Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency
From: Jonas Oberhauser
Date: Mon Oct 07 2024 - 07:55:31 EST
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