RE: [RFC PATCH v1 07/12] efi: Replace strstarts() by str_has_prefix().

From: David Laight <David.Laight@ACULAB.COM>
To: 'Arvind Sankar' <nivedita@alum.mit.edu>,
	Ard Biesheuvel <ardb@kernel.org>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>,
	"laniel_francis@privacyrequired.com" 
	<laniel_francis@privacyrequired.com>,
	linux-efi <linux-efi@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: RE: [RFC PATCH v1 07/12] efi: Replace strstarts() by str_has_prefix().
Date: Fri, 11 Dec 2020 09:45:15 +0000	[thread overview]
Message-ID: <4c251bcaf8234dd19ca0ecd0a756cf7f@AcuMS.aculab.com> (raw)
In-Reply-To: <X9Jlfqiu/vCOAm3U@rani.riverdale.lan>

From: Arvind Sankar
> Sent: 10 December 2020 18:14
...
> I wasn't aware of str_has_prefix() at the time. It does seem useful to
> eliminate the duplication of the string literal, I like the
> skip_prefix() API suggestion, maybe even
> 
> 	bool str_skip_prefix(const char **s, const char *pfx)
> 	{
> 		size_t len = str_has_prefix(*s, pfx);
> 		*s += len;
> 		return !!len;
> 	}
> 	...
> 	if (str_skip_prefix(&option, prefix)) { ... }
> 
> to avoid the intermediate variable.

That'll generate horrid code - the 'option' variable has to be
repeatedly reloaded from memory (unless it is all inlined).

Perhaps the #define

#define str_skip_prefix(str, prefix) \
{( \
	size_t _pfx_len = strlen(prefix)); \
	memcmp(str, pfx, _pfx_len) ? 0 : ((str) += _pfx_len, 1); \
)}

There's probably something that'll let you use sizeof() instead
of strlen() for quoted strings (if only sizeof pfx != sizeof (char *)).

	David

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