[Mesa-dev] [PATCH] i965: Do not render primitives in non-zero streams then TF is disabled

[Iago Toral Quiroga]

Haswell hardware seems to ignore Render Stream Select bits from
3DSTATE_STREAMOUT packet when the SOL stage is disabled even if
the PRM says otherwise. Because of this, all primitives are sent
down the pipeline for rasterization, which is wrong. If SOL is
enabled, Render Stream Select is honored and primitives bound to
non-zero streams are discarded after stream output.

Since the only purpose of primives sent to non-zero streams is to
be recorded by transform feedback, we can simply discard all geometry
bound to non-zero streams then transform feedback is disabled
to prevent it from ever reaching the rasterization stage.

Notice that this patch introduces a small change in the behavior we
get when a geometry shader emits more vertices than the maximum declared:
before, a vertex that was emitted to a non-zero stream when TF was
disabled would still count for the purposes of checking that we don't
exceed the maximum number of output vertices declared by the shader. With
this change, these vertices are completely ignored and won't increase
the output vertex count, making more room for other (hopefully more
useful) vertices.

Fixes piglit test arb_gpu_shader5-emitstreamvertex_nodraw in Haswell.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83962
---
 src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
index 2002ffd..01d8073 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
@@ -476,6 +476,22 @@ vec4_gs_visitor::visit(ir_emit_vertex *ir)
 {
    this->current_annotation = "emit vertex: safety check";
 
+   /* Haswell hardware seems to ignore Render Stream Select bits from
+    * 3DSTATE_STREAMOUT packet when the SOL stage is disabled even if
+    * the PRM says otherwise. Because of this, all primitives are sent
+    * down the pipeline for rasterization, which is wrong. If SOL is
+    * enabled, Render Stream Select is honored and primitives bound to
+    * non-zero streams are discarded after stream output.
+    *
+    * Since the only purpose of primives sent to non-zero streams is to
+    * be recorded by transform feedback, we can simply discard all geometry
+    * bound to these streams when transform feedback is disabled.
+    */
+   if (brw->is_haswell &&
+       ir->stream_id() > 0 &&
+       this->shader_prog->TransformFeedback.NumVarying == 0)
+      return;
+
    /* To ensure that we don't output more vertices than the shader specified
     * using max_vertices, do the logic inside a conditional of the form "if
     * (vertex_count < MAX)"
-- 
1.9.1