From: Joseph Myers <joseph@codesourcery.com>
To: Hans-Peter Nilsson <hans-peter.nilsson@axis.com>
Cc: <jwakely@redhat.com>, <rth@redhat.com>, <libstdc++@gcc.gnu.org>,
<gcc-patches@gcc.gnu.org>, <dodji@redhat.com>
Subject: Re: Issue 2 with "[libstdc++/65033] Give alignment info to libatomic"
Date: Mon, 13 Apr 2015 17:53:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.10.1504131749220.28684@digraph.polyomino.org.uk> (raw)
In-Reply-To: <201504130559.t3D5xnGu004491@ignucius.se.axis.com>
On Mon, 13 Apr 2015, Hans-Peter Nilsson wrote:
> 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?
When I was making sure C diagnosed unsupported extended alignments, my
conclusion was that the support for realigning the stack means that
MAX_OFILE_ALIGNMENT is supported everywhere (except for malloc, of
course). See <https://gcc.gnu.org/ml/gcc-patches/2013-11/msg02187.html>
for my analysis.
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2015-04-13 17:53 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 ` Issue 2 " Hans-Peter Nilsson
2015-04-13 17:53 ` Joseph Myers [this message]
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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