On Fri, Oct 3, 2014 at 8:41 AM, David Wohlferd <d...@limegreensocks.com> wrote:
>
>> You want
>>
>> "=m" (*( struct foo { char x[8]; } __attribute__((may_alias)) *)Dest)
>
>
> Thank you. With your help, that worse-than-useless sample in the docs is
> getting closer to something people can actually use.
>
> Except for one last serious problem: This trick only works for very
> specific (and very small) sizes of x (something else the existing docs fail
> to mention).
>
> On x64: Sizes of 1/2/4/8/16 correctly clobber Dest (and only Dest) as
> expected. Trying to use any other values (5, 12, 32, etc) seem to force a
> full memory clobber. This is the case REGARDLESS of the actual size of the
> underlying structure. For example clobbering 16 bytes of a 12 byte struct
> only clobbers the 12 bytes of Dest (as expected), but clobbering 12 bytes of
> a 12 byte struct performs a full memory clobber. This presents a problem
> for general purpose routines like memset, where the actual size of the block
> cannot be hardcoded into the struct.
>
> glibc (which uses this trick to avoid using the memory clobber) uses a size
> of 0xfffffff. Being larger than 16, this always causes a full memory
> clobber, not the "memory constraint" clobber I assume they were expecting.
> OTOH, since they don't use may_alias, this is may actually be a good
> thing...
>
> While I really like the idea of using memory constraints to avoid all out
> memory clobbers, 16 bytes is a pretty small maximum memory block, and x32
> only supports a max of 8. Unless there's some way to use larger sizes (say
> SSIZE_MAX), this feature hardly seems worth documenting.
I wonder how you figured out that a 12 byte clobber performs a full
memory clobber? I'm quite sure it does not.
Richard.
> dw
