Re: Machine description for volatile memory access

[William Tambe <tambewilliam at gmail dot com>]

On Mon, Sep 9, 2019 at 10:21 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Mon, Sep 09, 2019 at 09:48:46AM -0500, William Tambe wrote:
> > Does GCC support machine description for volatile memory accesses that
> > must not be cached ?
> > In other words, when using expressions such as theses, volatile memory
> > access instructions should be use:
> >
> > volatile uint8_t *m;
> > uint8_t n;
> >
> > *m = 5; // Should use a volatile memory store instruction.
> > n = *m; // Should use a volatile memory load instruction.
> >
> > If GCC has support for the above, are there any examples I could be
> > pointed to in the sources ?
>
> There is no such thing as a "volatile memory load" (or store).  What is
> it you really want to do?

I would like to be able to detect in the machine description that a
memory load or store is being done for a volatile memory location.

>
> "volatile" is a concept in the C language.  It doesn't translate to machine
> instructions very well.  You might want to use support for atomic (via
> builtins for example), or write assembler (or inline asm).

All I am looking for, is whether the "volatile" attribute can be
detected from the machine description.

>
>
> Segher