Mesa (master): i965: Do not render primitives in non-zero streams then TF is disabled

[Kenneth Graunke]

Module: Mesa
Branch: master
Commit: 2042a2f961a07e04eaca0347e42859c249325531
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=2042a2f961a07e04eaca0347e42859c249325531

Author: Iago Toral Quiroga <itoral at igalia.com>
Date:   Mon Mar  9 15:17:03 2015 +0100

i965: Do not render primitives in non-zero streams then TF is disabled

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 on Haswell
and Broadwell.

v2 (Ken): Drop is_haswell check in favor of doing this unconditionally.
Broadwell needs the workaround as well, and it doesn't hurt to do it in
general.  Also tweak comments - the Haswell PRM does actually mention
this ("Command Reference: Instructions" page 797).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83962
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
Cc: mesa-stable at lists.freedesktop.org

---

 src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp |   13 +++++++++++++
 1 file changed, 13 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..18a44be 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 +476,19 @@ vec4_gs_visitor::visit(ir_emit_vertex *ir)
 {
    this->current_annotation = "emit vertex: safety check";
 
+   /* Haswell and later hardware ignores the "Render Stream Select" bits
+    * from the 3DSTATE_STREAMOUT packet when the SOL stage is disabled,
+    * and instead sends all primitives down the pipeline for rasterization.
+    * If the SOL stage 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 (ir->stream_id() > 0 && 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)"