RFC: [ARM] Disable peeling

[Richard Earnshaw]

On 11/12/12 09:45, Richard Biener wrote:
> On Mon, Dec 10, 2012 at 10:07 PM, Andi Kleen <andi@firstfloor.org> wrote:
>> Jan Hubicka <hubicka@ucw.cz> writes:
>>
>>> Note that I think Core has similar characteristics - at least for string operations
>>> it fares well with unalignes accesses.
>>
>> Nehalem and later has very fast unaligned vector loads. There's still some
>> penalty when they cross cache lines however.
>>
>> iirc the rule of thumb is to do unaligned for 128 bit vectors,
>> but avoid it for 256bit vectors because the cache line cross
>> penalty is larger on Sandy Bridge and more likely with the larger
>> vectors.
>
> Yes, I think the rule was that using the unaligned instruction variants carries
> no penalty when the actual access is aligned but that aligned accesses are
> still faster than unaligned accesses.  Thus peeling for alignment _is_ a win.
> I also seem to remember that the story for unaligned stores vs. unaligned loads
> is usually different.

Yes, it's generally the case that unaligned loads are slightly more 
expensive than unaligned stores, since the stores can often merge in a 
store buffer with little or no penalty.

R.