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

[Alan Stern <stern@rowland.harvard.edu>]

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