intel/rt: Add a helper to create a trivial return shader

These are required for ray-tracing.  There are many cases where the
ray-tracing hardware may decide to execute some but not all of our
shaders.  In these cases, it needs a shader to execute at the end which
will pop the stack back to the shader which called traceRay().

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7356>

src/intel/compiler/brw_nir_lower_shader_calls.c

@@ -1181,3 +1181,26 @@ brw_nir_lower_shader_calls(nir_shader *shader,
 
    return true;
 }
+
+/** Creates a trivial return shader
+ *
+ * This is a callable shader that doesn't really do anything.  It just loads
+ * the resume address from the stack and does a return.
+ */
+nir_shader *
+brw_nir_create_trivial_return_shader(const struct brw_compiler *compiler,
+                                     void *mem_ctx)
+{
+   const nir_shader_compiler_options *nir_options =
+      compiler->glsl_compiler_options[MESA_SHADER_CALLABLE].NirOptions;
+
+   nir_builder b = nir_builder_init_simple_shader(MESA_SHADER_CALLABLE,
+                                                  nir_options,
+                                                  "RT Trivial Return");
+   ralloc_steal(mem_ctx, b.shader);
+   nir_shader *nir = b.shader;
+
+   NIR_PASS_V(nir, brw_nir_lower_shader_returns);
+
+   return nir;
+}

src/intel/compiler/brw_nir_rt.h

@@ -59,6 +59,10 @@ bool brw_nir_lower_shader_calls(nir_shader *shader,
 void brw_nir_lower_rt_intrinsics(nir_shader *shader,
                                  const struct gen_device_info *devinfo);
 
+nir_shader *
+brw_nir_create_trivial_return_shader(const struct brw_compiler *compiler,
+                                     void *mem_ctx);
+
 #ifdef __cplusplus
 }
 #endif