srw_mutex_impl<bool>::wait_and_lock(): In
commit a73eedbf3f we introduced
an std::atomic::fetch_or() in a loop. Alas, on the IA-32 and AMD64,
that was being translated into a loop around LOCK CMPXCHG.
To avoid a nested loop, it is better to explicitly invoke
std::atomic::compare_exchange_weak() in the loop, but only if
the attempt has a chance to succeed (the HOLDER flag is not set).
It is even more efficient to use LOCK BTS, but contemporary compilers
fail to translate std::atomic::fetch_or(x) & x into that when x is
a single-bit constant. On GCC-compatible compilers, we will use
inline assembler to achieve that.
On other ISA than IA-32 and AMD64, we will continue to use
std::atomic::fetch_or().
ssux_lock_impl<spinloop>::rd_wait(): Use rd_lock_try().
A loop around std::atomic::compare_exchange_weak() should be
cheaper than fetch_add(), fetch_sub() and a wakeup system call.
These deficiencies were pointed out and the use of LOCK BTS was
suggested by Thiago Macieira.
35f59bc4e1