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

Jonas Oberhauser
2024-10-07T11:55:21Z

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.

>
> 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)
>

jonas