Re: GCC ARM: aligned access

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Mon, Sep 1, 2014 at 9:14 AM, Peng Fan <van.freenix@gmail.com> wrote:
>
>
> On 09/01/2014 08:09 AM, Matt Thomas wrote:
>>
>> On Aug 31, 2014, at 11:32 AM, Joel Sherrill <joel.sherrill@OARcorp.com> wrote:
>>
>>>> Hi,
>>>>
>>>> I am writing some code and found that system crashed. I found it was
>>>> unaligned access which causes `data abort` exception. I write a piece
>>>> of code and objdump
>>>> it. I am not sure this is right or not.
>>>>
>>>> command:
>>>> arm-poky-linux-gnueabi-gcc -marm -mno-thumb-interwork -mabi=aapcs-linux
>>>> -mword-relocations -march=armv7-a -mno-unaligned-access
>>>> -ffunction-sections -fdata-sections -fno-common -ffixed-r9 -msoft-float
>>>> -pipe  -O2 -c 2.c -o 2.o
>>>>
>>>> arch is armv7-a and used '-mno-unaligned access'
>>>
>>> I think this is totally expected. You were passed a u8 pointer which is aligned for that type (no restrictions likely). You cast it to a type with stricter alignment requirements. The code is just flawed. Some CPUs handle unaligned accesses but not your ARM.
>>
> armv7 and armv6 arch except armv6-m support unaligned access. a u8 pointer is casted to u32 pointer, should gcc take the align problem into consideration to avoid possible errors? because -mno-unaligned-access.
>> While armv7 and armv6 supports unaligned access, that support has to be
>> enabled by the underlying O/S.  Not knowing the underlying environment,
>> I can't say whether that support is enabled.  One issue we had in NetBSD
>> in moving to gcc4.8 was that the NetBSD/arm kernel didn't enable unaligned
>> access for armv[67] CPUs.  We quickly changed things so unaligned access
>> is supported.
>
> Yeah. by set a hardware bit in arm coprocessor, unaligned access will not cause data abort exception.
> I just wonder is the following correct? should gcc take the responsibility to take care possible unaligned pointer `u8 *data`? because -mno-unaligned-access is passed to gcc.
I suppose no.  It explicit type conversion, the compiler assumes you
take the responsibility I think.
Actually you can dump the final rtl using -fdump-rtl-shorten,look at
the memory alignment information.  In my experiment, it's A32 with
-mno-unaligned-access, which means it's 32 bits aligned.

Thanks,
bin
>
> int func(u8 *data)
> {
>         return *(unsigned int *)data;
> }
>
> 00000000 <func>:
>    0: e5900000  ldr r0, [r0]
>    4: e12fff1e  bx  lr
>
> Regards,
> Peng.
>>