[Libclc-dev] [PATCH 2/3] amdgcn-amdhsa: Add get_num_groups implementation

[Tom Stellard via Libclc-dev]

On Wed, Sep 14, 2016 at 11:18:12AM -0400, Jan Vesely via Libclc-dev wrote:
> On Wed, 2016-09-14 at 10:58 -0400, Matt Arsenault via Libclc-dev wrote:
> > > 
> > > On Sep 14, 2016, at 07:22, Tom Stellard via Libclc-dev <libclc-dev@
> > > lists.llvm.org> wrote:
> > > 
> > > +_CLC_DEF size_t get_num_groups(uint dim) {
> > > +  size_t global_size = get_global_size(dim);
> > > +  size_t local_size = get_local_size(dim);
> > > +  size_t num_groups = global_size / local_size;
> > > +  if (global_size % local_size != 0) {
> > > +    num_groups++;
> > > +  }
> > > +  return num_groups;
> > LGTM. I hope the % does get optimized out?
> 
> AMDGPU implements DIVREM for 64bit integers, so / and % are computed at
> the same time. It's still rather expensive (~400 instructions).
> 
> (global_size + local_size -1) / local_size
> 
> allows elimination of REM only parts of DIVREM (although the savings
> are negligible).
> 

I would really prefer to have the runtime compute this, so I think we
can replace this with something better in the future.

-Tom

> Jan
> 
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> -- 
> Jan Vesely <jv356 at scarletmail.rutgers.edu>
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