[PATCH] D20116: Add speculatable function attribute

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added a comment.

In https://reviews.llvm.org/D20116#707103, @hfinkel wrote:

> > `readnone` etc. are different from `speculatable`, in that once you mark a call site as `speculatable` you've the said call site as speculatable throughout the lifetime of the program (since, by definition, it can be arbitrarily speculated).  `readnone`, `readonly` etc. do not have that property.
> > 
> > But thinking about it a bit, I concede that `speculatable` as a generic (i.e. both intrinsic and non-intrinsic) function attribute is fine.  However, it doesn't make sense as a call site attribute:  being speculatable only down a control flow path is basically the antithesis of `speculatable`.
>
> True, but it can still be a callsite attribute. It can represent a data dependency:
>
>   int div(int a, int b) {
>     return a/b;
>   }
>  
>   div(q, 1); /* speculatable */


But `div` is not well defined for any possible combination of arguments or global memory state.  I think I understand what you were getting at (that for all values of `q` the expression `div(q, 1)` is well defined), but the LangRef definition mentioned in this change does not phrase things that way.

However, this should be fine IMO:

  int div_specialized(int a) speculatable {
    return div(a, 1);
  }


https://reviews.llvm.org/D20116