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References: <20101209070938.GA3949@amd>
Date: Wed, 15 Dec 2010 15:09:26 -0800
Message-ID: <AANLkTim=Hd_xhk4egrTq96rk0eCQRWOxEjcfxM53HwXq@mail.gmail.com>
Subject: Re: Big git diff speedup by avoiding x86 "fast string" memcmp
From: Tony Luck <tony.luck@intel.com>
To: Nick Piggin <npiggin@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, linux-arch@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
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On Wed, Dec 8, 2010 at 11:09 PM, Nick Piggin <npiggin@kernel.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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