Re: Support for memcpy with equal source and destination

[Paul Eggert <eggert@cs.ucla.edu>]

On 2023-11-25 00:20, Ralf Jung wrote:
> For a memcpy that is implemented in C (rather than assembly), I don't 
> think it is possible to make this promise (of supporting src==dest) when 
> there is "restrict" in the signature.

Actually it is possible, though it costs a conditional branch compared 
to the naive approach that generally would work anyway.

Here's a sample implementation. When dest == src this function doesn't 
dereference either pointer, and this satisfies the C standard's rules 
for 'restrict'.

   #include <string.h>

   void *
   memcpy (void *restrict dest, void const *restrict src, size_t n)
   {
     if (dest != src)
       {
	char *d = dest;
	char const *s = src;
	for (; n != 0; n--)
	  *d++ = *s++;
       }
     return dest;
   }

When n is zero, this implementation also supports NULL dest or src, 
though that's a separate issue.


> when one sees restrict in a signature, it is impossible to tell what the actual constraint is without further documentation: the function needs to say which memory is being accesses through which pointer, and *that* is then where the disjointness comes from.

True, and one could document the new guarantee along those lines. 
Writing the documentation could be a bit tricky, though, as one needs to 
explain all this stuff clearly without being too prolix.