> 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.
> 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
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
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