Bug 1666404 - Invalidate depth attachments at end of render passes. r=gw

This must be done before the draw framebuffer is unbound, to allow
tiled GPUs to avoid writing the contents back to memory.

Differential Revision: https://phabricator.services.mozilla.com/D91261

gfx/wr/webrender/src/device/gl.rs

@@ -2396,9 +2396,6 @@ impl Device {
 
     /// Notifies the device that the contents of a render target are no longer
     /// needed.
-    ///
-    /// FIXME(bholley): We could/should invalidate the depth targets earlier
-    /// than the color targets, i.e. immediately after each pass.
     pub fn invalidate_render_target(&mut self, texture: &Texture) {
         let (fbos, attachments) = if texture.supports_depth() {
             (&texture.fbos_with_depth,
@@ -2418,6 +2415,16 @@ impl Device {
         self.bind_external_draw_target(original_bound_fbo);
     }
 
+    /// Notifies the device that the contents of the current framebuffer's depth
+    /// attachment is no longer needed. Unlike invalidate_render_target, this can
+    /// be called even when the contents of the colour attachment is still required.
+    /// This should be called before unbinding the framebuffer at the end of a pass,
+    /// to allow tiled GPUs to avoid writing the contents back to memory.
+    pub fn invalidate_depth_target(&mut self) {
+        assert!(self.depth_available);
+        self.gl.invalidate_framebuffer(gl::DRAW_FRAMEBUFFER, &[gl::DEPTH_ATTACHMENT]);
+    }
+
     /// Notifies the device that a render target is about to be reused.
     ///
     /// This method adds or removes a depth target as necessary.

gfx/wr/webrender/src/renderer.rs

@@ -4512,6 +4512,8 @@ impl Renderer {
             render_tasks,
             stats,
         );
+
+        self.device.invalidate_depth_target();
     }
 
     /// Draw an alpha batch container into a given draw target. This is used
@@ -5409,6 +5411,10 @@ impl Renderer {
                 stats,
             );
         }
+
+        if clear_depth.is_some() {
+            self.device.invalidate_depth_target();
+        }
     }
 
     /// Draw all the instances in a clip batcher list to the current target.