These calls are allowed to fail spuriously.
29.6.5.25:
Remark: A weak compare-and-exchange operation may fail spuriously.
That is, even when the contents of memory referred to by expected
and object are equal, it may return false and store back to expected
the same memory contents that were originally there. [ Note: This
spurious failure enables implementation of compare and-exchange on a
broader class of machines, e.g., load-locked store-conditional
machines. A consequence of spurious failure is that nearly all uses
of weak compare-and-exchange will be in a loop.
To fix this, we replace any assert() that expects
std::atomic::compare_exchange_weak() to return true with a loop. If the
call does not return true within N runs (with N currently equal to 10),
then the test fails.
http://llvm.org/bugs/show_bug.cgi?id=20546
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217319 91177308-0d34-0410-b5e6-96231b3b80d8
7ba3c57565