[PATCH] D18110: [OpenMP] Fix SEMA bug in the capture of global variables in template functions.

[Carlo Bertolli via cfe-commits]

carlo.bertolli added a comment.

Hi

If I understand correctly the problem, I would like to add something on top of Samuel's comment.
My understanding is that Alexey is suggesting that we pass a reference type to kernels for every pointer input parameter, regardless of the actual type and data sharing attribute of the related variable. Ignore the remained of this comment if this is not the case.

In my viewpoint, this violates the basic logic for which target firstprivate was introduced in the OpenMP specification: to avoid having to pass to GPU kernels a reference to every pointer variable. In fact, having to pass a reference results in ptxas generating an additional register for each input parameter. We saw significant performance differences in this respect, and reported about this in a paper at SC15. This has been reported by various members of the OpenMP committee multiple times as the reason why the target firstprivate logic is defined the way it is.

In conclusion, I do not see this patch as an optimization, but as a way of correctly implementing what the OpenMP specification clearly state. Any other implementation is wrong, not a simpler, non optimized version.


http://reviews.llvm.org/D18110