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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.lang.c
Subject: Re: "A diagram of C23 basic types"
Date: Thu, 3 Apr 2025 14:45:21 +0300
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On Thu, 3 Apr 2025 11:41:31 +0200
David Brown <david.brown@hesbynett.no> wrote:

> On 03/04/2025 10:45, Muttley@DastardlyHQ.org wrote:
> > On Wed, 02 Apr 2025 16:16:27 GMT
> > scott@slp53.sl.home (Scott Lurndal) wibbled:  
> >> Muttley@DastardlyHQ.org writes:  
> >>> On Wed, 2 Apr 2025 16:59:45 +0200
> >>> David Brown <david.brown@hesbynett.no> wibbled:  
> >>>> On 02/04/2025 16:05, Muttley@DastardlyHQ.org wrote:  
> >> ist first.  
> >>>  
> >>>> 18. "unreachable()" is now standard.  
> >>>
> >>> Googled it - don't see the point.  
> >>
> >> That's a defect in your understanding, not a defect in the
> >> standard.
> >>
> >> I've found  the gcc equivelent useful often in standalone
> >> applications (OS, Hypervisor, standalone utilities, etc).  
> > 
> > Enlighten me then.
> >   
> 
> I can't tell you what Scott uses it for, but I have used gcc's 
> __builtin_unreachable() a fair number of times in my coding.  I use
> it to inform both the compiler and human readers that a path is
> unreachable:
> 
> 	switch (x) {
> 		case 1 : ...
> 		case 2 : ...
> 		case 3 : ...
> 		default : __builtin_unreachable();
> 	}
> 
> I can also use it to inform the compiler about data :
> 
> 	if ((x < 0) || (x > 10)) __builtin_unreachable();
> 	// x must be 1 .. 10
> 
> Mostly I have it wrapped in macros that let me conveniently have 
> run-time checking during testing or debugging, and extra efficiency
> in the code when I am confident it is bug-free.
> 
> Good use of __builtin_unreachable() can result in smaller and faster 
> code, and possibly improved static error checking.  It is related to
> the C++23 "assume" attribute (which is also available as a gcc
> extension in any C and C++ version).
> 
> 

In theory, compilers can use unreachable() to generated better code.
In practice, every single time I looked at compiler output, it made no
difference.