On 10/05/2015 03:02 AM, Richard Biener wrote:
> On Fri, Oct 2, 2015 at 9:30 PM, Jeff Law <law@redhat.com> wrote:
>> On 10/02/2015 05:15 AM, Renlin Li wrote:
>>>
>>> Hi Jeff,
>>>
>>> Your patch causes an ICE regression.
>>> The test case is " gcc.c-torture/compile/pr27087.c", I observed it on
>>> aarch64-none-elf target when compiling the test case with '-Os' flag.
>>>
>>> A quick check shows, the cfg has been changed, but the loop information
>>> is not updated. Thus the information about the number of basic block in
>>> a loop is not reliable.
>>>
>>> Could you please have a look?
>>
>> As I mentioned, when we collapse a conditional inside a loop, we may change
>> the # of nodes in a loop which edges are exit edges and possibly other
>> stuff.  So we need to mark loops as needing fixups.
>>
>> Verified this fixes the aarch64-elf regression and did a bootstrap &
>> regression test on x86_64-linux-gnu.
>>
>> Installed on the trunk.
>>
>> jeff
>>
>> commit 992d281b2d1ba53a49198db44fee92a505e16f5d
>> Author: Jeff Law <law@tor.usersys.redhat.com>
>> Date:   Fri Oct 2 15:22:04 2015 -0400
>>
>>      Re: [PATCH] Improve DOM's optimization of control statements
>>
>>          * tree-ssa-dom.c (optimize_stmt): Note when loop structures need
>>          fixups.
>>
>> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
>> index 3f7561a..e541df3 100644
>> --- a/gcc/ChangeLog
>> +++ b/gcc/ChangeLog
>> @@ -1,3 +1,8 @@
>> +2015-10-02  Jeff Law  <law@redhat.com>
>> +
>> +       * tree-ssa-dom.c (optimize_stmt): Note when loop structures need
>> +       fixups.
>> +
>>   2015-10-02  Uros Bizjak  <ubizjak@gmail.com>
>>
>>          * system.h (ROUND_UP): New macro definition.
>> diff --git a/gcc/tree-ssa-dom.c b/gcc/tree-ssa-dom.c
>> index a8b7038..d940816 100644
>> --- a/gcc/tree-ssa-dom.c
>> +++ b/gcc/tree-ssa-dom.c
>> @@ -1843,6 +1843,12 @@ optimize_stmt (basic_block bb, gimple_stmt_iterator
>> si,
>>                /* Delete threads that start at BB.  */
>>                remove_jump_threads_starting_at (bb);
>>
>> +             /* If BB is in a loop, then removing an outgoing edge from BB
>> +                may cause BB to move outside the loop, changes in the
>> +                loop exit edges, etc.  So note that loops need fixing.  */
>> +             if (bb_loop_depth (bb) > 0)
>> +               loops_state_set (LOOPS_NEED_FIXUP);
>> +
>
> I would rather do this in remove_ctrl_stmt_and_useless_edges and only
> if taken_edge is a loop exit.  loop fixup is a pretty big hammer which
> we should avoid at all cost.
>
> So please try to be more specific on the cases you invoke it.
What's probably the most interesting is we don't actually have 
EDGE_LOOP_EXIT set in DOM.   So we can't use that, but that also implies 
it doesn't need updating.  Thankfully, there's an alternate test we can 
use instead.

And after more ponderings, I'm pretty sure the only case that's of 
concern right now is nodes moving out of the loop, which only happens 
when we have a BB with a loop exit edge and we delete the *other* edges. 
  We've got simple latches & preheaders and I don't think we destroy 
that property.   Both properties also simplify the things we need look for.

So I've minimized use of the hammer to just the case where blocks are 
going to be moving out of the loop.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu and 
verified the ARM test which originally spurred this change continues to 
work.

Installed on the trunk.

Jeff