[LLVMdev] [RFC] SCEV Enhancements

[Dan Gohman]

On Feb 23, 2012, at 8:20 AM, David Greene wrote:

> I have a few enhancements to SCEV to help it identify more types of
> recurrences.  I want to send these upstream but I need a little
> feedback.
> 
> I've added ptrtoint and inttoptr SCEV expressions so that recurrences
> involving these constructs can be recognized.  However, this scary
> comment gives me pause:
> 
>  // It's tempting to handle inttoptr and ptrtoint as no-ops, however this can
>  // lead to pointer expressions which cannot safely be expanded to GEPs,
>  // because ScalarEvolution doesn't respect the GEP aliasing rules when
>  // simplifying integer expressions.
> 
> Now, we don't handle them as no-ops because we create explicit
> expression for them, similar to how ZExt and SExt work.  So any pass
> using the analysis has to explicitly look through a ptrtoint or inttoptr
> while examining a SCEV expression.
> 
> 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.

 - If users are casting pointers to integers, do one or more of:
    (a) Tell them to fix their code.
    (b) Try to convert the code to use getelementptrs in instcombine.
    (c) Give up on higher-level optimizations, because such low-level
        code is can be considered to have already been hand-optimized.

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.

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

Dan