[Mesa-dev] [RFC 00/11] GL_ARB_gpu_shader_fp64

[Jason Ekstrand]

On Sat, Mar 11, 2017 at 9:50 AM, Jason Ekstrand <jason at jlekstrand.net>
wrote:
>
>
> As I said at the top, I'm really not going for NIR or nothing.  I agree
> that GLSL has advantages for chips like r600 which badly needs emulation
> and isn't moving to NIR any time soon.  Also, fp64 isn't a requirement in
> Vulkan and, given that Vulkan covers both desktop and mobile, likely won't
> be any time soon.  (In fact, if someone wanted Vulkan FP64 on hardware that
> didn't support it, I'd be tempted to tell them to pay someone to write a
> layer.)  However, *if* we decide that emulated fp64 is better on, for
> instance Ivy Bridge, *and* we had customers that cared about it (I don't
> know of any), then doing it in NIR could yield substantially better results
> (depending on initial shader quality) due to being able to run
> nir_opt_algebraic first.  Those are a lot of ifs so maybe I'm suggesting we
> design for a non-use-case, but I really don't want to paint ourselves into
> a corner that we have to crawl out of 2 years from now.
>

Chatting with people on IRC this morning, I realized there's a killer
argument for why we *need* NIR support: SPIR-V on OpenGL.  As soon as you
expose the GL_ARB_spirv extension on an OpenGL 4.0+ driver, you must
support fp64.  If we ever need emulated fp64 in such a driver, the lowering
has to be done in NIR because there is no GLSL IR in the path.
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