On Fri, 4 Dec 2020 at 19:02, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 2020-12-04 at 18:07 +0100, Ard Biesheuvel wrote:
> > On Fri, 4 Dec 2020 at 18:06, <laniel_francis@xxxxxxxxxxxxxxxxxxx>
> > wrote:
> > > From: Francis Laniel <laniel_francis@xxxxxxxxxxxxxxxxxxx>
> > >
> > > 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.
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.
