[PATCH vkd3d 2/4] vkd3d: Skip redundant descriptor copies.

[Henri Verbeet]

On Wed, 8 Dec 2021 at 16:00, Conor McCarthy <cmccarthy at codeweavers.com> wrote:
>
> Greatly improves performance in Control.
>
This doesn't tell someone reading the commit log all that much. What's
"greatly"? Is that a 5% increase? Or perhaps 20%? 500%? Perhaps at
least as important, what is the baseline? If it's running at 200 fps,
and this takes of 2.5ms, that's going to double fps, yes; on the other
hand, if it's running at 40 fps, and this takes of 10ms, that would be
a fair bit more interesting, even if the percentage increase is a bit
smaller. What hardware/driver/etc. is this on? Also, ideally that
explanation would go into a comment next to the code checking for
redundant copies, instead of into the commit message.

>      /* Shadow of the Tomb Raider and possibly other titles sometimes destroy
> -     * and rewrite a descriptor in another thread while it is being copied. */
> +     * and rewrite a descriptor in another thread while it is being copied.
> +     * We don't need to lock either descriptor for the update check. The purpose
> +     * of locking descriptors is not to make descriptor writes thread safe,
> +     * but only to prevent use-after-free of the vkd3d_view caused by a race
> +     * condition in the calling app. It's not important if the update check
> +     * falls out of date as it's their race condition, not ours. */
> +    if (dst->magic == src->magic)
> +    {
The comment probably belongs to the block actually taking the lock
more than the block checking for redundant updates.