Hi,
On Wed, 26 Feb 2014, Richard Biener wrote:
> > "\tstr wzr, [%2, #4]!\n"
> > "\tsub %1, %1, #1\n"
> > "\tcbnz %1, 0b\n"
> > : "=m"(*p), "+r"(len) : "r"(p));
> >
> > I presume this is wrong because *p only refers to p[0]. Is it
> > possible to tell GCC that the asm writes to the whole block of memory
> > reachable from p without a total memory clobber?
> >
> > I have attached some code that works but it is so fugly that I'm
> > reluctant to recommend it.
>
> Well, you are surely lucky at the moment because nothing in GCC
> disambiguates asms with memory operands against anything.
>
> But yes, technically you write p[0] here but as "m" merely builds
> an address to the memory I'd say that we have to treat any "m"
> operand as possibly reading from / writing to / clobbering the
> whole object that can be refered to using that address.
>
> Anything else would need extra syntax to specify a memory
> range that is accessed.
And we have that (as you found out downthread), and it's even documented
for inline asms, I'll cite:
---------------------------------------
If your assembler instructions access memory in an unpredictable
fashion, add `memory' to the list of clobbered registers.
...
If you know how large the accessed memory is, you can add it as input or
output but if this is not known, you should add `memory'. As an example,
if you access ten bytes of a string, you can use a memory input like:
{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.
---------------------------------------
So, I don't think there's any need to redefine '"m"(*p)' to mean anything
else than p[0].
> And yes, I probably should implement at least basic disambiguation
> against memory accessing asm()s ;)
>
> Summary: I consider your asm ok without changes.
I don't.
Ciao,
Michael.
