linux-kernel - Re: [RFC PATCH v1 07/12] efi: Replace strstarts() by str_has

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Message-ID: <X9OaCEPwNW3K++t+@rani.riverdale.lan>
Date:   Fri, 11 Dec 2020 11:10:48 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Arvind Sankar' <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "laniel_francis@...vacyrequired.com" 
        <laniel_francis@...vacyrequired.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 07/12] efi: Replace strstarts() by
 str_has_prefix().

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.