On 12/13/2011 01:25 PM, Jose Fonseca wrote:
----- Original Message -----
On 12/13/2011 03:09 PM, Jose Fonseca wrote:
----- Original Message -----
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.
But don't all the clip distances written by the shader need to be
declared?
E.g.:
DCL OUT[0], CLIPDIST[0]
DCL OUT[1], CLIPDIST[1]
DCL OUT[2], CLIPDIST[2]
DCL OUT[3], CLIPDIST[3]
therefore a trivial analysis of the declarations convey that?
No. Clip distance is an array of up to 8 floats in GLSL, but it's
represented in the hardware as 2 vec4s. You can tell by analyzing
the
declarations whether there are more than 4 clip distances in use, but
not which components the shader writes to.
TGSI_PROPERTY_NUM_CLIP_DISTANCES is the number of components in use,
not
the number of full vectors.
Lets imagine
out float gl_ClipDistance[6];
Each a clip distance is a scalar float.
Either all hardware represents the 8 clip distances as two 4 vectors, and we do:
DCL OUT[0].xywz, CLIPDIST[0]
DCL OUT[1].xy, CLIPDIST[1]
using the full range of struct tgsi_declaration::UsageMask [1] or we represent
them as as scalars:
DCL OUT[0].x, CLIPDIST[0]
DCL OUT[1].x, CLIPDIST[1]
DCL OUT[2].x, CLIPDIST[2]
DCL OUT[3].x, CLIPDIST[3]
DCL OUT[4].x, CLIPDIST[4]
DCL OUT[5].x, CLIPDIST[5]
If indirect addressing is allowed as I read bore, then maybe the later is
better.
As far as I'm aware, all hardware represents it as the former, and we
have a lowering pass to fix-up the float[] accesses to be vec4[] accesses.
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