[PATCH] wined3d: Implement icb dcl support in GLSL.

[Józef Kucia]

On Wed, Jun 15, 2016 at 1:35 PM, Henri Verbeet <hverbeet at gmail.com> wrote:
> On 13 June 2016 at 20:32, Józef Kucia <joseph.kucia at gmail.com> wrote:
>> This implementation has some issues, e.g. a generated GLSL shader will
>> produce a compilation error when the immediate constant buffer
>> contains an unsigned integer which represents NaN. Morever, from
>> testing a similar implementation, it seems that some drivers don't
>> handle shaders with huge arrays very well and such shaders perform
>> poorly. My current impression is that we should implement immediate
>> constant buffers using a uniform variable.
>>
> Which drivers are that? Using uniforms does have some advantages, but
> it also means the GLSL compiler potentially has less opportunities for
> optimisation. Perhaps that's ok if we can safely assume any such
> optimisations would have already been done by the HLSL compiler.
> There's also the consideration that accessing uniforms can potentially
> have higher latency than accessing immediate values, although I'm not
> aware of any hardware that supports arrays of immediate values, as
> opposed to single (vec4) values.

Switching from arrays to uniforms improved performance noticeably for
Gauntlet on OS X and on Linux with Nvidia binary drivers.

Mesa seems to lower const arrays to uniforms anyway. However, we
cannot declare these arrays as const when uintBitsToFloat is used (at
least in GLSL 1.30), so we are at risk of spilling these arrays to
scratch memory. An excerpt from a comment in the Mesa source code:
"Lower constant arrays to uniform arrays.
Some driver backends (such as i965 and nouveau) don't handle constant
arrays
gracefully, instead treating them as ordinary writable temporary
arrays.
Since arrays can be large, this often means spilling them to scratch
memory,
which usually involves a large number of instructions.".