[llvm-dev] Request suggestions about how to remove redundencies caused by SCEV expansion fundementally

[Wei Mi via llvm-dev]

Sanjoy and Andy, thanks a lot for your suggestions.

On Wed, Aug 24, 2016 at 8:53 AM, Andrew Trick <atrick at apple.com> wrote:
>
>
> > On Aug 23, 2016, at 11:30 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> >
> > Hi Wei,
> >
> > I've not seen GCC's SCEV so I cannot make a comparative comment here
> > (maybe Chris, Andy or Dan can chime in here), but I personally am in
> > the "make the cleanup passes smarter" camp.  We can also try to make
> > SCEV expansion smarter -- not by putting more things in SCEVExpander
> > (it is already complex enough!), but by splitting out a dedicated
> > SCEVSimplifier that you invoke on code generated from SCEVExpander to
> > strength reduce it.  SCEVSimplifier can then internally use routines
> > in SCEV, so that it is "as smart as" SCEV in most cases.
> >
> > — Sanjoy

Combine the efforts of making the cleanup passes smarter and
simplifying the expanded code to make it more IR canonical looks like
a promising way. But I may not understand SCEVSimplifier you suggested
here correctly: The input of SCEVSimplifier is IR or SCEV? The code
generated by SCEVExpander is IR. If the input of SCEVSimplifier is IR,
how does it use SCEV routine internally?

>
>
> SCEV is super useful as an analysis without SCEVExpander. The only real issue with SCEV itself is invalidating the expressions.
>
> I’ve always thought SCEVExpander is very dangerous to use directly. Ideally the SCEV expression should always be reformulated based on existing IR values before expanding. It would be nice if that was provided as a layer of functionality on top of SCEVExpander.
>
> -Andy
>

Yes, it makes me think that the original idea in D12090 to try to
reuse the existing value directly may not be the right way to solve
the problem fundamentally. Reformulating SCEV to make it more
consistent with cannonical IR form is more promising. If only the IR
expanded is cannonical compared with existing IR, cleanup pass can
have big chance to catch the reuse.

Thanks,
Wei.