Re: [patch libstdc++] Optimize synchronization in std::future if futexes are available.

[Jonathan Wakely <jwakely at redhat dot com>]

On 16/01/15 18:00 +0100, Torvald Riegel wrote:
> +namespace std _GLIBCXX_VISIBILITY(default)
> +{
> +_GLIBCXX_BEGIN_NAMESPACE_VERSION
> +
> +  bool
> +  __atomic_futex_unsigned_base::_M_futex_wait_until(unsigned *__addr,
> +      unsigned __val,
> +      bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns)
> +  {
> +    if (!__has_timeout)
> +      {
> +	// Ignore whether we actually succeeded to block because at worst,
> +	// we will fall back to spin-waiting.  The only thing we could do
> +	// here on errors is abort.
> +	int ret __attribute__((unused));
> +	ret = syscall (SYS_futex, __addr, futex_wait_op, __val);
> +	_GLIBCXX_DEBUG_ASSERT(ret == 0 || errno == EINTR || errno == EAGAIN);

I've just noticed these debug assertions in the futex.cc file, which
will never fire because the library isn't build with -D_GLIBCXX_DEBUG
defined. We should decide whether other errors are necessary to handle
and either throw an exception or std::terminate().