From: Hans-Peter Nilsson <hp@bitrange.com>
To: Richard Henderson <rth@redhat.com>
Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: [libstdc++/65033] Give alignment info to libatomic
Date: Fri, 03 Apr 2015 03:57:00 -0000 [thread overview]
Message-ID: <alpine.BSF.2.02.1504022240580.40679@arjuna.pair.com> (raw)
In-Reply-To: <54DD19B7.6060401@redhat.com>
On Thu, 12 Feb 2015, Richard Henderson wrote:
> When we fixed PR54005,
Hm, there's confusion. When was this fixed? (Not fixed
AFAICT.) Maybe you mean PR54004, but there was no note there
either. Or maybe there's a typo and you meant some other PR and
that PR54005 is supposedly fixed by this patch (committed as
r221701)
...but it doesn't seem right: you use a specific object when
deducing the alignment for the fake-pointer, so it's used anyway
and is_lock_free must not be object-specific (despite its name)
and only type-specific as mandated by the standard (see PR).
To wit, deduce from the known-alignment of _Tp, not
known-alignment of _M_i. Or is this what happens; they're the
same? Why then use __alignof(_M_i) (the object-alignment)
instead of _S_alignment (the deduced alas insufficiently
increased type-alignment)?
> making sure that atomic_is_lock_free returns the same
> value for all objects of a given type,
(That would work but it doesn't seem to be the case.)
> we probably should have changed the
> interface so that we would pass size and alignment rather than size and object
> pointer.
>
> Instead, we decided that passing null for the object pointer would be
> sufficient. But as this PR shows, we really do need to take alignment into
> account.
Regarding what's actually needed, alignment of an atomic type
should always be *forced to be at least the natural alignment of
of the object* (with non-power-of-two sized-objects rounded up)
and until then atomic types won't work for targets where the
non-atomic equivalents have less alignment (as straddling a
page-boundary won't be lock-less-atomic anywhere where
straddling a page-boundary may cause a non-atomic-access...) So,
not target-specific except for targets that require even
higher-than-natural alignment.
> The following patch constructs a fake object pointer that is maximally
> misaligned.
(With regards to the known object alignment of the _M_i object.)
> This allows the interface to both the builtin and to libatomic to
> remain unchanged. Which probably makes this back-portable to maintenance
> releases as well.
>
> I believe that for all of our current systems, size_t == uintptr_t, so the
> reinterpret_cast ought not generate warnings.
>
> The test case is problematic, as there's currently no good place to put it.
> The libstdc++ testsuite doesn't have the libatomic library path configured, and
> the libatomic testsuite doesn't have the libstdc++ include paths configured.
> Yet another example where we really need an install tree for testing. Thoughts?
brgds, H-P
next prev parent reply other threads:[~2015-04-03 3:57 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-12 21:23 Richard Henderson
2015-02-18 12:15 ` Jonathan Wakely
2015-03-25 16:22 ` Jonathan Wakely
2015-03-25 18:36 ` Richard Henderson
2015-03-25 18:49 ` Jonathan Wakely
2015-03-25 19:04 ` Richard Henderson
2015-03-26 13:21 ` Jonathan Wakely
2015-03-31 13:41 ` Jonathan Wakely
2015-03-31 14:54 ` Richard Henderson
2015-03-31 15:03 ` Jonathan Wakely
2015-03-31 15:13 ` Richard Henderson
2015-03-31 15:41 ` Jonathan Wakely
2015-04-06 22:59 ` Hans-Peter Nilsson
2015-04-13 4:45 ` patch fix issue 1 with "[libstdc++/65033] Give alignment info to libatomic" Hans-Peter Nilsson
2015-04-13 11:59 ` Jonathan Wakely
2015-04-13 5:59 ` Issue 2 " Hans-Peter Nilsson
2015-04-13 17:53 ` Joseph Myers
2015-03-25 18:39 ` [libstdc++/65033] Give alignment info to libatomic Richard Henderson
2015-04-03 3:04 ` Hans-Peter Nilsson
2015-03-26 11:54 ` Jonathan Wakely
2015-04-03 3:57 ` Hans-Peter Nilsson [this message]
2015-04-03 9:25 ` Hans-Peter Nilsson
2015-04-03 14:13 ` Jonathan Wakely
2015-04-03 19:13 ` Richard Henderson
2015-04-07 13:14 ` Jonathan Wakely
2015-04-09 11:17 ` Jonathan Wakely
2015-04-06 1:07 ` Hans-Peter Nilsson
2015-04-07 9:45 ` Jonathan Wakely
2015-04-07 10:52 ` Hans-Peter Nilsson
2015-04-07 13:12 ` Jonathan Wakely
2015-04-07 14:51 ` Hans-Peter Nilsson
2015-04-07 15:06 ` Jonathan Wakely
2015-04-08 3:58 ` Hans-Peter Nilsson
2015-04-08 9:35 ` Jonathan Wakely
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