From: Paul Eggert <eggert@cs.ucla.edu>
To: Joseph Myers <joseph@codesourcery.com>, gcc-patches@gcc.gnu.org
Cc: fweimer@redhat.com
Subject: Re: Make max_align_t respect _Float128
Date: Fri, 26 Aug 2016 21:51:00 -0000 [thread overview]
Message-ID: <56e04616-afd7-7fce-f2ba-00304ea5686b@cs.ucla.edu> (raw)
In-Reply-To: <alpine.DEB.2.20.1608262052110.1996@digraph.polyomino.org.uk>
On 08/26/2016 01:54 PM, Joseph Myers wrote:
> Such an increase is of course an ABI change, but a reasonably safe
> one, in that max_align_t doesn't tend to appear in library interfaces
> (rather, it's something to use when writing allocators and similar
> code;
It should be fine, though I would like to mention a thin-ice area. Emacs
uses max_align_t to infer whether malloc returns a pointer that is a
multiple of 8, with macros like this:
// in src/lisp.h:
#define GCALIGNMENT 8
// in src/alloc.c:
#define MALLOC_IS_GC_ALIGNED (__alignof__ (max_align_t) % GCALIGNMENT
== 0)
If __alignof__ (max_align_t) is greater than malloc alignment, the
assumption behind the above code is wrong, i.e., MALLOC_IS_GC_ALIGNED
might return 1 even though the correct value is 0. As it happens, Emacs
will work on x86 since GCALIGNMENT is 8; but if GCALIGNMENT were
increased to 16 the definition of MALLOC_IS_GC_ALIGNED would become
incorrect with the proposed GCC patch.
> (I think glibc malloc alignment should also increase to 16-byte on
> 32-bit x86 so it works for allocating objects of these types, which is
> now straightforward given the fix made for 32-bit powerpc.)
Yes. I hope the above note helps explain why malloc alignment should be
at least max_align_t.
> OK to commit?
For what it's worth, it looks good to me.
[in your followup email]
> Well, the patch could use __SIZEOF_FLOAT128__ just as well as __i386__
> (the effect would be an extra union member
s/union/struct/. Though I've always wondered why it is a struct and not
a union. Maybe change it to union while we're doing an ABI change anyway?
next prev parent reply other threads:[~2016-08-26 21:51 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-26 20:55 Joseph Myers
2016-08-26 21:10 ` Marc Glisse
2016-08-26 21:30 ` Joseph Myers
2016-08-26 21:45 ` Florian Weimer
2016-08-26 21:57 ` Paul Eggert
2016-08-26 22:25 ` Joseph Myers
2016-08-26 21:51 ` Paul Eggert [this message]
2016-08-29 13:29 ` Marek Polacek
2016-08-29 15:40 ` Joseph Myers
2016-09-01 14:18 ` Ping " Joseph Myers
2016-09-05 17:07 ` Make max_align_t respect _Float128 [version 2] Joseph Myers
2016-09-06 9:06 ` Richard Biener
2016-09-06 11:26 ` Joseph Myers
2016-09-06 9:19 ` Florian Weimer
2016-09-06 9:24 ` Richard Biener
2016-09-07 7:45 ` Florian Weimer
2016-09-07 17:53 ` Joseph Myers
2016-09-08 9:35 ` Florian Weimer
2016-09-06 11:40 ` Joseph Myers
2016-09-06 15:06 ` Florian Weimer
2016-09-06 15:20 ` Joseph Myers
2016-09-06 15:59 ` Paul Eggert
2016-09-06 20:47 ` Joseph Myers
2016-09-06 21:41 ` Paul Eggert
2016-09-07 9:22 ` Florian Weimer
2016-09-07 11:52 ` Mark Wielaard
2016-09-08 1:58 ` Paul Eggert
2016-09-08 11:58 ` Mark Wielaard
2016-09-08 12:22 ` Florian Weimer
2016-09-08 14:59 ` Paul Eggert
2016-09-08 12:30 ` Bernd Schmidt
2016-09-08 12:34 ` Florian Weimer
2016-09-07 9:15 ` Florian Weimer
2016-09-06 21:03 ` Jason Merrill
2016-09-06 21:18 ` Joseph Myers
2016-09-06 21:53 ` Jason Merrill
2016-09-06 21:56 ` Joseph Myers
2016-09-06 12:06 ` Bernd Schmidt
2016-09-06 14:59 ` Florian Weimer
2016-09-12 18:02 ` Make max_align_t respect _Float128 [version 3] Joseph Myers
2016-09-19 16:08 ` Ping " Joseph Myers
2016-09-19 17:11 ` Paul Eggert
2016-09-26 16:35 ` Jeff Law
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