Re: [PATCH] add support for placing variables in shared memory

[Nathan Sidwell <nathan@acm.org>]

On 04/21/16 10:25, Alexander Monakov wrote:
> On Thu, 21 Apr 2016, Nathan Sidwell wrote:
>> On 04/20/16 12:58, Alexander Monakov wrote:
>>> Allow using __attribute__((shared)) to place static variables in '.shared'
>>> memory space.
>>
>> What is the rationale for a new attribute, rather than leveraging the existing
>> section(".shared") machinery?
>
> Section switching does not work at all on NVPTX in GCC at present.  PTX
> assembly has no notion of different data sections, so the backend does not
> advertise section switching capability to the middle end.

Correct.  How is that relevant?  Look at DECL_SECTION_NAME in encode_section_info.

> CUDA C does it via attributes too, and there's no point in diverging
> gratuitously I think.

Also correct.  It seems you're trying to make the compiler look like CUDA rather 
than fit new features into existing idioms.

> I avoided using 'static' because it applies to external declarations as well.
> Other backends use "%qE attribute not allowed with auto storage class"; I'll
> be happy to switch to that for consistency.


Why can it not be applied to external declarations?  Doesn't '.extern .shared 
whatever' work?


Why can it not apply to  variables of auto storage?  I.e. function scope, 
function lifetime?  That would seem to be a useful property.

What happens if an initializer is present, is it silently ignored?  thinking 
further the uninitialized behaviour might be a reason  for not going with 
section(".shared").

nathan