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

David Laight
2024-10-02T08:14:12Z

From: 'Alan Stern'
> Sent: 01 October 2024 23:57
>
> 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".

After the cmp and bne r1 and r3 have the same value.
The compiler tracks that and will use either register later.
That can never matter.
Remember the compiler tracks values (in pseudo/internal registers)
not variables.

> > 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.

Look up OPTIMISER_HIDE_VAR(x) it basically x = f(x) where f() is
the identity operation:
asm ("" : "+r"(x))
I'll bet that gcc allocates a separate internal/pseudo register
for the result so wants to do y = f(x).
Probably generating y = x; y = f(y);
(The 'mov' might be after the asm, but I think that would get
optimised away - the listing file might help.)

So here the compiler has just decided to reuse the register that
held the other of a/b for the extra temporary.

David

>
> > 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

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