[Mesa-dev] mediump / half-precision support

Rob Clark robdclark at gmail.com
Sun May 15 20:04:56 UTC 2016

hmm, that is still a lot of sites of type comparison to fixup, unless
maybe some operator overloading hackery.. but that sounds ugly.. I
guess the pass to propagate precision works out equivalently, as far
as applying precision propagation, to my precision_visitor.  Not
*really* sure that makes it any easier to use for st_glsl_to_tgsi, and
still has all the same issues as my 2nd approach with having to fixup
intermediate var's that get created..


On Sun, May 15, 2016 at 3:47 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> Maybe instead you could do something in the middle -- add a precision
> field to glsl_type instead of adding to glsl_base_type. Then make
> different precision types compare equal in most places (since it is a
> hint to the compiler, rather than a full-fledged extra type) and have
> a pass that propagates mediump from variables to their
> uses/definitions in GLSL IR, so that non-NIR users also get the
> benefit. How does that sound?
> On Sun, May 15, 2016 at 3:05 PM, Rob Clark <robdclark at gmail.com> wrote:
>> So I've been hacking around on a couple approaches to getting
>> half-precision support going for gles (and btw, someday it might be
>> interesting to have an extension to respect precision qualifiers in gl
>> too)
>> I initially started by propagating ir_variable_data::precision to
>> nir_variable_data and fixing up glsl_get_bit_size() to take a
>> precision argument.  Ilia recommended to extend glsl_types to have
>> half-precision types instead.. so I gave that a try.  IMHO it turns
>> out to be a lot of churn (type explosion, with ripple-on effects to
>> builtin-functions, new glsl ir opcodes, etc).  What I have can at
>> least compile correctly *some* gles shaders.  And still having some
>> problems with things getting promoted to full precision when they
>> should not (or missing implicit conversion on function return types,
>> etc).  And builtin-functions is still missing a handling for a lot of
>> the various permutations of full and half-precision types, probably
>> falls over harder when there are samplers in the mix, etc... so still
>> lots of typing to do..  But here is what I've go:
>> https://github.com/freedreno/mesa/commits/wip-mediump
>> I may resurrect the other branch and see how far I get with that,
>> since I didn't get as far as to be able to compile a shader before I
>> switched. I did introduce a hierarchical-visitor to derive precision
>> from an ir_rvalue (which I think might be a more sane way to implement
>> the rules about precision promotion).  After that I think the main
>> thing was to fixup precision on generated intermediate variables (such
>> as for handling inlining of fxns, etc), but I think a lot of that
>> could just use the precision_visitor to figure out what to do.  For
>> most part, in nir the bitsize comes from the originating ir_variable
>> or glsl_struct_field for struct deref's..).  We'd have to somehow make
>> glsl_to_nir clever enough to insert f2h/h2f/u2h/.. conversion
>> instructions, since at the glsl ir level precision is separate from
>> type.  But I kinda think this might be the easier approach.
>> Thoughts?
>> BR,
>> -R
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