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From: Arvind Sankar <nivedita@alum.mit.edu>
To: David Laight <David.Laight@ACULAB.COM>
Cc: 'Arvind Sankar' <nivedita@alum.mit.edu>,
	Ard Biesheuvel <ardb@kernel.org>,
	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 11:10:48 -0500	[thread overview]
Message-ID: <X9OaCEPwNW3K++t+@rani.riverdale.lan> (raw)
In-Reply-To: <4c251bcaf8234dd19ca0ecd0a756cf7f@AcuMS.aculab.com>

On Fri, Dec 11, 2020 at 09:45:15AM +0000, David Laight wrote:
> 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
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

Avoiding the intermediate varable is for readability at the call-site,
not performance. The performance of this function shouldn't matter --
you shouldn't be parsing strings in a hotpath anyway, and nobody cares
if the EFI stub takes a few nanoseconds longer if that makes the code
more robust and/or readable.

That said,
- I'd be surprised if the overhead of an L1D cache access was measurable
  over the strncmp()/strlen() calls.
- You can't replace strncmp() with memcmp().
- Regarding sizeof() vs strlen(), you can simply use __builtin_strlen()
  to optimize string literals, there's no need for hacks using the
  preprocessor.

  reply	other threads:[~2020-12-11 17:20 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-04 17:03 [RFC PATCH v1 00/12] " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 01/12] arm: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 02/12] mips: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 03/12] crypto: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 04/12] device-mapper: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 05/12] renesas: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 06/12] omap: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 07/12] efi: " laniel_francis
2020-12-04 17:07   ` Ard Biesheuvel
2020-12-04 17:19     ` Francis Laniel
2020-12-04 18:02     ` James Bottomley
2020-12-05 19:08       ` Francis Laniel
2020-12-05 19:36       ` Ard Biesheuvel
2020-12-05 20:24         ` James Bottomley
2020-12-05 20:57           ` Ard Biesheuvel
2020-12-05 21:15             ` James Bottomley
2020-12-05 21:20               ` Ard Biesheuvel
2020-12-05 23:04                 ` James Bottomley
2020-12-07 15:10                   ` Steven Rostedt
2020-12-07 16:25                     ` David Laight
2020-12-05 20:28         ` Rasmus Villemoes
2020-12-10 18:14         ` Arvind Sankar
2020-12-11  9:45           ` David Laight
2020-12-11 16:10             ` Arvind Sankar [this message]
2020-12-04 17:03 ` [RFC PATCH v1 08/12] ide: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 09/12] mips: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 10/12] module: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 11/12] musb: " laniel_francis
2020-12-04 17:03 ` [RFC PATCH v1 12/12] string.h: Remove strstarts() laniel_francis
2020-12-04 17:56 ` [RFC PATCH v1 00/12] Replace strstarts() by str_has_prefix() James Bottomley

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