tra added inline comments.
================
Comment at: clang/lib/Sema/SemaType.cpp:2188
+ !Context.getTargetInfo().isVLASupported() &&
+ shouldDiagnoseTargetSupportFromOpenMP()) {
+ // Some targets don't support VLAs.
----------------
tra wrote:
> rjmccall wrote:
> > Please write this check so that it trips in an "ordinary" build on a target
> > that just happens to not support VLAs, something like:
> >
> > else if (!Context.getTargetInfo().isVLASupported() &&
> > shouldDiagnoseTargetSupportFromOpenMP())
> >
> > If you want to include the explicit OpenMP check there, it would need to be:
> >
> > else if (!Context.getTargetInfo().isVLASupported() &&
> > (!getLangOpts().OpenMP || shouldDiagnoseTargetSupportFromOpenMP()))
> >
> > but I think the first looks better.
> >
> > The CUDA and OpenMP paths here seem to be trying to achieve analogous
> > things; it's unfortunate that we can't find a way to unify their
> > approaches, even if we'd eventually want to use different diagnostic text.
> > I imagine that the target-environment language restrictions are basically
> > the same, since they arise for the same fundamental reasons, so all the
> > places using CUDADiagIfDeviceCode are likely to have a check for
> > shouldDiagnoseTargetSupportFromOpenMP() and vice-versa.
> The problem is that in CUDA we can't just do
> ```
> if (!Context.getTargetInfo().isVLASupported() &&
> shouldDiagnoseTargetSupportFromOpenMP())
> Diag(Loc, diag::err_vla_unsupported);
> ```
>
> In some situations diag messages will only be emitted if we attempt to
> generate unsupported code on device side.
> Consider
> ```
> __host__ __device__ void foo(int n) {
> int vla[n];
> }
> ```
>
> When Sema sees this code during compilation, it can not tell whether there is
> an error. Calling foo from the host code is perfectly valid. Calling it from
> device code is not. `CUDADiagIfDeviceCode` creates 'postponed' diagnostics
> which only gets emitted if we ever need to generate code for the function on
> device.
>
> So, while CUDA and OpenMP do similar things, they are not quite the same. If
> your goal to generalize CUDA and OpenMP handling, then it would have to be
> folded into diagnostic-emitting itself and we'll need an analog of
> CUDADiagIfDeviceCode which can handle both OpenMP and CUDA.
> E.g. something like this:
> ```
> ??? DiagIfDeviceCode(???) {
> if (OpenCL || (OpenMP && shouldDiagnoseTargetSupportFromOpenMP()))
> Diag(...);
> else if (CUDA)
> CUDADiagIfDeviceCode()
> }
>
> ...
>
> if (!Context.getTargetInfo().isVLASupported())
> DiagIfDeviceCode();
>
> ```
>
> Would that work for you?
There's another issue with this approach -- diagnostics itself. Each dialect
has its own. Specifically CUDA diags have details that are relevant only to
CUDA. I suspect OpenMP has something specific as well. If we insist emitting
only one kind of error for particular case across all dialects, we'll have to
stick to bare bones "feature X is not supported" which will not have sufficient
details to explain why the error was triggered in CUDA.
IMO dialect-specific handling of cuda errors in this case is the lesser evil.
I'll update the patch to handle non-cuda cases the way you suggested.
https://reviews.llvm.org/D40275
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