[LLVMdev] [RFC] New function attributes for errno-setting functions

Hal Finkel hfinkel at anl.gov
Tue Sep 17 12:43:51 PDT 2013

----- Original Message -----
> On 9/12/2013 7:44 PM, Hal Finkel wrote:
> >
> >    1. 'writes-only-errno'
> >    2. 'errno-ignored'
> Do we really need the first one?  We already know which standard
> functions could potentially set errno, and we know which standard
> functions don't modify any other storage (other than that which is
> passed via parameters).  I think it's safe to assume that standard
> functions can be recognized by name, since the standard prohibits
> user
> objects to use a predefined name.

I agree that we could put a list into LLVM instead of adding attributes in the frontend. We need to be a little careful that -fno-builtin is passed to the backend in this case.

> We could have an attribute like "side-effects-ignored" indicating
> that
> whatever modifications the given function makes, nobody will read
> them
> (at least not without an intervening store).  This would include
> errno,
> but could also be used in more general contexts.

That would depend on how this attribute was used; it could only be used by a pass that did actually understand what those side effects were (like the loop vectorizer understanding that, if errno is ignored, then vectorizing cos() is legal). I don't object to making the name generic, but I want to make sure that we're all on the same page regarding what it means.


> -K
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Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

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