http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54005
Hans-Peter Nilsson <hp at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |crowl at gcc dot gnu.org
--- Comment #6 from Hans-Peter Nilsson <hp at gcc dot gnu.org> 2012-08-14
03:52:07 UTC ---
(In reply to comment #4)
> A GCC port is incorrect if it is issuing any kind of lock. GCC is only
> allowed
> to issue a lock free sequence of some sort. If a lock is required, then a
> call
> to libatomic must be made.
>
> So if there is a type which is sometimes lock free and sometimes not due to
> alignment or anything else, __atomic_always_lock_free should return FALSE
> for
> that type.
Yes...
> I think your current problem is that the infrastructure change for
> __atomic_always_lock_free to handle alignment issues is not present yet. So it
> erroneously says TRUE for this type when in fact it shouldn't.
This PR is about the libstdc++ library is_lock_free function using the
*per-object* builtin query when it should use the *per-type* query as per the
referenced discussion. And that's been corrected.
It's separate to my other target woes; an incidental observation. I agree the
function has a name intuitively leading to thinking it should be per-object,
but it isn't.
> Its important that the libstdc++ routines call the __atomic_is_lock_free() so
> that the answer can be determined at runtime if libatomic is utilized for any
> atomic sequences.
Maybe elsewhere, but not this particular code. If your argument is that
is_lock_free should be per-object, then you'll have to argue with the
standard-guys; I'm just quoting the reference above.
> __atomic_always_lock_free() is *always* resolved to a 1 or a 0 at compile
> time.
> The compiler is only capable of answering the question, "Does the compiler
> always generate a lock free sequence".
Which is exactly the kind of answer sought here. Is the n2992 reference
incorrect or am I (and bkoz) misinterpreting it? Are things about to change
again standard-wise? Let's try and ask mr. Crowl.
