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

["Bin.Cheng" <amker dot cheng at gmail dot com>]

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?

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.
>
>> Thanks,
>> bin