[Bug middle-end/102829] New: Redundant null check after atomic load from that address

[Bug middle-end/102829] New: Redundant null check after atomic load from that address

[laurynas.biveinis at gmail dot com]

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)

[Bug tree-optimization/102829] Redundant null check after atomic load from that address

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization
     Ever confirmed|0                           |1
             Blocks|                            |85316
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2021-10-19

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We don't derive non-nullness for the pointer dereferenced by the atomic
builtins yet.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85316
[Bug 85316] [meta-bug] VRP range propagation missed cases

[Bug tree-optimization/102829] Redundant null check after atomic load from that address

[laurynas.biveinis at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829

--- Comment #2 from Laurynas Biveinis <laurynas.biveinis at gmail dot com> ---
FWIW adding "if (this == nullptr) __builtin_unreachable();" between
__atomic_load_n and return fails to workaround this issue

[Bug tree-optimization/102829] Redundant null check after atomic load from that address

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102829

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement