Re: [RFC v2 1/4] tls: add macros for coroutine-safe TLS variables

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 2 Dec 2021 at 14:44, Peter Maydell <peter.maydell@linaro.org> wrote:
> My compiler-developer colleagues present the following case where
> 'noinline' is not sufficient for the compiler to definitely
> use different values of the address-of-the-TLS-var across an
> intervening function call:
>
>   __thread int i;
>
>   __attribute__((noinline)) long get_ptr_i()
>   {
>     return (long)&i;
>   }
>
>   void switcher();
>
>   int g()
>   {
>     long a = get_ptr_i();
>     switcher();
>     return a == get_ptr_i();
>   }
>
> Trunk clang optimizes the comparison of the two pointers down
> to "must be always true" even though they might not be if the
> switcher() function has resulted in a change-of-thread:
>   https://godbolt.org/z/hd67zh6jW
>
> The 'optnone' attribute (clang-specific) seems to be able
> to suppress this

...no it doesn't -- although the get_ptr_i() function is
called both before and after, its return value is ignored
and g() always still returns 'true'.

-- PMM