Re: Patch to add builtin strncmp and builtin strncpy

[Jakub Jelinek <jakub at redhat dot com>]

On Sun, Nov 26, 2000 at 01:15:44PM -0800, Geoff Keating wrote:
> "Kaveh R. Ghazi" <ghazi@caip.rutgers.edu> writes:
> 
> > This patch adds builtins for strncpy and strncmp as described on the
> > GCC projects page.  The transformations done are:
> > 
> > strncpy(s1, s2, 0) -> NOP
> >   (handling side-effects of s1 & s2)
> > 
> > strncpy(s1, "string", n) -> memcpy (s1, "string", n)
> >   (where strlen("string")+1 >= n)
> 
> Why not also
> 
> strncpy(s1, "string", n) -> memcpy (s1, "string", sizeof ("string")+1)
>    (where strlen("string")+1 < n)

Because they do different thing, strncpy has to pad it with '\0' up to
length n.
The transformation could be really
-> slen = strlen ("string") + 1;
   memcpy (s1, "string", slen);
   memset (s1 + slen, 0, n - slen);

but then the question is if it is really an optimization (it could be if
both memcpy and memset are inlined (especially if it would be inlined
mempcpy), but doing 2 function calls would be bad.
Alternatively, if difference between slen and n was very small, we could
create new string cst which would have additional 0 at the end (or even
better cooperate with varasm if the string constant has not been output yet,
pad it at the end for everyone).

	Jakub