[LLVMdev] [RFC] SCEV Enhancements

[David A. Greene]

Dan Gohman <gohman at apple.com> writes:

>> These enhancements are critical for us because of the way our frontends
>> work (less than ideal but we have to deal with it), due to some language
>> quirks (casting in C, odd Fortran constructs, etc.) and because users
>> sometimes stretch the boundaries of good taste :).
>> 
>> Is this a reasonable approach?  Is this acceptable to upstream?
>
> Right now, the main strategy is:
>
>  - If front-ends are using ptrtoint+add+inttoptr to do addressing,
>    convert them to use bitcast+getelementptr+bitcast instead. You
>    can do the getelementptr with i8* so there's no scaling [0]
>    if all your offsets are raw byte offsets.

We already do that as much as possible but it doesn't cover every case
currently.  I will see if we can cover more cases.

>  - If users are casting pointers to integers, do one or more of:
>     (a) Tell them to fix their code.

Not possible, unfortunately.

>     (b) Try to convert the code to use getelementptrs in instcombine.

We added a pass to do this but it doesn't get everything.  I'll see if
we can enhance it.

>     (c) Give up on higher-level optimizations, because such low-level
>         code is can be considered to have already been hand-optimized.

We can't do that either.

> This is a compromise, but there are several reasons why it makes
> sense -- other passes like getelementptrs too, and it reduces complexity
> to avoid having everything understand inttoptr when most people
> don't need it. If you want to change this, it'd be helpful to show
> a code example to show motivation.

Sure.  I'll try tweaking instcombine first and see where that gets us.
I have some pretty good testcases.  I'd be thrilled if we could just
dump these SCEV changes.  That's why I did the original GEP instcombine
stuff.  I actually ripped out the SCEV changes after that and
performance tanked.  :(

> [0] or i1* if you feel like being a purist ;-).

Err?  What are the scaling semantics for i1?  I don't immediately see
that it should be the same as i8.

                               -Dave