Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

* Mathieu Desnoyers:

Something like this ?

#ifdef __cplusplus
# if  __cplusplus >= 201103L
#  define rseq_static_assert (expr, diagnostic)         static_assert (expr, diagnostic)
#  define rseq_alignof                                  alignof
# endif
#elif __STDC_VERSION__ >= 201112L
# define rseq_static_assert (expr, diagnostic)          _Static_assert (expr, diagnostic)
# define rseq_alignof                                   _Alignof
#endif

#ifndef rseq_static_assert
# define rseq_static_assert (expr, diagnostic)          /* nothing */
#endif

You can't have a space in #defines like that, no matter what GNU style
says. 8-)

/* Ensure the compiler supports __attribute__ ((aligned)).  */
rseq_static_assert ((rseq_alignof (struct rseq_cs) >= 32, "alignment"));
rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment"));

You need to move the ; into rseq_static_assert.  And if you use explicit
arguments, you can't use double parentheses.

quoted

And something similar for _Alignas/attribute aligned,

I don't see where _Alignas is needed here ?

For attribute aligned, what would be the oldest supported C and C++
standards ?

There are no standardized attributes for C, there is only _Alignas.
C++11 has an alignas specifier; it's not an attribute either.  I think
these are syntactically similar.

quoted

with an error for
older standards and !__GNUC__ compilers (because neither the type nor
__thread can be represented there).

By "type" you mean "struct rseq" here ? What does it contain that requires
a __GNUC__ compiler ?

__attribute__ and __thread support.

About __thread, I recall other compilers have other means to declare it.
In liburcu, I end up with the following:

#if defined (__cplusplus) && (__cplusplus >= 201103L)
# define URCU_TLS_STORAGE_CLASS thread_local
#elif defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
# define URCU_TLS_STORAGE_CLASS _Thread_local
#elif defined (_MSC_VER)
# define URCU_TLS_STORAGE_CLASS __declspec(thread)
#else
# define URCU_TLS_STORAGE_CLASS __thread
#endif

Would something along those lines be OK for libc ?

Yes, it would be okay (minus the Visual C++ part).  This part does not
have to go into UAPI headers first.  A fallback definition of __thread
should be okay.  Outside glibc, the TLS model declaration is optional, I
think.  The glibc *definition* ensures that the variable is
initial-exec.

Thanks,
Florian