Marking select(2) as restrict

[Konstantin Belousov]

On Thu, Feb 22, 2018 at 11:27:52AM +0100, Tijl Coosemans wrote:
> On Wed, 21 Feb 2018 12:44:00 +0200 Konstantin Belousov <kostikbel at gmail.com> wrote:
> > On Tue, Feb 20, 2018 at 10:14:05PM -0800, Eitan Adler wrote:
> >> On 20 February 2018 at 21:19, Warner Losh <imp at bsdimp.com> wrote:  
> >>> Once upon a time, this would break a lot of code. Perhaps times have
> >>> changed.  
> >> 
> >> I've seen very little code that this would break though some of it
> >> certainly exists.  
> > You certainly seen very little code, but the question was about the
> > existed code.
> 
> FWIW, it seems that glibc uses restrict since 2000 so there's unlikely to
> be much fallout:
> https://sourceware.org/git/?p=glibc.git;a=blob;f=misc/sys/select.h
> https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=98cbe360d947b59e7a5eda068581f4cfeb4b99b3

Clearly, nobody knowns.  At least, glibc is used with gcc compilation, not
with clang.

Consider the recently changed devd code:
	select(n + 1, &fd, &fd, &fd);
There, compiler can see that restrict is applied to arguments which are
given same values.  Since this leads to the self-contradicting statement
	fd != fd
which cannot be true, compliler in its optimizing wisdom can assume that
the code is never executing and remove it.  I do not know whether clang
actually makes such transformation, but it does not sound unfeasible
looking at its other advances.