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"volatile" on variables and current processors

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  "volatile" on variables and current processors Vir Campestris <vir.campestris@invalid.invalid> - 2019-08-19 21:56 +0100
    Re: "volatile" on variables and current processors red floyd <dont.bother@its.invalid> - 2019-08-19 15:20 -0700
      Re: "volatile" on variables and current processors red floyd <dont.bother@its.invalid> - 2019-08-19 15:21 -0700
        Re: "volatile" on variables and current processors "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-08-19 16:37 -0700
          Re: "volatile" on variables and current processors Vir Campestris <vir.campestris@invalid.invalid> - 2019-08-20 21:58 +0100
            Re: "volatile" on variables and current processors scott@slp53.sl.home (Scott Lurndal) - 2019-08-20 22:18 +0000
              Re: "volatile" on variables and current processors Vir Campestris <vir.campestris@invalid.invalid> - 2019-08-22 21:39 +0100
            Re: "volatile" on variables and current processors Paavo Helde <myfirstname@osa.pri.ee> - 2019-08-21 09:02 +0300
              Re: "volatile" on variables and current processors "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-08-20 23:10 -0700
            Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-21 08:34 +0200
              Re: "volatile" on variables and current processors Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-21 20:39 +0100
                Re: "volatile" on variables and current processors Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-21 21:02 +0100
                Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-22 13:30 +0200
          Re: "volatile" on variables and current processors Bonita Montero <Bonita.Montero@gmail.com> - 2019-08-21 08:43 +0200
        Re: "volatile" on variables and current processors "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-20 02:22 +0200
          Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-20 10:05 +0200
      Re: "volatile" on variables and current processors Keith Thompson <kst-u@mib.org> - 2019-08-19 17:08 -0700
    Re: "volatile" on variables and current processors Bonita Montero <Bonita.Montero@gmail.com> - 2019-08-20 06:08 +0200
    Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-20 10:14 +0200
    Re: "volatile" on variables and current processors Juha Nieminen <nospam@thanks.invalid> - 2019-08-20 13:03 +0000
      Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-20 16:18 +0200

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#67815

On 20/08/2019 15:03, Juha Nieminen wrote:
> Vir Campestris <vir.campestris@invalid.invalid> wrote:
>> And it's got lots of use of 
>> volatile in the old way - don't cache this please, another thread may 
>> touch it.
> 
> volatile does in no way make a variable thread-safe. And it has never been
> even really meant for that.
> 
> The main use for volatile is in certain architectures (eg. certain
> embedded systems) where eg. certain memory locations have more significance
> to the system than being merely dummy storage for a value. For example,
> a particular memory location might be mapped by the hardware to be input
> and output to a port (which may be, for instance, some kind of 7-segment
> LCD display or the like). Thus writing to that memory location will output
> the value to that port, and readint that location will read the port. In
> these architectures telling the compilar that eg. a pointer to that memory
> location is pointing to a volatile value may be important, as it (usually)
> prevents the compiler from optimizing away reads and writes to it.
> 

That was the original motivation for "volatile", and it is still a major
use-case.  (Usually the hardware will have features for treating the
memory locations of these hardware registers in special ways, such as
skipping caches and disallowing re-ordering in buffers or speculative
accesses - this gives you some of the effects of C++11 atomic accesses
on these addresses even when you just have a simple "volatile".)

"volatile" also finds use in single core systems - it has long been
essential in microcontroller programming for handling data between
interrupt functions and other code, and on most microcontrollers it is
still sufficient (while C++11 atomics would be overkill and slower).

Sometimes it is also used to force particular effects.  For example, if
you have a pure calculation function "foo" (i.e., it has no side effects
and does not access non-const global data), and you want to know how
long it takes to run, you might try:

	const int x = 123;
	auto start_time = get_time();
	for (int i = 0; i < 1000000; i++) {
		int y = foo(x);
	}
	auto end_time = get_time();
	std::cout << "Time for 1000000 runs is " <<
		(end_time - start_time) << "\n";

Enable optimisation, and the compiler may happily remove the entire loop.

But if you use:

	volatile const int x = 123;

and

	volatile int y = foo(x);

you get the timing you want.


> In general, for "normal" variables or pointers you don't want to
> needlessly declare then volatile, because they can become really inefficient,
> as the compiler usually obey the instruction and literally read and write
> to that variable in memory every single time it's done in the code.
> (For example, declaring a loop counter variable volatile for no good
> reason would be rather silly.)
> 

Yes.

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