[LLVMdev] [RFC] SCEV Enhancements
David A. Greene
dag at cray.com
Fri Feb 24 09:53:00 PST 2012
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 
> 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. :(
>  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.
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