i965/gen6/gs: Handle case where a GS doesn't allocate VUE

We can not use the VUE Dereference flags combination for EOT
message under ILK and SNB because the threads are not initialized
there with initial VUE handle unlike Pre-IL.
So to avoid GPU hangs on SNB and ILK we need
to avoid usage of the VUE Dereference flags combination.
(Was tested only on SNB but according to the specification
SNB Volume 2 Part 1: 1.6.5.3, 1.6.5.6
the ILK must behave itself in the similar way)

v2: Approach to fix this issue was changed.
Instead of different EOT flags in the program end
we will create VUE every time even if GS produces no output.

v3: Clean up the patch.
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105399
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>

src/intel/compiler/gen6_gs_visitor.cpp

@@ -350,27 +350,27 @@ gen6_gs_visitor::emit_thread_end()
    int max_usable_mrf = FIRST_SPILL_MRF(devinfo->gen);
 
    /* Issue the FF_SYNC message and obtain the initial VUE handle. */
+   this->current_annotation = "gen6 thread end: ff_sync";
+
+   vec4_instruction *inst = NULL;
+   if (prog->info.has_transform_feedback_varyings) {
+      src_reg sol_temp(this, glsl_type::uvec4_type);
+      emit(GS_OPCODE_FF_SYNC_SET_PRIMITIVES,
+           dst_reg(this->svbi),
+           this->vertex_count,
+           this->prim_count,
+           sol_temp);
+      inst = emit(GS_OPCODE_FF_SYNC,
+                  dst_reg(this->temp), this->prim_count, this->svbi);
+   } else {
+      inst = emit(GS_OPCODE_FF_SYNC,
+                  dst_reg(this->temp), this->prim_count, brw_imm_ud(0u));
+   }
+   inst->base_mrf = base_mrf;
+
    emit(CMP(dst_null_ud(), this->vertex_count, brw_imm_ud(0u), BRW_CONDITIONAL_G));
    emit(IF(BRW_PREDICATE_NORMAL));
    {
-      this->current_annotation = "gen6 thread end: ff_sync";
-
-      vec4_instruction *inst;
-      if (prog->info.has_transform_feedback_varyings) {
-         src_reg sol_temp(this, glsl_type::uvec4_type);
-         emit(GS_OPCODE_FF_SYNC_SET_PRIMITIVES,
-              dst_reg(this->svbi),
-              this->vertex_count,
-              this->prim_count,
-              sol_temp);
-         inst = emit(GS_OPCODE_FF_SYNC,
-                     dst_reg(this->temp), this->prim_count, this->svbi);
-      } else {
-         inst = emit(GS_OPCODE_FF_SYNC,
-                     dst_reg(this->temp), this->prim_count, brw_imm_ud(0u));
-      }
-      inst->base_mrf = base_mrf;
-
       /* Loop over all buffered vertices and emit URB write messages */
       this->current_annotation = "gen6 thread end: urb writes init";
       src_reg vertex(this, glsl_type::uint_type);
@@ -414,7 +414,7 @@ gen6_gs_visitor::emit_thread_end()
                dst_reg reg = dst_reg(MRF, mrf);
                reg.type = output_reg[varying][0].type;
                data.type = reg.type;
-               vec4_instruction *inst = emit(MOV(reg, data));
+               inst = emit(MOV(reg, data));
                inst->force_writemask_all = true;
 
                mrf++;
@@ -460,7 +460,7 @@ gen6_gs_visitor::emit_thread_end()
     *
     * However, this would lead us to end the program with an ENDIF opcode,
     * which we want to avoid, so what we do is that we always request a new
-    * VUE handle every time we do a URB WRITE, even for the last vertex we emit.
+    * VUE handle every time, even if GS produces no output.
     * With this we make sure that whether we have emitted at least one vertex
     * or none at all, we have to finish the thread without writing to the URB,
     * which works for both cases by setting the COMPLETE and UNUSED flags in
@@ -476,7 +476,7 @@ gen6_gs_visitor::emit_thread_end()
       emit(GS_OPCODE_SET_DWORD_2, dst_reg(MRF, base_mrf), data);
    }
 
-   vec4_instruction *inst = emit(GS_OPCODE_THREAD_END);
+   inst = emit(GS_OPCODE_THREAD_END);
    inst->urb_write_flags = BRW_URB_WRITE_COMPLETE | BRW_URB_WRITE_UNUSED;
    inst->base_mrf = base_mrf;
    inst->mlen = 1;