[Mesa-dev] [PATCH 21/37] i965/gen6/gs: Implement support for gl_PrimitiveIdIn.

[Iago Toral Quiroga]

For this we will need to move PrimitiveID information, delivered in the thread
payload in r0.1, to a separate register (we use GS_OPCODE_SET_PRIMITIVE_ID
for this), then map the corresponding varying slot to that register in the
setup_payload() method.

Notice that we cannot use a virtual register as the destination for the
PrimitiveID because we need to map all input attributes to hardware registers
in setup_payload(), which happens before virtual registers are mapped to
hardware registers. We could work around that issue if we were able to compute
the first non-payload register in emit_prolog() and move the PrimitiveID
information to that register, but we can't because at that point we still
don't know the final number uniforms that will be included in the payload.

So, what we do is to place PrimitiveID information in r1, which is always
delivered as part of the payload but its only populated with data
relevant for transform feedback when we set GEN6_GS_SVBI_PAYLOAD_ENABLE
in the 3DSTATE_GS state packet.

When we implement transform feedback, we wil make sure to move the value of r1
to another register before we overwrite it with the PrimitiveID.
---
 src/mesa/drivers/dri/i965/gen6_gs_visitor.cpp | 69 ++++++++++++++++++++++++++-
 src/mesa/drivers/dri/i965/gen6_gs_visitor.h   |  2 +
 2 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/gen6_gs_visitor.cpp b/src/mesa/drivers/dri/i965/gen6_gs_visitor.cpp
index 4a440eb..b45c381 100644
--- a/src/mesa/drivers/dri/i965/gen6_gs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/gen6_gs_visitor.cpp
@@ -31,6 +31,8 @@
 
 #include "gen6_gs_visitor.h"
 
+const unsigned MAX_GS_INPUT_VERTICES = 6;
+
 namespace brw {
 
 void
@@ -38,6 +40,7 @@ gen6_gs_visitor::emit_prolog()
 {
    vec4_gs_visitor::emit_prolog();
 
+   this->current_annotation = "gen6 prolog";
    /* Gen6 geometry shaders require to allocate an initial VUE handle via
     * FF_SYNC message, however the documentation remarks that only one thread
     * can write to the URB simultaneously and the FF_SYNC message provides the
@@ -59,7 +62,6 @@ gen6_gs_visitor::emit_prolog()
     * flags for the next vertex come right after the data items and flags for
     * the previous vertex.
     */
-   this->current_annotation = "gen6 prolog";
    this->vertex_output = src_reg(this,
                                  glsl_type::uint_type,
                                  (prog_data->vue_map.num_slots + 1) *
@@ -94,6 +96,30 @@ gen6_gs_visitor::emit_prolog()
     */
    this->prim_count = src_reg(this, glsl_type::uint_type);
    emit(MOV(dst_reg(this->prim_count), 0u));
+
+   /* PrimitveID is delivered in r0.1 of the thread payload. If the program
+    * needs it we have to move it to a separate register where we can map
+    * the atttribute.
+    *
+    * Notice that we cannot use a virtual register for this, because we need to
+    * map all input attributes to hardware registers in setup_payload(),
+    * which happens before virtual registers are mapped to hardware registers.
+    * We could work around that issue if we were able to compute the first
+    * non-payload register here and move the PrimitiveID information to that
+    * register, but we can't because at this point we don't know the final
+    * number uniforms that will be included in the payload.
+    *
+    * So, what we do is to place PrimitiveID information in r1, which is always
+    * delivered as part of the payload, but its only populated with data
+    * relevant for transform feedback when we set GEN6_GS_SVBI_PAYLOAD_ENABLE
+    * in the 3DSTATE_GS state packet. That information can be obtained by other
+    * means though, so we can safely use r1 for this purpose.
+    */
+   if (c->prog_data.include_primitive_id) {
+      this->primitive_id =
+         src_reg(retype(brw_vec8_grf(1, 0), BRW_REGISTER_TYPE_UD));
+      emit(GS_OPCODE_SET_PRIMITIVE_ID, dst_reg(this->primitive_id));
+   }
 }
 
 void
@@ -410,4 +436,45 @@ gen6_gs_visitor::emit_thread_end()
    inst->mlen = 1;
 }
 
+void
+gen6_gs_visitor::setup_payload()
+{
+   int attribute_map[BRW_VARYING_SLOT_COUNT * MAX_GS_INPUT_VERTICES];
+
+   /* Attributes are going to be interleaved, so one register contains two
+    * attribute slots.
+    */
+   int attributes_per_reg = 2;
+
+   /* If a geometry shader tries to read from an input that wasn't written by
+    * the vertex shader, that produces undefined results, but it shouldn't
+    * crash anything.  So initialize attribute_map to zeros--that ensures that
+    * these undefined results are read from r0.
+    */
+   memset(attribute_map, 0, sizeof(attribute_map));
+
+   int reg = 0;
+
+   /* The payload always contains important data in r0. */
+   reg++;
+
+   /* r1 is always part of the payload and it holds information relevant
+    * for transform feedback when we set the GEN6_GS_SVBI_PAYLOAD_ENABLE bit in
+    * the 3DSTATE_GS packet. We will overwrite it with the PrimitiveID
+    * information (and move the original value to a virtual register if
+    * necessary).
+    */
+   if (c->prog_data.include_primitive_id)
+      attribute_map[VARYING_SLOT_PRIMITIVE_ID] = attributes_per_reg * reg;
+   reg++;
+
+   reg = setup_uniforms(reg);
+
+   reg = setup_varying_inputs(reg, attribute_map, attributes_per_reg);
+
+   lower_attributes_to_hw_regs(attribute_map, true);
+
+   this->first_non_payload_grf = reg;
+}
+
 } /* namespace brw */
diff --git a/src/mesa/drivers/dri/i965/gen6_gs_visitor.h b/src/mesa/drivers/dri/i965/gen6_gs_visitor.h
index 7af6405..8d2386c 100644
--- a/src/mesa/drivers/dri/i965/gen6_gs_visitor.h
+++ b/src/mesa/drivers/dri/i965/gen6_gs_visitor.h
@@ -52,6 +52,7 @@ protected:
                                       int base_mrf,
                                       int last_mrf,
                                       int urb_offset);
+   virtual void setup_payload();
 
 private:
    src_reg vertex_output;
@@ -59,6 +60,7 @@ private:
    src_reg temp;
    src_reg first_vertex;
    src_reg prim_count;
+   src_reg primitive_id;
 };
 
 } /* namespace brw */
-- 
1.9.1