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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Daniel Kiper <daniel.kiper@oracle.com>,
	grub-devel@gnu.org, xen-devel@lists.xenproject.org
Cc: jgross@suse.com, eric.snowberg@oracle.com, arvidjaar@gmail.com,
	phcoder@gmail.com, seth.goldberg@oracle.com
Subject: Re: [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section
Date: Thu, 9 Jun 2016 23:07:12 +0100	[thread overview]
Message-ID: <1c1e54de-2f19-20b6-b8c8-229619b95038__17568.0356486284$1465510103$gmane$org@citrix.com> (raw)
In-Reply-To: <1465504244-17175-9-git-send-email-daniel.kiper@oracle.com>

On 09/06/2016 21:30, Daniel Kiper wrote:
> Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
> ---
>  doc/multiboot.texi |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/doc/multiboot.texi b/doc/multiboot.texi
> index c81b2ea..bf02a1b 100644
> --- a/doc/multiboot.texi
> +++ b/doc/multiboot.texi
> @@ -1384,6 +1384,7 @@ document, but are included for prospective operating system and boot
>  loader writers.
>  
>  @menu
> +* C structure alignment and padding consideration::
>  * Notes on PC::                 
>  * BIOS device mapping techniques::  
>  * Example OS code::             
> @@ -1391,6 +1392,22 @@ loader writers.
>  @end menu
>  
>  
> +@node C structure alignment and padding consideration
> +@section C structure alignment and padding consideration
> +
> +Many C compilers try to optimize memory accesses aligning structure

"by aligning"

> +members properly. Usually they reach the goal by adding some padding.

What does "properly" mean here?  The default padding will be specified
by the default ABI the compiler conforms to.

> +This is very useful thing in general. However, if you try to mix assembler
> +with C or use C to implement structure low level access this behavior
> +may lead, at least, to quite surprising results. Hence, compiler should
> +be instructed to not optimize such accesses. Usually it is done by special
> +attribute added to structure definition, e.g. GCC compatible sources use
> +@samp{__attribute__ ((__packed__))} for this purpose. However, this is not
> +required if it is known that its members are properly aligned and compiler
> +does not do any optimization. Very good example of this is shown below in
> +@file{multiboot2.h} file.

I am not sure what you are trying to say.

~Andrew

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  reply	other threads:[~2016-06-09 22:07 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1465504244-17175-1-git-send-email-daniel.kiper@oracle.com>
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 01/10] multiboot2: Remove redundant if Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 02/10] multiboot2: Clarify meaning of information request header tag Daniel Kiper
2016-06-09 21:14   ` Andrew Cooper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 03/10] multiboot2: Fix description of EFI boot services tag Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 04/10] multiboot2: Add description of support for EFI boot services Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 05/10] multiboot2: Add description of EFI image handle tags Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 06/10] multiboot2: Add description of support for relocatable images Daniel Kiper
2016-06-09 21:36   ` Andrew Cooper
2016-06-10 17:36     ` Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 07/10] multiboot2: Say that memory maps may not be available on EFI platforms Daniel Kiper
2016-06-09 21:37   ` Andrew Cooper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section Daniel Kiper
2016-06-09 22:07   ` Andrew Cooper [this message]
     [not found]   ` <1c1e54de-2f19-20b6-b8c8-229619b95038@citrix.com>
2016-06-10 17:58     ` Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 09/10] multiboot2: Add me to authors Daniel Kiper
2016-06-09 20:30 ` [MULTIBOOT2 DOC PATCH 10/10] multiboot2: Bump version to 2.0 Daniel Kiper
     [not found] ` <1465504244-17175-2-git-send-email-daniel.kiper@oracle.com>
2016-06-09 21:02   ` [MULTIBOOT2 DOC PATCH 01/10] multiboot2: Remove redundant if Andrew Cooper
     [not found] ` <1465504244-17175-5-git-send-email-daniel.kiper@oracle.com>
2016-06-09 21:31   ` [MULTIBOOT2 DOC PATCH 04/10] multiboot2: Add description of support for EFI boot services Andrew Cooper

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