[LLVMdev] some superoptimizer results

[escha]

> 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?

—escha