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

[James Bottomley]

On Sat, 2020-12-05 at 20:36 +0100, Ard Biesheuvel wrote:
> 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.

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