[PATCH] [ARM] Post-indexed addressing for NEON memory access

[Ramana Radhakrishnan]

On Tue, Jun 17, 2014 at 4:03 PM, Charles Baylis
<charles.baylis@linaro.org> wrote:
> On 5 June 2014 07:27, Ramana Radhakrishnan <ramana.gcc@googlemail.com> wrote:
>> On Mon, Jun 2, 2014 at 5:47 PM, Charles Baylis
>> <charles.baylis@linaro.org> wrote:
>>> This patch adds support for post-indexed addressing for NEON structure
>>> memory accesses.
>>>
>>> For example VLD1.8 {d0}, [r0], r1
>>>
>>>
>>> Bootstrapped and checked on arm-unknown-gnueabihf using Qemu.
>>>
>>> Ok for trunk?
>>
>> This looks like a reasonable start but this work doesn't look complete
>> to me yet.
>>
>> Can you also look at the impact on performance of a range of
>> benchmarks especially a popular embedded one to see how this behaves
>> unless you have already done so ?
>
> I ran a popular suite of embedded benchmarks, and there is no impact
> at all on Chromebook (including with the additional attached patch)

Thanks for the due diligence

>
> The patch was developed to address a performance issue with a new
> version of libvpx which uses intrinsics instead of NEON assembler. The
> patch results in a 3% improvement for VP8 decode.

Good - 3% not to be sneezed at.

>
>> POST_INC, POST_MODIFY usually have a funny way of biting you with
>> either ivopts or the way in which address costs work. I think there
>> maybe further tweaks needed but for a first step I'd like to know what
>> the performance impact is.
>
>> I would also suggest running this through clyon's neon intrinsics
>> testsuite to see if that catches any issues especially with the large
>> vector modes.

Thanks.

>
> No issues found in clyon's tests.

Please keep an eye out for any regressions.

>
> Your mention of larger vector modes prompted me to check that the
> patch has the desired result with them. In fact, the costs are
> estimated incorrectly which means the post_modify pattern is not used.
> The attached patch fixes that. (used in combination with my original
> patch)
>
>
> 2014-06-15  Charles Baylis  <charles.bayls@linaro.org>
>
>         * config/arm/arm.c (arm_new_rtx_costs): Reduce cost for mem with
>         embedded side effects.

I'm not too thrilled with putting in more special cases that are not
table driven in there. Can you file a PR with some testcases that show
this so that we don't forget and CC me on it please ?


Ramana