<atomic>: On x64, atomic_ref::is_lock_free() incorrectly returns true when it shouldn't #4728
Description
Repros with VS 2022 17.11 Preview 2 x64:
C:\Temp>type meow.cpp
#include <atomic>
#include <print>
using namespace std;
struct Large {
char str[100]{};
};
int main() {
Large lg{};
atomic_ref<Large> atom_ref{lg};
println("atomic_ref<Large> is_lock_free(): {}", atom_ref.is_lock_free());
atom_ref.store(Large{"cute fluffy kittens"});
println("lg.str: {}", lg.str);
}C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /Zi /Fdmeow.pdb meow.cpp
meow.cpp
C:\Temp>meow
atomic_ref<Large> is_lock_free(): true
lg.str: cute fluffy kittens
atomic_ref<Large>::store() definitely takes a lock:
Lines 630 to 635 in e36ee6c
atomic_ref::is_lock_free() is bogus because:
Lines 2367 to 2376 in e36ee6c
This meant to say: "If we're always lock-free then true, otherwise we have to be potentially lock-free and actually have cmpxchg16b."
I have a fix.