Re: Widening multiplication, but no narrowing division [i386/AMD64]

[Paul Koning <paulkoning@comcast.net>]

> On Jan 9, 2023, at 11:27 AM, Stefan Kanthak <stefan.kanthak@nexgo.de> wrote:
> 
> "Paul Koning" <paulkoning@comcast.net> wrote:
> 
>>> ...
> 
>> Yes, I was thinking the same.  But I spent a while on that pattern -- I
>> wanted to support div/mod as a single operation because the machine has
>> that primitive.  And I'm pretty sure I saw it work before I committed
>> that change.  That's why I'm wondering if something changed.
> 
> I can't tell from the past how GCC once worked, but today it can't
> (or doesn't) use such patterns, at least not on i386/AMD64 processors.

It turns out I was confused by the RTL generated by my pattern.  That pattern is for divmodhi, so it works as desired given same-size inputs.  

I'm wondering if the case of longer dividend -- which is a common thing for several machines -- could be handled by a define_peephole2 that matches the sign-extend of the divisor followed by the (longer) divide.  I made a stab at that but what I wrote wasn't valid.

So, question to the list:  suppose I want to write RTL that matches what Stefan is talking about, with a div or mod or divmod that has si results and a di dividend (or hi results and an si dividend), how would you do that?  Can a define_peephole2 do it, and if so, what would it look like?

	paul