[lttng-dev] [PATCH 02/11] urcu/uatomic: Use atomic builtins if configured

[Olivier Dion]

On Thu, 22 Jun 2023, "Paul E. McKenney" <paulmck at kernel.org> wrote:
> On Thu, Jun 22, 2023 at 11:55:55AM -0400, Mathieu Desnoyers wrote:
>> On 6/21/23 19:19, Paul E. McKenney wrote:
> I suggest C11 volatile atomic load/store.  Load/store fusing is permitted
> for non-volatile atomic loads and stores, and such fusing can ruin your
> code's entire day.  ;-)

After some testing, I got a wall of warnings:

  -Wignored-qualifiers:

    Warn if the return type of a function has a type qualifier such as
    "const".  For ISO C such a type qualifier has no effect, since the
    value returned by a function is not an lvalue.  For C++, the warning
    is only emitted for scalar types or "void".  ISO C prohibits
    qualified "void" return types on function definitions, so such
    return types always receive a warning even without this option.

Since we are using atomic builtins, for example load:

  type __atomic_load_n (type *ptr, int memorder)

If we put the qualifier volatile to TYPE, we end up with the same
qualifier on the return value, triggering a warning for each atomic
operation.

This seems to be only a problem when compiling in C++ [0] while in C it
seems the compiler is more relaxed on this [1].

Ideas to make the toolchains happy? :-)

  [0] https://godbolt.org/z/3nW14M3v1
  [1] https://godbolt.org/z/TcTeMeKbW

-- 
Olivier Dion
EfficiOS Inc.
https://www.efficios.com