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Re: Question about always executed info computed in tree-ssa-loop-im.c


On Wed, Jul 8, 2015 at 12:01 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Wed, Jul 8, 2015 at 5:58 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>> On Wed, Jul 8, 2015 at 5:51 PM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>> On Wed, Jul 8, 2015 at 8:52 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>> Hi,
>>>> Function fill_always_executed_in_1 computes basic blocks' always
>>>> executed information, and it has below code and comment:
>>>>
>>>>           /* In a loop that is always entered we may proceed anyway.
>>>>          But record that we entered it and stop once we leave it.  */
>>>>           inn_loop = bb->loop_father;
>>>>
>>>> Then in following iterations, it breaks the loop if basic block not
>>>> belonging to the inner loop is encountered.  This means basic blocks
>>>> after inner loop won't have always executed information computed, even
>>>> they dominates the original loop's latch.
>>>>
>>>> Am I missing something?  Why is that?
>>>
>>> To improve here it would need to verify that all exits of the inner loop
>>> exit to the same BB of the outer loop and that no exit skips any blocks
>>> in the outer loop.  Like for
>> But we are working on dominating tree anyway.  Won't dominating latch be enough?

Hmm, yes.

>> Thanks,
>> bin
>>>
>>>   for (;;)
>>>     {
>>>       for (;;) { if (x) goto skip; if (y) break }
>>>       foo();
>>>     skip:
>>>     }
>>>
>>> so it is just a simple and conservative algorithm it seems.  It's also
>>> quadratic in the number of BBs of the outermost loop.
> What we need to do is check if inner loop's exit goes to basic block
> not belonging to outer loop?

Doesn't

          FOR_EACH_EDGE (e, ei, bb->succs)
            if (!flow_bb_inside_loop_p (loop, e->dest))
              break;
          if (e)
            break;

catch this?

Richard.

> Thanks,
> bin


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