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

David Laight
2024-10-07T13:20:00Z

From: Jonas Oberhauser
> Sent: 07 October 2024 12:55
>
> Am 10/3/2024 um 3:23 PM schrieb Mathieu Desnoyers:
> > What _does_ work however are the following two approaches:
> >
> > 1) Perform the equality check on the original variables, creating
> > new versions (with OPTIMIZER_HIDE_VAR) of both variables for the
> > rest of their use, therefore making sure the pointer dereference
> > are not derived from versions of the variables which were compared
> > with another pointer. (as suggested by Boqun)
>
> This should not be guaranteed to work, because right after the
> comparison the compiler can do b=a, then it doesn't matter how much you
> hide afterwards.
>
> However it might work if you escape the addresses of a and b first, in
> which case the compiler will not do b=a anymore, but it might force the
> compiler to put a and b on the stack, which has some performance impact.

Nope, as pointed out last week, the compiler can move the 'a == b'
check to before the OPTIMISER_HID_VAR() and then use the same register
for both of them.

> > 2) Perform the equality check on the versions resulting of hiding
> > both variables, making sure those versions of the variables are
> > not dereferenced afterwards. (as suggested by Linus)

That (and other things) could usefully use:
#define OPTIMISER_HIDE_VALUE(x) \
({ __auto_type _x = x; OPTIMISER_HIDE_VAR(_x); _x; })
You'll almost certainly end up with a register-register move
even if 'x' isn't used afterwards.

The calling could just become:
if (a == OPTIMISER_HIDE_VALUE(b) ...
since it is likely that you only care about one of the pointers.
(Actually isn't hiding one of them always enough?)

David

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