Forwarding -foffload=[...] from the driver (compile-time) to libgomp (run-time)

[Bernd Schmidt]

On 10/20/2015 01:17 PM, Thomas Schwinge wrote:
>
> As explained a few times already: GOMP_offload_register_ver constructors
> will only be generated if there actually are offloaded code regions, but
> for example:
>
>      #include <openacc.h>
>      int main()
>      {
>        __builtin_printf("%d\n", acc_get_num_devices(acc_device_nvidia));
>        return 0;
>      }
>
> ... is a valid OpenACC program (untested), which doesn't contain any
> offloaded code regions.  As a user I'd expect it to return different
> answers if compiled with -foffload=nvptx-none in contrast to
> -foffload=disable.  Actually, I can foresee exactly such code to be used
> to probe for offloading being available, for example in testsuites.  And,
> I guess we agree that under -foffload=disable we'd like the
> compilation/runtime system to be configured in a way that no offloading
> will happen?

Both of you can ignore me if you feel I'm not making sense, but what 
exactly is the use case for -foffload=disable? Isn't it slightly 
redundant with -fno-openacc? IMO it's not an option that alters the 
available devices, that's a question that is answered at run-time and 
doesn't (or shouldn't) really depend on compiler switches. As a user I'd 
expect -foffload=disable to just prevent generation of offloaded code 
for the things I'm compiling. As Jakub pointed out, shared libraries may 
still contain other pieces that are offloadable.

I guess I don't fully understand why you want to go to great lengths to 
disable devices at run-time based on a compile-time switch. What's the 
reasoning here?

> Nathan has used this term before (libgomp/openacc.h:acc_device_t), and he
> told me this means "High Water Mark".  I have no strong opinion on the
> name to use, just want to mention that "*_LAST" sounds to me like that
> one still is part of the accepted set, whereas in this case it'd be the
> first enumerator outside of the accepted ones.  (And I guess, we agree
> that "OFFLOAD_TARGET_TYPE_INTEL_LAST = 6" followed by
> "OFFLOAD_TARGET_TYPE_INTEL_MIC = OFFLOAD_TARGET_TYPE_INTEL_LAST" is
> ugly?)

Nah, just rename HWM to LAST, that's fairly common usage I think.


Bernd