Re: Implement C11 _Atomic

["Joseph S. Myers" <joseph@codesourcery.com>]

On Thu, 21 Nov 2013, Hans-Peter Nilsson wrote:

> Oh right, gcc still doesn't remove target-introduced "manual"
> alignment checks (when expanding atomic intrinsics), but at
> least gcc makes sure it's aligned on stack, when options doesn't
> say it's aligned.  And a.c:plugh2 doesn't seem to perform an
> atomic assignment, but just assignment through an
> _Atomic-aligned stack temporary.  Might be my C11-ignorance
> showing.

It appears to me on x86_64 to produce an __atomic_store_4 to *x (in the 
GIMPLE dumps, what happens after that is a matter for the back end).

Note that atomic variable initialization is *not* atomic (see 7.17.2.1 - 
in general ATOMIC_VAR_INIT needs using with the initializer, or the 
initialization needs to be carried out with atomic_init, though GCC 
doesn't require that).  (In C11, the effect of a plain initialization 
without ATOMIC_VAR_INIT is I think that the initializer is evaluated for 
its side effects, but if the variable gets used as either rvalue or lvalue 
without one of the special forms of initialization being used first then 
the behavior is undefined.  The idea is to support implementations with 
extra bits in atomic objects used for locking purposes.)  So no atomic 
store to y is expected - although there are atomic loads from y.

-- 
Joseph S. Myers
joseph@codesourcery.com