https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829
Bug ID: 102829
Summary: Redundant null check after atomic load from that
address
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: laurynas.biveinis at gmail dot com
Target Milestone: ---
The following source code
----
struct d {
long b;
d *e() {
__atomic_load_n(&b, 0);
return this;
}
};
d *j;
void i();
void k() {
auto l = j->e();
if (l)
i();
}
----
produces the following bit of assembly with -O3 on x86_64
(https://godbolt.org/z/G1ThvnMWP):
...
mov rdx, QWORD PTR [rax]
test rax, rax
je .L1
...
It first dereferences the address at RAX, and later checks whether RAX == 0.
Since we already tried accessing memory at that address, the nullptr check
seems redundant. Moreover, since the address happens to be of 'this' of
variable j, it being equal to nullptr is UB anyway and may be assumed to be !=
nullptr?
Clang tests show the redundant test being present up to version 11 inclusive
(https://godbolt.org/z/rT3ha4enb) and absent from version 12 onwards
(https://godbolt.org/z/Gfb9EWMfq)
