Am 09.10.19 um 18:12 schrieb Klaus Doldinger:
>
>
> Am 09.10.19 um 11:19 schrieb Jonathan Wakely:
>> On Wed, 9 Oct 2019 at 04:49, Klaus Doldinger wrote:
>>>
>>> Hi all,
>>>
>>> following is an example regarding the generation of guards for the
>>> static m in template X.
>>
>> I didn't check the implementation, but according to the documentation,
>> -fno-threadsafe-statics only affects local static, so it's
>> unsurprising that it doesn't have any effect on a static member
>> variable.
>>
>> For the local static variable, GCC and Clang still emit the guard
>> variable with -fno-threadsafe-statics but they do not emit the code to
>> use that variable (there are no calls to __cxa_guard_acquire or
>> __cxa_guard_release). I believe the guard variable stills gets be
>> emitted for ABI reasons, because another translation unit compiled
>> without -fno-threadsafe-statics might be expecting it to exist.
>>
>
> Ok. Looks like constinit (C++20) and a constexpr user-defined ctor could
> solve the problem.
> Thanks for pointing me into the right direction.
>
The full example:
The key-feature to suppress guards are constinit and constexpr ctors:
#include <cstdint>
struct A {
inline constexpr A(uint8_t v) : m{v} {} // without constexpr it
should not compile, but does anymay
auto m1() const {
return m;
}
private:
uint8_t m{0};
};
template<typename T>
struct X {
static auto foo() {
return m.m1();
}
constinit inline static T m{2}; // requires constexpr ctor
};
int main() {
return X<A>::foo();
}
With constinit the initialization must be performed on compile-time, so
there is no need to generate guards. This requires a constexpr ctor. In
the above example the ctor (at least for gcc) could be declared without
constexpr, but this may be a pending bug.
