On Mon, Sep 01, 2014 at 09:14:31AM +0800, Peng Fan wrote:
>
>
> On 09/01/2014 08:09 AM, Matt Thomas wrote:
> >
> > On Aug 31, 2014, at 11:32 AM, Joel Sherrill <joel.sherr...@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.
>
> int func(u8 *data)
>
> {
>
> return *(unsigned int *)data;
>
> }
I guess -mno-unaligned-access only applies to (potentially) unaligned addresses
that the compiler itself is aware of, like packed struct members.
Otherwise gcc would have to consider *every* memory load/store as
unaligned and break them down into byte loads/stores. In the above case,
you are telling the compiler that you know it is word aligned (by
casting), and the compiler believes you :).
Regards
Senthil
