[Mesa-dev] [PATCH] i965: Allocate at least some URB space even when max_vertices = 0.

[Kenneth Graunke]

Allocating zero URB space is a really bad idea.  The hardware has to
give threads a handle to their URB space, and threads have to use that
to terminate the thread.  Having it be an empty region just breaks a
lot of assumptions.  Hence, why we asserted that it isn't possible.

Unfortunately, it /is/ possible prior to Gen8, if max_vertices = 0.
In theory a geometry shader could do SSBO/image access and maybe
still accomplish something.  In reality, this is tripped up by
conformance tests.

Gen8+ already avoids this problem by placing the vertex count DWord
in the URB entry header.  This fixes things on earlier generations.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

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 900d9d3..3894a63 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
@@ -781,7 +781,13 @@ brw_compile_gs(const struct brw_compiler *compiler, void *log_data,
    if (compiler->devinfo->gen >= 8)
       output_size_bytes += 32;
 
-   assert(output_size_bytes >= 1);
+   /* Shaders can technically set max_vertices = 0, at which point we
+    * may have a URB size of 0 bytes.  Nothing good can come from that,
+    * so enforce a minimum size.
+    */
+   if (output_size_bytes == 0)
+      output_size_bytes = 1;
+
    unsigned max_output_size_bytes = GEN7_MAX_GS_URB_ENTRY_SIZE_BYTES;
    if (compiler->devinfo->gen == 6)
       max_output_size_bytes = GEN6_MAX_GS_URB_ENTRY_SIZE_BYTES;
-- 
2.10.2