https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110799
--- Comment #9 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #7)
> Can you elaborate on what you consider a correct approach?
I think this optimization is incorrect and should be active only under -Ofast.
I can offer two arguments. First, even without considering correctness,
breaking TSan and Helgrind is a substantial QoI issue and we should consider
shielding -O2 users from that (otherwise they'll discover it the hard way,
curse at us, stick -fno-hoist-adjacent-loads in their build system and consider
switching to another compiler).
Second, I can upgrade the initial example to an actual miscompilation. The
upgrade is based on two considerations: the optimization works on
possibly-trapping accesses, and relies on types of memory references to decide
if it's safe, but it runs late where the types are not what they were in the C
source. Hence, the following example:
struct S {
int a;
};
struct M {
int a, b;
};
int f(struct S *p, int c, int d)
{
int r;
if (c)
if (d)
r = p->a;
else
r = ((struct M*)p)->a;
else
r = ((struct M*)p)->b;
return r;
}
is miscompiled to
f:
mov eax, DWORD PTR [rdi+4]
test esi, esi
cmovne eax, DWORD PTR [rdi]
ret
even though the original program never accesses beyond struct S if 'c && d'.
Phi-opt incorrectly performs hoisting after PRE collapses 'if (d) ... else
...'.
