Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 12/27/22 00:52, Alejandro Colomar wrote:
> However, and this is interesting, calling strnlen(3) results in faster code even 
> when no truncation occurs; at least for the short string I tested.
> 
> I suspect it might be because it is already heavily optimized in glibc, and it 
> implicitly simplifies surrounding code:
> 
> -       slen = strlen(src);
>          dsize = end - dst;
> -       trunc = (slen >= dsize);
> +       slen = strnlen(src, dsize);
> +       trunc = (slen == dsize);
> 
> The generated assembly code is one line smaller (on my system, and phase of the 
> moon), and some small percent faster.  :)

I found the reason; it helps simplify considerably the code.  Here's the 
resulting optimized code:


char *stp_nullable
stpecpy(char *stp_nullable dst, char *end, const char *restrict src)
{
	bool    trunc;
	size_t  dsize, dlen, slen;

	if (dst == end)
		return end;
	if (stp_unlikely(dst == NULL))  // Allow chaining with stpeprintf().
		return NULL;
	stp_impossible(dst > end);

	dsize = end - dst;
	slen = strnlen(src, dsize);
	trunc = (slen == dsize);
	dlen = slen - trunc;
	dst[dlen] = '\0';

	return mempcpy(dst, src, dlen) + trunc;
}


See how using strnlen(3) removed the ternary operator.  That's a great 
optimization.  :)

-- 
<http://www.alejandro-colomar.es/>

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