[Spice-devel] [PATCH spice-common 2/3] lz: Optimise SAME_PIXEL for RGB16

[Jonathon Jongsma]

On Thu, 2018-05-10 at 13:03 -0400, Frediano Ziglio wrote:
> > 
> > On Wed, 2018-05-09 at 14:10 +0100, Frediano Ziglio wrote:
> > > Do not extract all components and compare one by one, can be
> > > easily
> > > compared together.
> > 
> > Do you have some data about how performance compares between the
> > two
> > implementations? The new implementation clearly looks more
> > efficient,
> > but the code is not as straightforward to understand at a glance
> > (at
> > least for me). If it's called a lot, it clearly makes sense to
> > optimize
> > the implementation, but if not, I'm a bit ambivalent about the
> > change.
> > 
> > Jonathon
> > 
> 
> Looked at produced assembly code and is smaller.

I'm sure that's true. I was more curious about whether you can actually
see any practical performance impact in real-world scenarios.

> 
> A similar macro (but a bit more readable) can be:
> 
> #define SAME_PIXEL(p1, p2) (((p1) & 0x7fffu) == ((p2) & 0x7fffu))
> 
> A #define RGB16_BITMASK 0x7fffu can be defined to make even more
> clear, like
> 
> #define RGB16_BITMASK 0x7fffu
> #define SAME_PIXEL(p1, p2) (((p1) & RGB16_BITMASK) == ((p2) &
> RGB16_BITMASK))

Yes, this seems a bit more readable to me. 

> 
> or something like
> 
> #define GET_rgb(pix) ((pix) & 0x7fffu)
> #define SAME_PIXEL(p1, p2) (GET_rgb(p1) == GET_rgb(p2))


Not sure this is necessary.

> 
> > 
> > > 
> > > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > > ---
> > >  common/lz_compress_tmpl.c | 6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/common/lz_compress_tmpl.c
> > > b/common/lz_compress_tmpl.c
> > > index 69e69a6..b778e9d 100644
> > > --- a/common/lz_compress_tmpl.c
> > > +++ b/common/lz_compress_tmpl.c
> > > @@ -105,9 +105,7 @@
> > >  #ifdef LZ_RGB16
> > >  #define PIXEL rgb16_pixel_t
> > >  #define FNAME(name) lz_rgb16_##name
> > > -#define GET_r(pix) (((pix) >> 10) & 0x1f)
> > > -#define GET_g(pix) (((pix) >> 5) & 0x1f)
> > > -#define GET_b(pix) ((pix) & 0x1f)
> > > +#define SAME_PIXEL(p1, p2) ((((p1)^(p2)) & 0x7fffu) == 0)
> > >  #define ENCODE_PIXEL(e, pix) {encode(e, (pix) >> 8); encode(e,
> > > (pix)
> > > & 0xff);}
> > >  
> > >  #define HASH_FUNC(v, p) {                 \
> > > @@ -153,7 +151,7 @@
> > >      }
> > >  #endif
> > >  
> > > -#if defined(LZ_RGB16) || defined(LZ_RGB24) || defined(LZ_RGB32)
> > > +#if defined(LZ_RGB24) || defined(LZ_RGB32)
> > >  #define SAME_PIXEL(p1, p2) (GET_r(p1) == GET_r(p2) && GET_g(p1)
> > > ==
> > > GET_g(p2) && \
> > >                              GET_b(p1) == GET_b(p2))
> > >  
> 
> Frediano