>
> On Thu, Jun 28, 2018 at 07:40:19AM +0100, Frediano Ziglio wrote:
> > Make code faster on platforms not supporting unaligned access
> > by default.
>
> How does this work exactly? Rather than silencing the warning with
> SPICE_UNALIGNED_CAST, and then potentially dereferencing a non-aligned
> 32 bit value, and getting a trap from the kernel/cpu, we instead give a
> lot of hints to the compiler to let it know the value is not going to be
> aligned, and we expect it's going to emit the right code?
>
> Christophe
>
Same "trick" used in the marshaller. Yes, compiler will generate the
right code. The SPICE_UNALIGNED_CAST marks that the code is doing
a misaligned access (or helps discovering it), does not solve the problem.
This solves the problem :-)
OT: yes, strangely this is encoded as big endian!
Frediano
> >
> > Signed-off-by: Frediano Ziglio <fziglio@xxxxxxxxxx>
> > ---
> > common/canvas_base.c | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/common/canvas_base.c b/common/canvas_base.c
> > index 6bf6e5d..36546b3 100644
> > --- a/common/canvas_base.c
> > +++ b/common/canvas_base.c
> > @@ -45,6 +45,14 @@
> > #include "mem.h"
> > #include "macros.h"
> >
> > +#include <spice/start-packed.h>
> > +typedef struct SPICE_ATTR_PACKED {
> > + uint32_t v;
> > +} uint32_unaligned_t;
> > +#include <spice/end-packed.h>
> > +
> > +#define read_uint32_be(ptr) ntohl(((uint32_unaligned_t *)(ptr))->v)
> > +
> > #define ROUND(_x) ((int)floor((_x) + 0.5))
> >
> > static inline int fix_to_int(SPICE_FIXED28_4 fixed)
> > @@ -572,7 +580,7 @@ static pixman_image_t *canvas_get_lz4(CanvasBase
> > *canvas, SpiceImage *image)
> >
> > do {
> > // Read next compressed block
> > - enc_size = ntohl(*SPICE_UNALIGNED_CAST(uint32_t *, data));
> > + enc_size = read_uint32_be(data);
> > data += 4;
> > dec_size = LZ4_decompress_safe_continue(stream, (const char *)
> > data,
> > (char *) dest, enc_size,
> > available);
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