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Subject: Re: [PATCH] general style: replaces memcmp() with proper starts_with()
Date: Wed, 12 Mar 2014 21:51:24 +0100
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Cc: Quint Guvernator <quintus.public@gmail.com>, git@vger.kernel.org
To: Junio C Hamano <gitster@pobox.com>, Jeff King <peff@peff.net>
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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 attempt=
 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=20
also needed for correctness, however, to avoid running over the end of=20
the buffer.

> If one really wants to remove the magic constants from this, then
> one must take advantage of the pattern
>
> 	len =3D=3D strlen(S) - 1 && !memcmp(field, S, strlen(S))
>
> that appears here, and come up with a simple abstraction to express
> that we are only using the string S (e.g. "tree "), length len and
> location field of the counted string.

This might be a job for kwset.

Ren=E9