From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Kastrup <dak@gnu.org>
Subject: Re: [PATCH] general style: replaces memcmp() with proper starts_with()
Date: Wed, 12 Mar 2014 22:16:13 +0100
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Cc: Junio C Hamano <gitster@pobox.com>, Jeff King <peff@peff.net>,
	Quint Guvernator <quintus.public@gmail.com>,
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To: =?iso-8859-1?Q?Ren=E9?= Scharfe <l.s.r@web.de>
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In-Reply-To: <5320C8CC.3010404@web.de> (=?iso-8859-1?Q?=22Ren=E9?=
 Scharfe"'s message of "Wed, 12
	Mar 2014 21:51:24 +0100")
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Ren=E9 Scharfe <l.s.r@web.de> writes:

> Am 12.03.2014 20:39, schrieb Junio C Hamano:
>> Jeff King <peff@peff.net> writes:
>>
>>>>   static inline int standard_header_field(const char *field, size_=
t len)
>>>>   {
>>>> -	return ((len =3D=3D 4 && !memcmp(field, "tree ", 5)) ||
>>>> -		(len =3D=3D 6 && !memcmp(field, "parent ", 7)) ||
>>>> -		(len =3D=3D 6 && !memcmp(field, "author ", 7)) ||
>>>> -		(len =3D=3D 9 && !memcmp(field, "committer ", 10)) ||
>>>> -		(len =3D=3D 8 && !memcmp(field, "encoding ", 9)));
>>>> +	return ((len =3D=3D 4 && starts_with(field, "tree ")) ||
>>>> +		(len =3D=3D 6 && starts_with(field, "parent ")) ||
>>>> +		(len =3D=3D 6 && starts_with(field, "author ")) ||
>>>> +		(len =3D=3D 9 && starts_with(field, "committer ")) ||
>>>> +		(len =3D=3D 8 && starts_with(field, "encoding ")));
>>>
>>> These extra "len" checks are interesting.  They look like an attemp=
t to
>>> optimize lookup, since the caller will already have scanned forward=
 to
>>> the space.
>
> I wonder what the performance impact might be.  The length checks are
> also needed for correctness, however, to avoid running over the end o=
f
> the buffer.

Depends on whether memcmp is guaranteed to stop immediately on mismatch=
=2E
Then memcmp(field, "tree ", 5) cannot walk across a NUL byte in field.

--=20
David Kastrup