Re: PR tree-optimization/57337

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, May 27, 2013 at 10:20 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Sun, May 26, 2013 at 5:53 AM, Easwaran Raman <eraman@google.com> wrote:
>> On Sat, May 25, 2013 at 4:46 AM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>> Easwaran Raman <eraman@google.com> wrote:
>>>
>>>>In that case, if my insert_stmt immediately follows dep_stmt and both
>>>>have the same UID, not_dominated_by would return true and I will end
>>>>up updating insert_stmt to dep_stmt which is wrong.
>>>
>>> But there should be a safe default answer for
>>> Equal uids. Unless we are asking different questions in different places.
>>> Thus, I do not like the stmt walking but rather have a safe fallback.
>>
>> I am lost here. I don't see how we could avoid doing the stmt walking
>> to resolve the equal uid case. How to ensure that not_dominated_by (a,
>> b) returns true and not_dominated_by (b, a) returns false if A and B
>> have the same UID and A appears before B without doing the statement
>> walk. And, I don't see why the statement walk is bad. It is not likely
>> that there is a long sequence of statements with the same UID.
>
> Sure, but if you are always asking a question like "is placing X before Y ok?"
> then you can conservatively answer "no" and code should handle that ok.
> If you are asking questions both way then of course no conservative answer is
> possible.  Both current uses of not_dominated_by are of the same kind,
> if the placement is not ok then either the insert point needs adjustment or
> the debug stmt reset.

Ok, thinking about this more I come to the conclusion that a safe default for
equal UIDs is not possible as we may be not able to order two dep_stmts.
I still dislike walking though, but to get the quite frequent regressions fixed
the patch is ok as-is.

Please install.

Thanks,
Richard.