iris: ensure null render target for specific cases

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Reviewed-by: Ivan Briano <ivan.briano@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31196>

src/gallium/drivers/iris/iris_context.h

@@ -635,6 +635,9 @@ struct iris_binding_table {
    uint64_t used_mask[IRIS_SURFACE_GROUP_COUNT];
 
    uint64_t samplers_used_mask;
+
+   /** Whether the first render target is a null fb surface */
+   uint8_t use_null_rt;
 };
 
 /**

src/gallium/drivers/iris/iris_program.c

@@ -1381,7 +1381,8 @@ iris_setup_binding_table(const struct intel_device_info *devinfo,
                          struct iris_binding_table *bt,
                          unsigned num_render_targets,
                          unsigned num_system_values,
-                         unsigned num_cbufs)
+                         unsigned num_cbufs,
+                         bool use_null_rt)
 {
    const struct shader_info *info = &nir->info;
 
@@ -1404,6 +1405,8 @@ iris_setup_binding_table(const struct intel_device_info *devinfo,
          bt->used_mask[IRIS_SURFACE_GROUP_RENDER_TARGET_READ] =
             BITFIELD64_MASK(num_render_targets);
       }
+
+      bt->use_null_rt = use_null_rt;
    } else if (info->stage == MESA_SHADER_COMPUTE) {
       bt->sizes[IRIS_SURFACE_GROUP_CS_WORK_GROUPS] = 1;
    }
@@ -1858,7 +1861,7 @@ iris_compile_vs(struct iris_screen *screen,
 
    struct iris_binding_table bt;
    iris_setup_binding_table(devinfo, nir, &bt, /* num_render_targets */ 0,
-                            num_system_values, num_cbufs);
+                            num_system_values, num_cbufs, false);
 
    const char *error;
    const unsigned *program;
@@ -2086,7 +2089,7 @@ iris_compile_tcs(struct iris_screen *screen,
    iris_setup_uniforms(devinfo, mem_ctx, nir, 0, &system_values,
                        &num_system_values, &num_cbufs);
    iris_setup_binding_table(devinfo, nir, &bt, /* num_render_targets */ 0,
-                            num_system_values, num_cbufs);
+                            num_system_values, num_cbufs, false);
 
    const char *error = NULL;
    const unsigned *program;
@@ -2276,7 +2279,7 @@ iris_compile_tes(struct iris_screen *screen,
 
    struct iris_binding_table bt;
    iris_setup_binding_table(devinfo, nir, &bt, /* num_render_targets */ 0,
-                            num_system_values, num_cbufs);
+                            num_system_values, num_cbufs, false);
 
    const char *error;
    const unsigned *program;
@@ -2458,7 +2461,7 @@ iris_compile_gs(struct iris_screen *screen,
 
    struct iris_binding_table bt;
    iris_setup_binding_table(devinfo, nir, &bt, /* num_render_targets */ 0,
-                            num_system_values, num_cbufs);
+                            num_system_values, num_cbufs, false);
 
    const char *error;
    const unsigned *program;
@@ -2627,16 +2630,14 @@ iris_compile_fs(struct iris_screen *screen,
     */
    brw_nir_lower_fs_outputs(nir);
 
-   /* On Gfx11+, shader RT write messages have a "Null Render Target" bit
+   int null_rts = brw_nir_fs_needs_null_rt(devinfo, nir,
-    * and do not need a binding table entry with a null surface.  Earlier
+                                           key->multisample_fbo,
-    * generations need an entry for a null surface.
+                                           key->alpha_to_coverage) ? 1 : 0;
-    */
-   int null_rts = devinfo->ver < 11 ? 1 : 0;
 
    struct iris_binding_table bt;
    iris_setup_binding_table(devinfo, nir, &bt,
                             MAX2(key->nr_color_regions, null_rts),
-                            num_system_values, num_cbufs);
+                            num_system_values, num_cbufs, null_rts != 0);
 
    const char *error;
    const unsigned *program;
@@ -2966,7 +2967,7 @@ iris_compile_cs(struct iris_screen *screen,
 
    struct iris_binding_table bt;
    iris_setup_binding_table(devinfo, nir, &bt, /* num_render_targets */ 0,
-                            num_system_values, num_cbufs);
+                            num_system_values, num_cbufs, false);
 
    const char *error;
    const unsigned *program;

src/gallium/drivers/iris/iris_state.c

@@ -5696,7 +5696,7 @@ iris_populate_binding_table(struct iris_context *ice,
             }
             push_bt_entry(addr);
          }
-      } else if (GFX_VER < 11) {
+      } else if (bt->use_null_rt) {
          uint32_t addr = use_null_fb_surface(batch, ice);
          push_bt_entry(addr);
       }

src/intel/compiler/brw_nir.h

@@ -137,6 +137,28 @@ brw_nir_ubo_surface_index_get_bti(nir_src src)
    return nir_src_as_uint(intrin->src[1]);
 }
 
+/* Returns true if a fragment shader needs at least one render target */
+static inline bool
+brw_nir_fs_needs_null_rt(const struct intel_device_info *devinfo,
+                         nir_shader *nir,
+                         bool multisample_fbo, bool alpha_to_coverage)
+{
+   assert(nir->info.stage == MESA_SHADER_FRAGMENT);
+
+   /* Null-RT bit in the render target write extended descriptor is only
+    * available on Gfx11+.
+    */
+   if (devinfo->ver < 11)
+      return true;
+
+   uint64_t relevant_outputs = 0;
+   if (multisample_fbo)
+      relevant_outputs |= BITFIELD64_BIT(FRAG_RESULT_SAMPLE_MASK);
+
+   return (alpha_to_coverage ||
+           (nir->info.outputs_written & relevant_outputs) != 0);
+}
+
 void brw_preprocess_nir(const struct brw_compiler *compiler,
                         nir_shader *nir,
                         const struct brw_nir_compiler_opts *opts);