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

[Charles Baylis <charles dot baylis at linaro dot org>]

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)

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.

> 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.

No issues found in clyon's tests.

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.

Attachment: 0002-Adjust-costs-for-mem-with-post_modify.patch
Description: application/download