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

'Alan Stern'
2024-10-01T22:57:19Z

On Tue, Oct 01, 2024 at 05:11:05PM +0000, David Laight wrote:
> From: Alan Stern
> > Sent: 30 September 2024 19:53
> >
> > 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.
>
> The 'mov r3, r1' has nothing to do with 'a'.

What do you mean by that? At this point in the program, a is the
variable whose value is stored in r1 and b is the variable whose value
is stored in r3. "mov r3, r1" copies the value from r1 into r3 and is
therefore equivalent to executing "b = a". (That is why I said one
could argue that the "return *b" statement uses the value of *a.) Thus
it very much does have something to do with "a".

> It is a more general problem that OPTIMISER_HIDE_VAR() pretty much
> always ends up allocating a different internal 'register' for the
> output and then allocating a separate physical rehgister.

What output are you referring to? Does OPTIMISER_HIDE_VAR() have an
output? If it does, the source program above ignores it, discarding any
returned value.

> There doesn't seem to be a later optimisation path to remove
> 'pointless' register moves.

That would be a good thing to add, then.

Alan