[PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

[David Laight]

From: Willy Tarreau
> Sent: 06 December 2024 19:39
> On Fri, Dec 06, 2024 at 11:15:20AM -0800, Linus Torvalds wrote:
> > On Fri, 6 Dec 2024 at 11:07, David Laight <David.Laight at aculab.com> wrote:
> > >
> > > I'm missing the compiler version and options to generate the error.
> >
> > Just -Wall with most recent gcc versions seems to do it. At least I
> > can repro it with gcc-14.2.1 and something silly like this:
> >
> >   $ cat t.c
> >   int fn(int a) { return (a<<2)?1:2; }
> >   $ gcc -Wall -S t.c
> >   t.c: In function 'fn':
> >   t.c:1:26: warning: '<<' in boolean context, did you mean '<'?
> > [-Wint-in-bool-context]
> >
> > > Does a '+ 0' help?  "(var << 2) + 0 ? 0 : 0"
> >
> > Yeah, that actually works.
> >
> > And "+0" is nice in that it should work in any context.
> 
> I've already used "+0" to shut certain warnings, I don't really remember
> which one, but also remember it was OK everywhere I needed.

I've often used +0u when -Wsign-compare is enabled.
Much safer than a cast.

> 
> Another trick I've been using to shut up the compiler is a cast via typeof
> and an intermediary variable:
> 
>   #define shut_up(expr)                           \
>           ({                                      \
>                   typeof(expr) _expr_ = expr;     \
>                   _expr_;                         \
>                   })

That is like OPTIMISER_HIDE_VAR() and can't be used in a 'constant integer expression'.

I suspect it also has the same nasty habit of adding an extra register move.

	David

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