linux-kernel - Re: Big git diff speedup by avoiding x86 "fast string" memcmp

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Message-ID: <AANLkTim=Hd_xhk4egrTq96rk0eCQRWOxEjcfxM53HwXq@mail.gmail.com>
Date:	Wed, 15 Dec 2010 15:09:26 -0800
From:	Tony Luck <tony.luck@...el.com>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Big git diff speedup by avoiding x86 "fast string" memcmp

On Wed, Dec 8, 2010 at 11:09 PM, Nick Piggin <npiggin@...nel.dk> wrote:
> +static inline int dentry_memcmp(const unsigned char *cs,
> +                               const unsigned char *ct, size_t count)
> +{
> +       while (count) {
> +               int ret = (*cs != *ct);
> +               if (ret)
> +                       return ret;
> +               cs++;
> +               ct++;
> +               count--;
> +       }
> +       return 0;
> +}

Since you are proposing a routine that only compares file
names - I wonder whether it would be faster to start at the
end and work backwards? If the filenames are the same,
it makes no difference - you have to look at all the bytes.
But if they are different you might bail out earlier. There
are many applications that stick a common prefix onto
the start of filenames (just look in "/lib" !), but I think it is
less common to add a suffix (longer than "." single letter).

-Tony
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