region): Likewise.
(verify_dominators): Invoke calc_dfs_tree and calc_idoms with false
argument.
* dominance.h: Add prototype for introduced functions
calculate_dominance_info_for_region and
free_dominance_info_for_region.
tree-if-conv.c: Add to local variables ifc_sese_bbs & fake_postheader.
(build_sese_region): New function.
(if_convertible_loop_p_1): Invoke local version of post-dominators
calculation, free it after basic block predication and delete created
fake post-header block if any.
(tree_if_conversion): Delete call of free_dominance_info for
post-dominators, free ifc_sese_bbs which represents SESE region.
(pass_if_conversion::execute): Delete detection of infinite loops
and fake edges to exit block since post-dominator calculation is
performed per if-converted loop only.