On 12/13/2011 12:26 PM, Bryan Cain wrote:
On 12/13/2011 02:11 PM, Jose Fonseca wrote:
----- Original Message -----
This is an updated version of the patch set I sent to the list a few
hours
ago.
There is now a TGSI property called
TGSI_PROPERTY_NUM_CLIP_DISTANCES
that drivers can use to determine how many of the 8 available clip
distances
are actually used by a shader.
Can't the info in TGSI_PROPERTY_NUM_CLIP_DISTANCES be easily derived from the
shader, and queried through src/gallium/auxiliary/tgsi/tgsi_scan.h ?
No. The clip distances can be indirectly addressed (there are up to 2
of them in vec4 form for a total of 8 floats), which makes it impossible
to determine which ones are used by analyzing the shader.
The description is almost complete. :) The issue is that the shader may
declare
out float gl_ClipDistance[4];
the use non-constant addressing of the array. The compiler knows that
gl_ClipDistance has at most 4 elements, but post-hoc analysis would not
be able to determine that. Often the fixed-function hardware (see
below) needs to know which clip distance values are actually written.
Could you also elaborate on why TGSI_SEMANTIC_CLIPDIST is useful for the
drivers? I personally don't have nothing against it, but just like to
understand why it makes a difference.
Jose
Unless my understanding of clip distances is wrong, the GPU uses clip
distances to decide whether a vertex should be clipped, and thus needs
to know which of the vertex shader outputs are clip distances.
Right. Clip distance is generally written to a special register that
some fixed-function hardware uses to do clipping.
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