Dare I ... another volatile discussion ?

[Iain Buclaw via Digitalmars-d]

On 7 May 2015 at 18:04, Jens Bauer via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> I'm sorry for opening such a topic; I've heard it's not liked a lot, but I
> think it might be necessary.
>
> I'm not asking for a 'volatile' keyword, but rather to find out what the
> right thing to use is.
> After reading a few different threads related to microcontrollers, I started
> wondering how to program the following in D:
>
> 1: System level drivers, which writes directly to hardware registers (any
> architecture, PC/i386 [Linux, Windows, others], Atari ST, IBM BladeCenter,
> etc.)
> 2: Interrupts that needs to share variables.
>
> 1) is what we basically need on microcontrollers. If it's possible to write
> a driver in D, which has no problems with accessing hardware, then it should
> be possible to do it for any microcontroller as well.
>
> 2) shared variables could be used for interrupts, but what happens if the
> TLS is disabled; will shared variables work ? -Interrupts are not threads.
>
> Regarding (1), because marking a variable 'shared' is not enough (it allows
> instructions to be moved around), Johannes already made a volatileLoad and
> volatileStore, which will be usable for microcontrollers, though for
> convenience, it requires writing additional code.
> -But this solution ... I do not know if it would work, when writing a driver
> for Debian running on a i586 platform or PowerMac G3 for instance.
>
> If variable 'alice' and variable 'bob' are both shared, and reading from
> 'bob', then writing to 'alice'; would instructions be moved around, so
> reading from 'bob' could actually occur after writing to 'alice' ?
>

Yes.  Take this example:

---
shared int alice, bob;
void foo()
{
    alice = bob + 1;
    bob = 0;
}
---

gdc without optimisations produces:
---
; load bob into %eax
movl    _D7reorder3bobOi(%rip), %eax
; + 1
addl    $1, %eax
; store to alice
movl    %eax, _D7reorder5aliceOi(%rip)
; bob = 0
movl    $0, _D7reorder3bobOi(%rip)
---

gdc with optimisations produces:
---
; load bob into eax
movl    _D7reorder3bobOi(%rip), %eax
; bob = 0
movl    $0, _D7reorder3bobOi(%rip)
; + 1
addl    $1, %eax
; store to alice
movl    %eax, _D7reorder5aliceOi(%rip)
---

Now, this does not change the behaviour of the program if we consider
that bob and alice do not have any side effects.  However on a
micro-controller dealing with direct memory I/O ...


Iain.