Re: Which OpenACC 'acc_device_t' to use for AMD GPU offloading (was: [PATCH 4/7 libgomp,amdgcn] GCN libgomp port)

[Julian Brown <julian@codesourcery.com>]

On Tue, 3 Dec 2019 10:32:57 +0100
Thomas Schwinge <thomas@codesourcery.com> wrote:

> Hi!
> 
> On 2019-12-02T14:50:42+0000, Julian Brown <julian@codesourcery.com>
> wrote:
> > On Mon, 2 Dec 2019 15:43:29 +0100
> > Thomas Schwinge <thomas@codesourcery.com> wrote:
> >  
> >> > --- a/libgomp/openacc.h
> >> > +++ b/libgomp/openacc.h
> >> > @@ -55,6 +55,7 @@ typedef enum acc_device_t {
> >> >    /* acc_device_host_nonshm = 3 removed.  */
> >> >    acc_device_not_host = 4,
> >> >    acc_device_nvidia = 5,
> >> > +  acc_device_gcn = 8,    
> >> 
> >> There is no 'acc_device_gcn' in OpenACC.  
> 
> My point is, the OpenACC specification defines 'acc_device_t', and
> we're now adding/using a non-standard value, 'acc_device_gcn'.
> Depending on how you read the specification, it may be allowed for an
> implementation to provide additional/different values, but for good
> reason, OpenACC 3.0, A. "Recommendations for Implementors", still
> "gives recommendations for standard names [...] to use for
> implementations for specific targets and target platforms, to promote
> portability across such implementations".
> 
> >> Per OpenACC 3.0, A.1.2. "AMD
> >> GPU Targets", for example, there is 'acc_device_radeon' (and "the
> >> case-insensitive name 'radeon' for the environment variable
> >> 'ACC_DEVICE_TYPE'").  If that is not appropriate to use (I have not
> >> read up how exactly AMD's "GCN" and "radeon" relate to each other),
> >> we should get that clarified in the OpenACC specification.  
> >
> > FWIW, I'm pretty sure there are Radeon devices that do not use the
> > GCN ISA.  
> 
> But does an OpenACC user really care?  Are users likely to state: "I
> have a GCN card", or rather: "I have an AMD GPU card", or even just:
> "I have a card with a big AMD logo on it"?
> 
> Admittedly, users are probably unlikely to state: "I have a Radeon
> card", heh?  ;-) (But it's still the standard name defined in the
> OpenACC specification.)
> 
> > OTOH, there are also Nvidia devices that are not PTX-compatible.  
> 
> (But not any recent (as in a few years old) ones, capable for GPGPU
> computing, are there?)
> 
> But again: "I have a card with a big Nvidia logo on it" is probably
> what users care about, not whether that is internally using PTX or
> anything else, which then is the implementation's job to sort out,
> when 'acc_device_nvidia' is requested by the user.
> 
> > No strong opinion on the acc_device_foo name from me, though (maybe
> > "acc_device_amdgcn"?).  
> 
> Once/if we have established that the standard 'acc_device_radeon' is
> not suitable for us here, only then we should think of a new name, in
> my opinion.  You wouldn't just add a 'copy_mem' function to glibc
> given that 'memcpy' already exists?  ;-)

Oops, I clearly didn't read that email carefully! Since this is a
user-visible, not internal-only thing and there is a recommended name
given in the spec, we should use that name. Still, it's a pity the
official naming is such a jumble.

Julian