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Message-ID: <20130409055436.GU20323@brightrain.aerifal.cx>
Date: Tue, 9 Apr 2013 01:54:36 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: multibyte performance findings
On Sat, Apr 06, 2013 at 02:08:52AM -0400, Rich Felker wrote:
> On Sat, Apr 06, 2013 at 01:21:21AM -0400, Rich Felker wrote:
> > Hi all,
> >
> > I've been examining performance in the multibyte conversion functions
> > (as part of the POSIX locale controversy), and have some interesting
> > findings so far:
> > [...]
>
> And here's a diff of the proposed changes so far..
>
> Rich
> diff --git a/src/multibyte/mbrtowc.c b/src/multibyte/mbrtowc.c
> index cc49781..d552652 100644
> --- a/src/multibyte/mbrtowc.c
> +++ b/src/multibyte/mbrtowc.c
> @@ -18,6 +18,7 @@ size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate
> const unsigned char *s = (const void *)src;
> const unsigned N = n;
>
> + if (!n) return -2;
> if (!st) st = (void *)&internal_state;
> c = *(unsigned *)st;
>
> @@ -27,9 +28,9 @@ size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate
> n = 1;
> } else if (!wc) wc = (void *)&wc;
>
> - if (!n) return -2;
This change turned out to be wrong (it's an invalid transformation
when s is null) and I found a better improvement anyway, which I've
committed. The commit log message is actually rather interesting:
http://git.musl-libc.org/cgit/musl/commit/?id=a49e038bab7b3927b6a9c7d0c52f9e1a9cb82629
and I think this finding serves as a warning about writing 'clever'
code for special cases that "falls through" to the general code,
rather than just writing the special case code explicitly.
> + /* This condition can only be true if *s<0x80 and c==0 */
> + if (*s + c < 0x80) return !!(*wc = *s);
> if (!c) {
> - if (*s < 0x80) return !!(*wc = *s);
> if (*s-SA > SB-SA) goto ilseq;
> c = bittab[*s++-SA]; n--;
> }
I omitted this for now too since the improvement seems difficult to
measure. In principle it should be better, so I may revisit this
later.
Rich
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