Although in principle there is no hardware limitation that prevents
gl_MaxGeometryInputComponents from being set to 128 on Gen7, we have
the following limitations in the vec4 compiler back end:
- Registers assigned to geometry shader inputs can't be spilled or
later re-used for any other purpose.
- The last 16 registers are set aside for the "MRF hack", meaning they
can only be used to send messages, and not for general purpose
computation.
- Up to 32 registers may be reserved for push constants, even if there
is sufficient register pressure to make this impractical.
A shader using 128 geometry input components, and having an input type
of triangles_adjacency, would use up:
- 1 register for r0 (which holds URB handles and various pieces of
control information).
- 1 register for gl_PrimitiveID.
- 102 registers for geometry shader inputs (17 registers per input
vertex, assuming DUAL_INSTANCED dispatch mode and allowing for one
register of overhead for gl_Position and gl_PointSize, which are
present in the URB map even if they are not used).
- Up to 32 registers for push constants.
- 16 registers for the "MRF hack".
That's a total of 152 registers, which is well over the 128 registers
the hardware supports.
Fortunately, the GLSL 1.50 spec allows us to reduce
gl_MaxGeometryInputComponents to 64. Doing that frees up 48
registers, brining the total down to 104 registers, leaving 24
registers available to do computation.
Fixes piglit test
spec/glsl-1.50/execution/geometry/max-input-components.
---
src/mesa/drivers/dri/i965/brw_context.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/mesa/drivers/dri/i965/brw_context.c
b/src/mesa/drivers/dri/i965/brw_context.c
index 307292d..2bd494f 100644
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -400,7 +400,7 @@ brw_initialize_context_constants(struct brw_context *brw)
if (brw->gen >= 6) {
ctx->Const.MaxVarying = 32;
ctx->Const.VertexProgram.MaxOutputComponents = 128;
- ctx->Const.GeometryProgram.MaxInputComponents = 128;
+ ctx->Const.GeometryProgram.MaxInputComponents = 64;
ctx->Const.GeometryProgram.MaxOutputComponents = 128;
ctx->Const.FragmentProgram.MaxInputComponents = 128;
}
--
1.8.4.1
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