[llvm-dev] [cfe-dev] [RFC] Expose user provided vector function for auto-vectorization.

[Francesco Petrogalli via llvm-dev]

> On May 31, 2019, at 11:57 AM, Renato Golin <rengolin at gmail.com> wrote:
> 
> On Fri, 31 May 2019 at 17:19, Francesco Petrogalli via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> TOPIC 2: metadata vs attribute
>> 
>> Also, @Renato expressed concern that metadata might be dropped by optimization passes - would using attributes prevent that?
> 
> I think it would, thanks!
> 
> 
>> TOPIC 3: "there are no special arguments / flags / status regs that are used / changed in the vector version that the compiler will have to "just know”
>> 
>> I believe that this concern is raised by the problem of handling FP exceptions? If that’s the case, the compiler is not allowed to do any assumption on the vector function about that, and treat it with the same knowledge of any other function, depending on the visibility it has in the compilation unit. @Renato, does this answer your question?
> 
> So, if there are side-effects on the scalar version, there will be
> also in the vector version? Unfortunately, this does not work in
> practice by default (different units have different rules).
> 

The OpenMP use of  `declare simd` and `declare variant` informs the compiler that the underlying scalar function is “safe to vectorize”. I believe that side-effects are excluded from the list of things that are “safe to vectorize”. 

See https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf, page 118, lines 23-24:

“The execution of the function or subroutine cannot have any side effects that would alter its execution for concurrent iterations of a SIMD chunk."

> If we want to enforce this, it's up to the library implementation to
> provide similar behaviour (either hide or create side-effects) and it
> will be "library error" if they do not.
> 
> This seems a bit heavy handed, though...
> 
> --renato