linux-kernel - Re: [RFC-PATCH 1/5] unaligned: introduce common header

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Date:	Mon, 10 Nov 2008 11:49:22 +0000
From:	"Will Newton" <will.newton@...il.com>
To:	"Harvey Harrison" <harvey.harrison@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC-PATCH 1/5] unaligned: introduce common header

On Mon, Nov 10, 2008 at 4:22 AM, Harvey Harrison
<harvey.harrison@...il.com> wrote:

(add back lkml cc that I mistakenly dropped)

> On Sat, 2008-11-08 at 12:47 +0000, Will Newton wrote:
>> On Wed, Nov 5, 2008 at 6:16 PM, Harvey Harrison
>> <harvey.harrison@...il.com> wrote:
>>
>> > The memmove-based arches (m32r, xtensa, h8300) are likely going to be fine with this change
>> > barring compiler bugs that made them go with memmove in the first place.
>>
>> As I understand it the need for the memmove implementation is not
>> compiler bugs but default struct alignment. The packed struct
>> implementation will only work with compilers where structs can be
>> aligned on byte boundaries, it's fairly common for RISC architectures
>> to align structs to 4 or 8 byte boundaries.
>
> Which I believe is disabled entirely using __attribute__((packed)), no?

As far as I am aware the packed attribute is handled in this way for
some toolchains (arm in particular). Not everybody does it, and for
good reasons. For example if I have this struct on an architecture
with 8 byte default struct alignment:

struct foo {
    u64 big_data;
    u8 small_data;
    u32 medium_data;
} __attribute__((packed));

Should big_data be accessed as 8 byte load instructions rather than
one 64bit load instruction? It's a pretty large performance penalty to
pay when all I really want is for medium_data to be accessed
correctly.
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