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From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: 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: Sat, 05 Dec 2020 12:24:10 -0800	[thread overview]
Message-ID: <3161fc13d69c388b1f51f59c6ecea48dcd0a7856.camel@HansenPartnership.com> (raw)
In-Reply-To: <CAMj1kXFvCZKH-Bbvu-V5-GxcTun8yz98igeFxRc1tTswbttM0w@mail.gmail.com>

On Sat, 2020-12-05 at 20:36 +0100, Ard Biesheuvel wrote:
> On Fri, 4 Dec 2020 at 19:02, James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > On Fri, 2020-12-04 at 18:07 +0100, Ard Biesheuvel wrote:
> > > On Fri, 4 Dec 2020 at 18:06, <laniel_francis@privacyrequired.com>
> > > wrote:
> > > > From: Francis Laniel <laniel_francis@privacyrequired.com>
> > > > 
> > > > The two functions indicates if a string begins with a given
> > > > prefix. The only difference is that strstarts() returns a bool
> > > > while str_has_prefix() returns the length of the prefix if the
> > > > string begins with it or 0 otherwise.
> > > > 
> > > 
> > > Why?
> > 
> > I think I can answer that.  If the conversion were done properly
> > (which it's not) you could get rid of the double strings in the
> > code which are error prone if you update one and forget
> > another.  This gives a good example: 3d739c1f6156 ("tracing: Use
> > the return of str_has_prefix() to remove open coded numbers"). so
> > in your code you'd replace things like
> > 
> >     if (strstarts(option, "rgb")) {
> >         option += strlen("rgb");
> >         ...
> > 
> > with
> > 
> >     len = str_has_prefix(option, "rgb");
> >     if (len) {
> >         option += len
> >         ...
> > 
> > Obviously you also have cases where strstart is used as a boolean
> > with no need to know the length ... I think there's no value to
> > converting those.
> > 
> 
> This will lead to worse code being generated. strlen() is evaluated
> at build time by the compiler if the argument is a string literal, so
> your 'before' version gets turned into 'option += 3', whereas the
> latter needs to use a runtime variable.

str_has_prefix() is an always_inline function so it should be build
time evaluated as well.  I think most compilers see len as being a
constant and unchanged, so elide the variable.  This means the code
generated should be the same.

> So I don't object to using str_has_prefix() in new code in this way,
> but I really don't see the point of touching existing code.

That's your prerogative as a Maintainer ... I was just explaining what
the original author had in mind when str_has_prefix() was created.

James



  reply	other threads:[~2020-12-05 20:24 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 [this message]
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
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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