From: Hans-Peter Nilsson <hans-peter.nilsson@axis.com>
To: jwakely@redhat.com
Cc: rth@redhat.com, libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org,
dodji@redhat.com
Subject: Issue 2 with "[libstdc++/65033] Give alignment info to libatomic"
Date: Mon, 13 Apr 2015 05:59:00 -0000 [thread overview]
Message-ID: <201504130559.t3D5xnGu004491@ignucius.se.axis.com> (raw)
In-Reply-To: <20150326132147.GL9755@redhat.com> (message from Jonathan Wakely on Thu, 26 Mar 2015 14:21:47 +0100)
(check_cxx_fundamental_alignment_constraints is Dodji's, others
CC:ed were already in the thread)
Looking into those atomic things and running tests for cris-elf,
I get FAIL for
libstdc++-v3/testsuite/29_atomics/atomic/65147.cc, specifically
struct S16 {
char c[16];
};
static_assert( alignof(std::atomic<S16>) >= 16,
"atomic<S16> must be aligned to at least its size" );
which just isn't aligned for cris-elf. Its aligmnent is 1; the
default. Trying to investigate using:
#include <iostream>
using std::cout;
using std::endl;
struct xx {
alignas (16) char x[16];
};
xx ai;
int main(void)
{
cout << "alignof(ai): " << __alignof__(ai)
<< endl;
}
yields:
b.cc:5:25: warning: requested alignment 16 is larger than 8 [-Wattributes]
alignas (16) char x[16];
which is mysterious (where does the 8 come from?), until I grep
the error string and find
c-family/c-common.c:check_cxx_fundamental_alignment_constraints.
In there, I see target macros used, among them
BIGGEST_ALIGNMENT. This is 8 for cris-elf: the *bit alignment*
(there's a bug there already; bits not bytes) of the biggest
*required* alignment (modulo atomics) for types, not the biggest
*supported* alignment. So, the wrong macro (and unit) is used.
Similarly, BIGGEST_FIELD_ALIGNMENT is about *require*, not
*support*. Changing either macro is also an ABI change.
Why not allow the presumably most relaxed value for types, like
for __attribute__ ((__aligned__())), i.e. MAX_OFILE_ALIGNMENT,
then a tighter alignment check when declaring an object?
Right now, MAX_OFILE_ALIGNMENT is only used in
check_cxx_fundamental_alignment_constraints when the scope is
*known* to be file-scope and I know MAX_OFILE_ALIGNMENT sounds
like it's only for file-scope variables, but well, that's what
we have here, so the error is wrong.
So, into what shall BIGGEST_ALIGNMENT change in
check_cxx_fundamental_alignment_constraints?
brgds, H-P
next prev parent reply other threads:[~2015-04-13 5:59 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-12 21:23 [libstdc++/65033] Give alignment info to libatomic 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 ` Hans-Peter Nilsson [this message]
2015-04-13 17:53 ` Issue 2 " 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
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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