[LLVMdev] some superoptimizer results

[Sean Silva]

On Wed, Jul 22, 2015 at 10:54 AM, escha <escha at apple.com> wrote:

>
> > On Jul 22, 2015, at 10:15 AM, John Regehr <regehr at cs.utah.edu> wrote:
> >
> > We (the folks working on Souper) would appreciate any feedback on these
> IR-level superoptimizer results:
> >
> >  http://blog.regehr.org/extra_files/souper-jul-15.html
> >
> > My impression is that while there's clearly plenty of material in here
> that doesn't want to get implemented in an opt pass, there are a number of
> gems hiding in there that are worth implementing.
> >
> > Blog post containing additional explanation and caveats is here:
> >
> >  http://blog.regehr.org/archives/1252
> >
> > Thanks!
> >
> > John
>
> I was reading about these; it looks really interesting! One thing that I
> worry about is that we want to make sure not to IR-transform into “weird
> constructs” that might be very suboptimal on certain targets (and hard or
> impossible to invert later); e.g. you noted that selects had cost 3 in your
> model, but I work on a GPU where select_cc (fcmp/icmp + select) has cost 1.
> Doing sneaky bit math to avoid a select might be good in some cases, but
> not others. Maybe it could use target hooks of some sort so it only does
> transformations that makes sense given a certain cost model?
>

Even on x86, materializing i1's residing in the condition flags is a pain
(e.g. to sext) but CMOVcc based on them is easy.

-- Sean Silva


>
> —escha
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