[Clang] Convergent Attribute

[Matt Arsenault via cfe-commits]

On 05/06/2016 02:42 PM, David Majnemer via cfe-commits wrote:
> This example looks wrong to me. It doesn't seem meaningful for a 
> function to be both readonly and convergent, because convergent means 
> the call has some side-effect visible to other threads and readonly 
> means the call has no side-effects visible outside the function.
This s not correct. It is valid for convergent operations to be 
readonly/readnone. Barriers are a common case which do have side 
effects, but there are also classes of GPU instructions which do not 
access memory and still need the convergent semantics.

-Matt