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

Mathieu Desnoyers
2024-10-07T13:23:14Z

On 2024-10-07 15:18, David Laight wrote:
> 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.

Yes.

>
>>> 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.

Yes.

>
> 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?)

Linus asked that we hide both inputs, otherwise it's really asking
for trouble in terms of API misuse.

Thanks,

Mathieu

>
> David
>
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Mathieu Desnoyers
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