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

David Laight
2024-10-01T17:12:03Z

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'.
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.

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

David

>
> 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

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