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

David Laight
2024-10-03T17:08:01Z

...
> 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)

If that is
a1 = a; OPTIMISER_HIDE_VAR(a1);
b1 = b; OPTIMISER_HIDE_BAR(b1);
if (a != b}
return;
// code using a1 and b1
then can't the compiler first flip it to:
if (a != b)
return;
a1 = a; OPTIMISER_HIDE_VAR(a1);
b1 = b; OPTIMISER_HIDE_VAR(b1);
and then replace the last line with:
b1 = a; OPTIMISER_HIDE_VAR(b1);
which isn't intended at all.

OTOH if you do:
a1 = a; OPTIMISER_HIDE_VAR(a1);
b1 = b; OPTIMISER_HIDE_VAR(b1);
if (a1 != b1)
return;
// code using a and b
(which I think is)

> 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)

then the compiler can't possibly reverse the asm blocks.

David

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