[Spice-devel] [PATCH] Remove X == TRUE tests for non-boolean values

[Christophe de Dinechin]

> On 27 Feb 2017, at 17:10, Frediano Ziglio <fziglio at redhat.com> wrote:
> 
> On 27 Feb 2017, at 13:12, Christophe Fergeau <cfergeau at redhat.com <mailto:cfergeau at redhat.com>> wrote:
> 
> On Mon, Feb 27, 2017 at 06:58:47AM -0500, Frediano Ziglio wrote:
> 
> Using X == TRUE is fragile, since the input value is a uint8_t. It would be
> surprising if the value was set to 2 or 0xFF and the test failed.
> 
> Signed-off-by: Christophe de Dinechin <dinechin at redhat.com <mailto:dinechin at redhat.com>>
> 
> Acked-by: Frediano Ziglio <fziglio at redhat.com <mailto:fziglio at redhat.com>>
> 
> This apparently is going to cause warnings with gcc 7, see
> https://www.redhat.com/archives/libvir-list/2017-February/msg01199.html <https://www.redhat.com/archives/libvir-list/2017-February/msg01199.html>
> 
> if (dcc->priv->surface_client_created[surface_id] != 0) {} should work,
> or
> diff --git a/server/dcc-private.h b/server/dcc-private.h
> index 64b32a7..1cf3b0d 100644
> --- a/server/dcc-private.h
> +++ b/server/dcc-private.h
> @@ -51,7 +51,7 @@ struct DisplayChannelClientPrivate
>         int num_pixmap_cache_items;
>     } send_data;
> 
> -    uint8_t surface_client_created[NUM_SURFACES];
> +    bool surface_client_created[NUM_SURFACES];
>     QRegion surface_client_lossy_region[NUM_SURFACES];
> 
>     StreamAgent stream_agents[NUM_STREAMS];
> 
> which seems to be a more accurate type for 'surface_client_created’.
> 
> Looking at the declaration, I just noticed we pre-allocate 10000 surfaces. Why is this not dynamic allocation? Is it sane to allocate 250K statically with each DisplayChannelClientPrivate?
> I think mostly old style pool allocations. Considering that usually we use only 1 surface would be indeed much smaller.
> 
> I’m tempted to replace this with a dynamic array of QRegion *, where a non-NULL pointer indicates that the client has been created. OK with that? I expect some operations will be faster because they won’t iterate 
> I would use a new structure like
> 
> typedef struct {
>     QRegion region;
> } RedClientSurfaceInfo;
> 
> make source code a bit longer (not the binary) but can be easily extended and it's easier to
> understand a dcc->client_surface_info[i] != NULL than a dcc->surface_client_lossy_region[i] != NULL.

Yes. Agreed.

> 
> over 10000 mostly empty surfaces. There will be one extra indirection when accessing the surface_client_lossy region, but x86_64 became pretty darn good at chasing pointers like that thanks to C++ vtables ;-)
> Last sentence is a bit cryptic I think :-)

Sorry. Did not mean to be. What I meant is that chasing pointers like a->b->c is required to perform well for virtual function calls in C++ (this->foo() translates into this->vptr->vtbl[n]()). At some point, it was a huge performance hit for many workloads, because CPUs were not that good at prefetching this kind of double-dereferences. But C++ and other similar programming techniques with double indirection became so prevalent that CPUs are now quite good at it.


> Could be true.
> I don't think the extra indirection is a big deal, it's mostly used on surface/channel creation/destroying.

OK

Christophe

> 
> 
> Christophe
> 
> Chistophe
> 
> 
> 
> 
> ---
> server/dcc.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/server/dcc.c b/server/dcc.c
> index afe37b1..7cfa72b 100644
> --- a/server/dcc.c
> +++ b/server/dcc.c
> @@ -398,7 +398,7 @@ static void
> add_drawable_surface_images(DisplayChannelClient *dcc, Drawable *dra
> 
>         surface_id = drawable->surface_deps[x];
>         if (surface_id != -1) {
> -            if (dcc->priv->surface_client_created[surface_id] == TRUE) {
> +            if (dcc->priv->surface_client_created[surface_id]) {
>                 continue;
>             }
>             dcc_create_surface(dcc, surface_id);
> @@ -407,7 +407,7 @@ static void
> add_drawable_surface_images(DisplayChannelClient *dcc, Drawable *dra
>         }
>     }
> 
> -    if (dcc->priv->surface_client_created[drawable->surface_id] == TRUE) {
> +    if (dcc->priv->surface_client_created[drawable->surface_id]) {
>         return;
>     }
> Frediano

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