[Spice-devel] [PATCH spice-common] codegen: Make the compiler work about better way to write unaligned memory

[Christophe Fergeau]

On Tue, Sep 12, 2017 at 03:14:22AM -0400, Frediano Ziglio wrote:
> > 
> > On Mon, Sep 11, 2017 at 11:13:04AM +0100, Frediano Ziglio wrote:
> > > Instead of assuming that the system can safely do unaligned access
> > > to memory use packed structures to allow the compiler generate
> > > best code possible.
> > 
> > It will generate the best code possible, but assuming all accesses are
> > unaligned I believe? Ie a perfectly fine 32 bit aligned field may still
> > be read using byte-sized accesses if the arch/cpu does not support
> > unaligned 32 bit reads?
> > 
> 
> Yes. A packed structure tells the compiler that the structure can be
> not aligned so any field can be unaligned and compiler will read/write
> the fields byte by byte if needed.
> 
> > The big missing part in this commit log is what this commit is doing in
> > human readable form...
> 
> 
> 
> > This changes the current read_intX methods from:
> > 
> > #ifdef WORDS_BIGENDIAN
> > #define read_uint32(ptr) ((uint32_t)SPICE_BYTESWAP32(*((uint32_t *)(ptr))))
> > #define write_uint32(ptr, val) *(uint32_t *)(ptr) =
> > SPICE_BYTESWAP32((uint32_t)val)
> > #else
> > #define read_uint32(ptr) (((uint32_unaligned_p)(ptr))->v)
> > #define read_uint32(ptr) (*((uint32_t *)(ptr)))
> > #endif
> > 
> > to:
> > #include <spice/start-packed.h>
> > typedef struct {
> >     uint32_t v;
> > } *uint32_unaligned_p;
> > include <spice/end-packed.h>
> > 
> > #ifdef WORDS_BIGENDIAN
> > #define read_uint32(ptr)
> > ((uint32_t)SPICE_BYTESWAP32(((uint32_unaligned_p)(ptr))->v))
> > #define write_uint32(ptr, val) ((uint32_unaligned_p)(ptr))->v =
> > SPICE_BYTESWAP32((uint32_t)val)
> > #else
> > #define read_uint32(ptr) (((uint32_unaligned_p)(ptr))->v)
> > #define write_uint32(ptr, val) (((uint32_unaligned_p)(ptr))->v) = val
> > #endif
> > 
> 
> Should I copy this section entirely in the commit message?

Sure!