[PATCH] D20116: Add speculatable function attribute

[Hal Finkel via Phabricator via llvm-commits]

hfinkel added a comment.

In https://reviews.llvm.org/D20116#708273, @sanjoy wrote:

> In https://reviews.llvm.org/D20116#708268, @hfinkel wrote:
>
> > > It also breaks "code compression" type optimizations:
> > > 
> > >   if (a == 1 || a == 2) {
> > >     switch (a) {
> > >     case 1:
> > >       div(m, 1) speculatable;
> > >       break;
> > >     case 2:
> > >       div(m, 2) speculatable;
> > >       break;
> > >     }
> > >   }
> > > 
> > > 
> > > to
> > > 
> > >   if (a == 1 || a == 2) {
> > >     div(m, a) speculatable;
> > >   }
> > > 
> > > 
> > > to
> > > 
> > >   if (a == 1 || a == 2) {
> > >     if (a == 0)
> > >       div(m, 0) speculatable;
> >
> > Your "code compression" optimization just introduced dead code ;)
>
>
> Yea, I don't even know why I called it "code compression". :)
>
> > 
> > 
> >>   else
> >>     div(m, a) speculatable;
> >> 
> >> }
> >> 
> >>   
> > 
> > I think that all of this is right, you can't apply some of these optimizations to call sites with the speculatable attribute. I agree, however, that we need to think carefully about how to define what speculatable means on an individual call site. Perhaps they're like convergent functions in this regard: you can't introduce new control dependencies (at least not in general).
>
> I'd say as an initial step support for intrinsics that we _know_ are `speculatable` (like we _know_ that `add` is speculatable) can land without any further discussion.


I'm fine with restricting speculatable to only appear where it appears on a function declaration/definition unless/until we can figure out semantics for it on a call site in general. I don't want it restricted to intrinsics specifically, but I don't think that's the problem.

> As for a generic `speculatable` attribute -- I need some time to think about this.  Perhaps if done with sufficient care, it is possible.
> 
> I'd also like to ping @whitequark for comments.  I had blocked https://reviews.llvm.org/D18738 some time back on similar grounds, but if we can reach a conclusion on `specuatable`, perhaps some of that can be transferable to `!unconditionally_dereferenceable` as well.




https://reviews.llvm.org/D20116