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Re: GCC ARM: aligned access

On 09/01/2014 08:09 AM, Matt Thomas wrote:
> On Aug 31, 2014, at 11:32 AM, Joel Sherrill <> 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.

int func(u8 *data)                                                              
        return *(unsigned int *)data;                                           

00000000 <func>:                                                                
   0: e5900000  ldr r0, [r0]                                                    
   4: e12fff1e  bx  lr


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