[Mesa-dev] [PATCH 05/12] nir: rename global/local to private/function memory

[Jason Ekstrand]

On Fri, Jan 11, 2019 at 10:19 AM Kenneth Graunke <kenneth at whitecape.org>
wrote:

> On Wednesday, January 9, 2019 5:33:22 PM PST Ian Romanick wrote:
> > On 1/8/19 9:57 PM, Kenneth Graunke wrote:
> > > On Tuesday, December 4, 2018 10:26:43 AM PST Karol Herbst wrote:
> > >> the naming is a bit confusing no matter how you look at it. Within
> SPIR-V
> > >> "global" memory is memory accessible from all threads. glsl "global"
> memory
> > >> normally refers to shader thread private memory declared at global
> scope. As
> > >> we already use "shared" for memory shared across all thrads of a work
> group
> > >> the solution where everybody could be happy with is to rename
> "global" to
> > >> "private" and use "global" later for memory usually stored within
> system
> > >> accessible memory (be it VRAM or system RAM if keeping SVM in mind).
> > >> glsl "local" memory is memory only accessible within a function,
> while SPIR-V
> > >> "local" memory is memory accessible within the same workgroup.
> > >>
> > >> v2: rename local to function as well
> > >>
> > >> Signed-off-by: Karol Herbst <kherbst at redhat.com>
> > >
> > > I strongly dislike this patch, and I think we ought to revert it.
> > >
> > > This probably makes sense from an OpenCL memory-model view of the
> world,
> > > but it's really confusing from a compiler or general programming point
> > > of view.
> > >
> > > /Everybody/ knows what a local variable is.  It's one of the most used
> > > concepts in programming.  Calling it nir_var_function is very
> confusing.
> > > The variable is a...function?  Maybe it's a function pointer?  Neither
> > > of those things even exist in GLSL, so...what the heck is it?
> > >
> > > Renaming global scope variables to "private" is also confusing IMO.
> > > They're certainly not private to a function.  They're globally
> > > accessible by anything in the whole shader.  I'll admit "global" isn't
> > > a great name either.
> >
> > It seems like the concepts we're after a function local and thread
> > local, so why not nir_var_thread_local (for old nir_var_global) and
> > nir_var_function_local (for old nir_var_local).  When "global" is
> > reintroduced to mean thread global, we could add it as
> > nir_var_thread_global.  That seems to match at least one reasonable view
> > of a storage hierarchy.
>
> Those names (nir_var_func_local, nir_var_thread_local, and
> nir_var_thread_global) make more sense to me than private/function.
>
> Another option is `nir_var_local_temp` and `nir_var_shader_temp`,
> indicating that they're just temporary variables, and not anything
> with special semantics like memory.  shader_temp would pair well with
> the existing shader_in/shader_out, since they have the same scope.
>
> I might also consider adding 'mem' to variables representing memory.
>
> So that would look like...
>
>    nir_var_shader_in
>    nir_var_shader_out
>    nir_var_shader_temp  (formerly local/function)
>    nir_var_local_temp   (formerly global/private)
>

Are those flipped?


>    nir_var_uniform
>    nir_var_system_value
>    nir_var_mem_ubo      (added mem)
>    nir_var_mem_ssbo     (added mem)
>    nir_var_mem_shared   (added mem)
>    nir_var_mem_global   (the new global memory type being introduced)
>
> How does that look?
>

I think I kind of like having "mem" be on external things.  Shared is a
little weird there because it never leaves the chip so is it mem or shader?


> We may also want to rename the nir->globals list, or
> nir_lower_global_vars_to_local and nir_opt_global_to_local.  Not sure.
>

Yes, whatever we do, we should make those lists more consistent.
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