> On 7 Mar 2018, at 16:14, Frediano Ziglio <fzig...@redhat.com> wrote:
> 
>> 
>> On Wed, Mar 07, 2018 at 03:01:43PM +0100, Christophe de Dinechin wrote:
>>> Also, on an arch like ARM. the packed attribute gives the compiler the
>>> opportunity to use special mis-aligned load and store instructions. So
>>> it’s a good thing, isn’t it?
>> 
>> Iirc the compiler was not doing that much magic last time the issue came up.
>> 
>> Christophe
>> 
> 
> Don't exactly agree.
> Was quite confusing. There were 2 patches, one to fix ARMv7 and alignment of
> 64 bit and one to fix old ARMv5.
> We didn't have much test environment and the reply for testing was very
> few so we didn't have much confirmations.
> When I tried directly using a ARM machine the code and speed improved a lot.

Very interesting. Last time the issue of alignment came up, I did some tests 
for misaligned accesses on various generations of x86 and ARM. On x86, I could 
see no performance impact at all for any Core-i5 and i7 machine, and barely 
noticeable for earlier ones. For ARM, the performance impact was more serious, 
but much less than I anticipated. I can try to re-do the experiment if anybody 
is interested.

My conclusion at the time, which was a surprise to me, was that alignment no 
longer mattered at all on x86.

> Is confusing actually that the unaligned access code is generated only for
> structures.
> 
> Frediano
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