[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Måns Rullgård]

Lucas Stach <dev at lynxeye.de> writes:

> Am Montag, den 02.07.2012, 10:53 +0100 schrieb Måns Rullgård:
>> Tetsuyuki Kobayashi <koba at kmckk.co.jp> writes:
>> 
>> > Recent compiler generates unaligned memory access in armv7 default.
>> > But current U-Boot does not allow unaligned memory access, so it causes
>> > data abort exception.
>> > This patch add compile option "-mno-unaligned-access" if it is available.
>> 
>> Why not allow unaligned accesses instead?
>> 
> IMHO, our recent discussion showed that both ways are wrong.
> "-mno-unaligned-access" works around misaligned data on the software
> level, while allowing unaligned access does on the hardware level.
>
> What we really want is no unaligned access in U-Boot at all. Just
> because "-mno-unaligned-access" is the default on ARMv5, we should not
> consider it a gold standard.

It's slightly more complicated than that.  Data can be misaligned for a
variety of reasons:

1. Errors in software.
2. Specified by a file format or communication protocol.
3. Deliberately misaligned by the compiler.

Misaligned data of type 1 should of course be fixed properly, not worked
around in any way.

Type 2 happens all the time, and has to be dealt with one way or
another.  If the hardware supports unaligned accesses, this is usually
faster than reading a byte at a time.

When targeting ARMv6 and later, recent gcc versions have started issuing
deliberate unaligned accesses where previously byte by byte accesses
would have been done.  This happens with "packed" structs and sometimes
to write multiple smaller values at once, typically when
zero-initialising things.  These unaligned accesses are *good*.  They
make code smaller and faster.

The real problem here is that u-boot is setting the strict alignment
checking flag, invalidating the assumption of the compiler that the
system allows unaligned accesses.  For ARMv5 and earlier, setting this
flag is usually advisable since it makes finding accidental unaligned
accesses much easier.

This was debated in the context of the kernel a while ago, ultimately
leading to strict alignment being disabled for ARMv6 and up [1].

[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=8428e84d42179c2a00f5f6450866e70d802d1d05

-- 
Måns Rullgård
mans at mansr.com