Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency

Alan Stern
2024-09-30T18:53:07Z

On Mon, Sep 30, 2024 at 07:05:06PM +0200, Jonas Oberhauser wrote:
>
>
> Am 9/30/2024 um 6:43 PM schrieb Alan Stern:
> > On Mon, Sep 30, 2024 at 01:26:53PM +0200, Jonas Oberhauser wrote:
> > >
> > >
> > > Am 9/28/2024 um 4:49 PM schrieb Alan Stern:
> > >
> > > I should also point out that it is not enough to prevent the compiler from
> > > using @a instead of @b.
> > >
> > > It must also be prevented from assigning @b=@a, which it is often allowed to
> > > do after finding @a==@b.
> >
> > Wouldn't that be a bug?
>
> That's why I said that it is often allowed to do it. In your case it
> wouldn't, but it is often possible when a and b are non-atomic &
> non-volatile (and haven't escaped, and I believe sometimes even then).
>
> It happens for example here with GCC 14.1.0 -O3:
>
> int fct_hide(void)
> {
> int *a, *b;
>
> do {
> a = READ_ONCE(p);
> asm volatile ("" : : : "memory");
> b = READ_ONCE(p);
> } while (a != b);
> OPTIMIZER_HIDE_VAR(b);
> return *b;
> }
>
>
>
> ldr r1, [r2]
> ldr r3, [r2]
> cmp r1, r3
> bne .L6
> mov r3, r1 // nay...

A totally unnecessary instruction, which accomplishes nothing other than
to waste time, space, and energy. But nonetheless, allowed -- I agree.

The people in charge of GCC's optimizer might like to hear about this,
if they're not already aware of it...

> ldr r0, [r3] // yay!
> bx lr

One could argue that in this example the compiler _has_ used *a instead
of *b. However, such an argument would have more force if we had
described what we are talking about more precisely.

Yes, we do want to prevent compilers from doing this. I'm not sure that
it really needs to be mentioned in the comments or commit description,
though.

Alan