Re: [PATCH V4] Optimize '(X - N * M) / N' to 'X / N - M' if valid

[Aldy Hernandez <aldyh@redhat.com>]

On 7/14/23 15:37, Richard Biener wrote:
> On Fri, 14 Jul 2023, Aldy Hernandez wrote:
> 
>> I don't know what you're trying to accomplish here, as I haven't been
>> following the PR, but adding all these helper functions to the ranger header
>> file seems wrong, especially since there's only one use of them. I see you're
>> tweaking the irange API, adding helper functions to range-op (which is only
>> for code dealing with implementing range operators for tree codes), etc etc.
>>
>> If you need these helper functions, I suggest you put them closer to their
>> uses (i.e. wherever the match.pd support machinery goes).
> 
> Note I suggested the opposite beacuse I thought these kind of helpers
> are closer to value-range support than to match.pd.

Oh sorry, I missed that.

> 
> But I take away from your answer that there's nothing close in the
> value-range machinery that answers the question whether A op B may
> overflow?

Not currently.

I vaguely recall we talked about some mechanism for doing range 
operations in a wider precision and comparing them with the result of 
doing it in the natural precision, and if the results differ, it must 
have overflowed.

*hunts down PR*

Comment 23 here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100499#c23

Would something like that work?

I would prefer something more general, rather than having to re-invent 
every range-op entry to check for overflow.

Aldy