[Mesa-dev] [WIP 04/13] mesa: add a storage for inactive/removed uniform variables

[Ian Romanick]

I've been thinking about this a bit, and I think this is more complex
than it needs to be.  I don't have a complete design, but here are some
thoughts on the matter.

When linking is all done, we just want the UniformRemapTable.  It's
entires should have one of three kinds of values:

 - NULL: Invalid entry.  Any function that gets this as a uniform handle
should generate an error.

 - (intptr_t)-1: Explicitly assigned location for a uniform that was
eliminated.  Any function that gets this as a uniform handle should exit
without error.

 - Anything else: Valid.  No changes need to any of the functions that
operate on uniforms.

At the beginning of linking, we can prime the table.  Make a pass over
all the uniforms in all the shaders.  Set elements in the table that
correspond to the explicit location to (intptr_t)-1.  Set every other
location to NULL.  A later pass in linking can set the active slots to
the "real" values.

The timing of building the initial table is important.  Optimization
passes in the linker can trim arrays, and we want the remap slots for
the trimmed elements to get (intptr_t)-1.

To further simplify things, I think we can put all the uniforms that
lack explicit locations after the last explicit location.  If we want to
pack non-explicit location uniforms in among the explicit location
uniforms, I think we'll want a different data structure to track the
avaialble ranges.  I don't know how often explicit/non-explicit mixing
will occur in real apps.

On 03/27/2014 11:45 PM, Tapani Pälli wrote:
> GL_ARB_explicit_uniform_location extension requires driver to reserve
> locations for all uniforms (including inactive ones) when they have
> explicit location set. Patch adds additional storage structure and a
> list of locations reserved by inactive uniforms as part of the shader
> program structure.
> 
> Signed-off-by: Tapani Pälli <tapani.palli at intel.com>
> ---
>  src/mesa/main/mtypes.h    | 13 +++++++++++++
>  src/mesa/main/shaderobj.c |  7 +++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/src/mesa/main/mtypes.h b/src/mesa/main/mtypes.h
> index c8b8fa7..87d69c1 100644
> --- a/src/mesa/main/mtypes.h
> +++ b/src/mesa/main/mtypes.h
> @@ -2710,6 +2710,19 @@ struct gl_shader_program
>     struct gl_uniform_storage **UniformRemapTable;
>  
>     /**
> +    * This list contains explicit locations defined by inactive uniforms that
> +    * were removed during the optimization pass (by opt_dead_code). This is
> +    * used by linker to be able to still preserve locations for those
> +    * uniforms.
> +    *
> +    * All removed uniforms have common storage 'RemovedUniformStorage' which
> +    * is allocated once using ReservedUniformLocations as the ralloc context.
> +    */
> +   unsigned *ReservedUniformLocations;
> +   unsigned NumReservedLocations;
> +   struct gl_uniform_storage *RemovedUniformStorage;
> +
> +   /**
>      * Size of the gl_ClipDistance array that is output from the last pipeline
>      * stage before the fragment shader.
>      */
> diff --git a/src/mesa/main/shaderobj.c b/src/mesa/main/shaderobj.c
> index b0f0bfa..7ec2827 100644
> --- a/src/mesa/main/shaderobj.c
> +++ b/src/mesa/main/shaderobj.c
> @@ -287,6 +287,13 @@ _mesa_clear_shader_program_data(struct gl_context *ctx,
>        shProg->UniformStorage = NULL;
>     }
>  
> +   if (shProg->ReservedUniformLocations) {
> +      ralloc_free(shProg->ReservedUniformLocations);
> +      shProg->NumReservedLocations = 0;
> +      shProg->ReservedUniformLocations = NULL;
> +      shProg->RemovedUniformStorage = NULL;
> +   }
> +
>     if (shProg->UniformRemapTable) {
>        ralloc_free(shProg->UniformRemapTable);
>        shProg->NumUniformRemapTable = 0;
>