[PATCH i386][google]With -mtune=core2, avoid generating the slow unaligned vector load/store (issue5488054)

[Sriraman Tallam]

On Tue, Dec 13, 2011 at 10:56 AM, Richard Henderson <rth@redhat.com> wrote:
> On 12/13/2011 10:26 AM, Sriraman Tallam wrote:
>> Cool, this works for stores!  It generates the movlps + movhps. I have
>> to also make a similar change to another call to gen_sse2_movdqu for
>> loads. Would it be ok to not do this when tune=core2?
>
> We can work something out.
>
> I'd like you to do the benchmarking to know if unaligned loads are really as expensive as unaligned stores, and whether there are reformatting penalties that make the movlps+movhps option for either load or store less attractive.

I can confirm that movhps+movlps is *not at all* a good substitute for
movdqu on core2. It makes it much worse. MOVHPS/MOVLPS has a very high
penalty (~10x) for unaligned load/stores.

>
>
> r~