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Date: Sat, 1 Dec 2018 09:15:56 +1100
From: Xan Phung <xan.phung@...il.com>
To: musl@...ts.openwall.com
Subject: Re: stdio glitch & questions

Thanks for the quick answer, and I've taken a look at the pre-2011 fwrite.c
code, and using SYS_writev is indeed much cleaner code!

See below on my proposed answer to your question about what the "cutoff"
should be for copying.  (SYS_writev is fully retained, but the 2nd iovec
element will very often be zero length in this proposal, which makes
emulation of SYS_writev much more efficient).

On Sat, 1 Dec 2018 at 03:10, Rich Felker <dalias@...c.org> wrote:

>
> It would probably be welcome to make __stdio_write make use of
> SYS_write when it would be expected to be faster (len very small), but
> I'm not sure what the exact cutoff should be.
>
>
My proposal is the cutoff be 5-8 bytes (on 32 bit CPUs) , and on 64 bit
CPUs, 9-16 bytes.

The cutoffs are selected in such a way that the "no copy" loop (searching
for '\n') always ends on a word aligned position (opening the door to
future optimisations by using word-at-a-time search for '\n' instead of
byte-at-a-time).  The "copy" branch is also guaranteed to only be a double
word at most, but a minimum of a single word (allowing a two word memcpy to
be done with just a 2x load/mask/store word code sequence).  Some example
code is shown to give a general idea of word-aligning the cutoff amount
(but not yet doing word-at-a-time searching of '\n', or optimised two word
memcpy).

*CURRENT __fwritex.c CODE (lines 12-20)*:


if (f->lbf >= 0) {

/* Match /^(.*\n|)/ */

for (i=l; i && s[i-1] != '\n'; i--);

if (i) {

size_t n = f->write(f, s, i);

if (n < i) return n;

s += i;

l -= i;

}

}



*PROPOSED*:

	size_t i, len;
	if (f->lbf >= 0) {
		const unsigned char *t = ALIGN(s+sizeof(size_t)*2);
		for (i = l+s-t; ; i--) {
			if (i <= 0) {   /* SHORT LINE - copy up to 16 bytes into f->wpos
buffer and then flush line */
				for (j = t-s; j && s[j-1] != '\n'; j--);
				if (j) {
			  		memcpy(f->wpos, s, j);  f->wpos += j;
					size_t n = f->write(f, t, 0);
					if (n < 0) return n;
					s += j;
					l -= j;
				}				break;
			}
			if (t[i-1] == '\n') {
				size_t n = f->write(f, s, len = i+t-s);
				if (n < len) return n;
				s += len;
				l -= len;
				break;
			}
		}
	}

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