v3d: don't emit point coordinates varyings if the FS doesn't read them

We still need to emit them in V3D 3.x since there there is no mechanism to
disable them.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/broadcom/compiler/nir_to_vir.c

@@ -1462,6 +1462,17 @@ var_needs_point_coord(struct v3d_compile *c, nir_variable *var)
                   (1 << (var->data.location - VARYING_SLOT_VAR0)))));
 }
 
+static bool
+program_reads_point_coord(struct v3d_compile *c)
+{
+        nir_foreach_variable(var, &c->s->inputs) {
+                if (var_needs_point_coord(c, var))
+                        return true;
+        }
+
+        return false;
+}
+
 static void
 ntq_setup_fs_inputs(struct v3d_compile *c)
 {
@@ -2300,15 +2311,17 @@ nir_to_vir(struct v3d_compile *c)
                 c->payload_w_centroid = vir_MOV(c, vir_reg(QFILE_REG, 1));
                 c->payload_z = vir_MOV(c, vir_reg(QFILE_REG, 2));
 
-                /* XXX perf: We could set the "disable implicit point/line
+                /* V3D 4.x can disable implicit point coordinate varyings if
-                 * varyings" field in the shader record and not emit these, if
+                 * they are not used.
-                 * they're not going to be used.
                  */
-                if (c->fs_key->is_points) {
+                if (c->fs_key->is_points &&
+                    (c->devinfo->ver < 40 || program_reads_point_coord(c))) {
                         c->point_x = emit_fragment_varying(c, NULL, 0, 0);
                         c->point_y = emit_fragment_varying(c, NULL, 0, 0);
-                } else if (c->fs_key->is_lines) {
+                        c->uses_implicit_point_line_varyings = true;
+                } else if (c->fs_key->is_lines && c->devinfo->ver < 40) {
                         c->line_x = emit_fragment_varying(c, NULL, 0, 0);
+                        c->uses_implicit_point_line_varyings = true;
                 }
                 break;
         case MESA_SHADER_COMPUTE:

src/broadcom/compiler/v3d_compiler.h

@@ -513,6 +513,7 @@ struct v3d_compile {
 
         bool uses_center_w;
         bool writes_z;
+        bool uses_implicit_point_line_varyings;
 
         /* State for whether we're executing on each channel currently.  0 if
          * yes, otherwise a block number + 1 that the channel jumped to.
@@ -689,6 +690,7 @@ struct v3d_fs_prog_data {
         bool writes_z;
         bool disable_ez;
         bool uses_center_w;
+        bool uses_implicit_point_line_varyings;
 };
 
 struct v3d_compute_prog_data {

src/broadcom/compiler/vir.c

@@ -688,6 +688,8 @@ v3d_fs_set_prog_data(struct v3d_compile *c,
         prog_data->writes_z = c->writes_z;
         prog_data->disable_ez = !c->s->info.fs.early_fragment_tests;
         prog_data->uses_center_w = c->uses_center_w;
+        prog_data->uses_implicit_point_line_varyings =
+                c->uses_implicit_point_line_varyings;
 }
 
 static void

src/gallium/drivers/v3d/v3dx_draw.c

@@ -236,6 +236,11 @@ v3d_emit_gl_shader_state(struct v3d_context *v3d,
                 shader.fragment_shader_uses_real_pixel_centre_w_in_addition_to_centroid_w2 =
                         v3d->prog.fs->prog_data.fs->uses_center_w;
 
+#if V3D_VERSION >= 40
+               shader.disable_implicit_point_line_varyings =
+                        !v3d->prog.fs->prog_data.fs->uses_implicit_point_line_varyings;
+#endif
+
                 shader.number_of_varyings_in_fragment_shader =
                         v3d->prog.fs->prog_data.fs->num_inputs;