Christophe de Dinechin authored Jun 08, 2017 and Christophe Fergeau committed Jun 20, 2017

For example, something like this:
    uint8_t  *p8;
    uint32_t *p32 = (uint32_t *) p8;

generates a warning like this:
  spice-channel.c:1350:10: error: cast from 'uint8_t *' (aka 'unsigned char *') to
      'uint32_t *' (aka 'unsigned int *') increases required alignment from 1 to
      4 [-Werror,-Wcast-align]

The warning indicates that we end up with a pointer to data that
should be 4-byte aligned, but its value may be misaligned. On x86,
this does not make much of a difference, except a relatively minor
performance penalty. However, on platforms such as older ARM, misaligned
accesses are emulated by the kernel, and support for them is optional.
So we may end up with a fault.

The intent of the fix here is to make it easy to identify and rework
places where actual mis-alignment occurs. Wherever casts raise the warning,
they are replaced with a macro:

- SPICE_ALIGNED_CAST(type, value) casts value to type, and indicates that
  we believe the resulting pointer is aligned. If it is not, a runtime
  warning will be issued if you configure with --enable-alignment-warnings.

- SPICE_UNALIGNED_CAST(type, value) casts value to type, and indicates that
  we believe the resulting pointer is not always aligned. A debug message
  is issued if you configure with --enable-alignment-warnings.

Any code using SPICE_UNALIGNED_CAST may need to be revisited in order
to improve performance, e.g. by using memcpy. A few easy fixes have been
made in his patch.

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>