Re: [PATCH v2] vsprintf: introduce %dE for error constants

[Juergen Gross]

On 29.08.19 10:12, Petr Mladek wrote:
> On Wed 2019-08-28 21:18:37, Uwe Kleine-KÃnig  wrote:
> > Hello Petr,
> >
> > On 8/28/19 1:32 PM, Petr Mladek wrote:
> > > On Tue 2019-08-27 23:12:44, Uwe Kleine-KÃnig  wrote:
> > > > Petr Mladek had some concerns:
> > > > > There are ideas to make the code even more tricky to reduce
> > > > > the size, keep it fast.
> > > >
> > > > I think Enrico Weigelt's suggestion to use a case is the best
> > > > performance-wise so that's what I picked up. Also I hope that
> > > > performance isn't that important because the need to print an error
> > > > should not be so common that it really hurts in production.
>
> This is contadicting. The "best" performance-wise solution was
> choosen in favor of space. The next sentence says that performance
> is not important.
>
> > > I personally do not like switch/case. It is a lot of code.
> > > I wonder if it even saved some space.
> >
> > I guess we have to die either way. Either it is quick or it is space
> > efficient.
>
> I am more concerned about the size. Well, array of strings will
> be both fast and size efficient.
>
> > With the big case I trust the compiler to pick something
> > sensible expecting that it adapts for example to -Osize.
>
> I am not sure what are the expectations here. I can't imagine
> another translation than:
>
>     if (val == 1)
>       str = "EPERM";
>     else if (val == 2)
>       str = "ENOENT"
>     else if (val == 3)
>       str = "ESRCH"
>     ...
>
> It means that all constans will be hardcoded in the code instead
> of in data section. Plus there will be instructions for each
> if/else part.
>
> > > If you want to safe space, I would use u16 to store the numbers.
> > > Or I would use array of strings. There will be only few holes.
> > >
> > > You might also consider handling only the most commonly
> > > used codes from errno.h and errno-base.h (1..133). There will
> > > be no holes and the codes are stable.
> >
> > I'd like to postpone the discussion about "how" until we agreed about
> > the "if at all".
>
> It seems that all people like this feature.

Hmm, what about already existing format strings conatining "%dE"?

Yes, I could find only one (drivers/staging/speakup/speakup_bns.c), but
nevertheless...

> BTW: I though more about generating or cut&pasting the arrary.
> I can't find any reasonable way how to generate it.

Generate the array and errno.h/errno-base.h from the same source?


Juergen