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Groups > comp.lang.c > #42275

Re: Padding involved

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: Padding involved
Date 2014-03-29 00:57 -0700
Organization None to speak of
Message-ID <lnlhvtjv2n.fsf@nuthaus.mib.org> (permalink)
References (9 earlier) <lh4g05$uu7$1@speranza.aioe.org> <5335CC2C.4000401@verizon.net> <lh4p1u$nd6$1@speranza.aioe.org> <lh4rr1$bhd$1@dont-email.me> <lh57rc$ra9$1@dont-email.me>

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James Kuyper <jameskuyper@verizon.net> writes:
> On 03/28/2014 06:07 PM, Stephen Sprunk wrote:
>> On 28-Mar-14 16:19, glen herrmannsfeldt wrote:
>>> James Kuyper <jameskuyper@verizon.net> wrote:
> ...
>>>> On the platform you describe, must every double be aligned on a 16
>>>> byte address, so the SSE instructions can always be used?
>>>
>>> Pairs of doubles are aligned on 16 byte boundaries.
>> 
>> Standard C has no type "pair of doubles".
>
> Actually, that's precisely what double[2] is; and _Alignof(double[2])
> probably would be 16 on such a platform.

N1570 6.5.3.4p3:

    The _Alignof operator yields the alignment requirement of its
    operand type. The operand is not evaluated and the result is
    an integer constant. When applied to an array type, the result
    is the alignment requirement of the element type.

So _Alignof(double[2]) == _Alignof(double), by definition.

[...]

> While that's the only guarantee, an implementation is free to impose
> stricter alignment requirements on arrays of a type than on the type itself.

It's not free to *require* a stricter alignment.  Given:

    double arr[3];

both arr[0] and arr[1] can be treated as the first element of a
double[2] object.  (I'm *fairly* sure of that, but I don't have a
citation to prove it.)

Of course a compiler can allocate arrays at stricter alignments than
required, but it must still be able to (generate code to) access them at
the alignment of their element type.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

Padding involved anish singh <anish198519851985@gmail.com> - 2014-03-07 13:13 -0800
  Re: Padding involved jacob navia <jacob@spamsink.net> - 2014-03-07 22:17 +0100
  Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-07 16:26 -0500
    Re: Padding involved anish singh <anish198519851985@gmail.com> - 2014-03-07 13:49 -0800
      Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-07 18:11 -0500
        Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 15:19 -0800
          Re: Padding involved Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-03-07 16:26 -0700
            Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 15:41 -0800
              Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-07 18:50 -0500
            Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-08 00:10 +0000
              Re: Padding involved Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2014-03-08 09:27 -0700
          Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-07 16:24 -0800
            Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 16:45 -0800
              Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-08 01:03 +0000
                Re: Padding involved David Thompson <dave.thompson2@verizon.net> - 2014-03-28 06:11 -0400
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-28 18:45 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-28 15:23 -0400
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-28 12:40 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-28 21:19 +0000
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-28 17:07 -0500
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-28 18:25 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-28 21:15 -0500
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-29 01:03 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 14:23 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-30 01:43 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-28 21:32 -0400
                Re: Padding involved Richard Damon <Richard@Damon-Family.org> - 2014-03-28 21:59 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-29 00:46 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-29 08:51 -0400
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-29 08:57 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-30 18:53 -0500
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-30 20:25 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-31 11:34 -0500
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-31 11:27 -0700
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-31 11:36 -0700
                Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-31 19:14 +0000
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-31 15:43 -0500
                Re: Padding involved Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-04-01 03:12 +0300
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-04-03 00:45 -0500
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-29 13:02 -0500
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-29 14:44 -0400
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-29 19:49 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-30 19:12 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-31 16:20 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-31 17:53 -0400
                Re: Padding involved Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2014-04-01 03:22 +0300
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-04-04 14:36 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-04-04 17:33 -0400
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 13:46 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-14 22:00 +0000
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-04-14 15:37 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-17 09:11 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 13:25 -0700
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-29 00:57 -0700
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-29 08:57 -0400
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-29 14:18 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-30 12:02 -0700
                Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-30 19:35 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 12:54 -0700
                Re: Padding involved Tim Rentsch <txr@alumni.caltech.edu> - 2014-03-29 14:44 -0700
                Re: Padding involved glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-03-29 03:50 +0000
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-29 09:02 -0400
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-29 13:19 -0500
                Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-28 21:21 -0400
                Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-28 15:27 -0400
                Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-28 19:54 +0000
                Re: Padding involved Stephen Sprunk <stephen@sprunk.org> - 2014-03-28 15:02 -0500
          Re: Padding involved Kaz Kylheku <kaz@kylheku.com> - 2014-03-08 00:43 +0000
          Re: Padding involved Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-03-07 20:20 -0500
    Re: Padding involved anish singh <anish198519851985@gmail.com> - 2014-03-07 13:49 -0800
  Re: Padding involved James Kuyper <jameskuyper@verizon.net> - 2014-03-07 16:46 -0500
  Re: Padding involved "BartC" <bc@freeuk.com> - 2014-03-07 21:59 +0000
    Re: Padding involved anish kumar <yesanishhere@gmail.com> - 2014-03-07 14:34 -0800
      Re: Padding involved "BartC" <bc@freeuk.com> - 2014-03-07 23:59 +0000
    Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-07 15:15 -0800
      Re: Padding involved "BartC" <bc@freeuk.com> - 2014-03-08 10:08 +0000
        Re: Padding involved Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-03-08 12:39 +0000
        Re: Padding involved Keith Thompson <kst-u@mib.org> - 2014-03-08 14:03 -0800
  Re: Padding involved Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-03-08 09:01 -0800

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