[PATCH][ARM] PR target/70473: Reduce size of Cortex-A8 automaton

[Ramana Radhakrishnan]

On Fri, Aug 26, 2016 at 11:14 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
> Hi all,
>
> The scheduling automata sizes are getting a bit out of control (as the PR
> complains about) and the Cortex-A8
> one is one of the largest offenders. An easy, low-hanging fruit in dealing
> with this are some of the FP/NEON operations
> that have very large reservation durations specified for them. They bloat
> the state space by quite a lot and it's not
> likely that there is enough parallelism present in the program to fill the
> (for example) 64 cycles that are modelled
> for the double-precision division. In the past we've dealt with this by
> decreasing the modelled reservation duration
> to keep the state space down.
>
> This patch does that for the cortex_a8_neon automaton and caps the
> reservation duration for a particular reservation
> to 15 cycles. This should be plenty to demonstrate that these are high
> latency instructions.
> With this patch the number of NDFA states is massively reduced by more than
> 70% (26796 -> 6020).
>
> As I don't have access to reasonable Cortex-A8 hardware I benchmarked it on
> SPEC2000 on a Cortex-A15.
> The idea (from Ramana) is that since Cortex-A8 tuning is the default tuning
> for armv7-a the patch shouldn't hurt
> the more widely accessible Cortex-A15 targets. There were no regressions in
> performance there.
>
> Bootstrapped and tested on arm-none-linux-gnueabihf.
> Ok for trunk?
>


OK,

regards
Ramana
> Thanks,
> Kyrill
>
> 2016-08-26  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     PR target/70473
>     * config/arm/cortex-a8-neon.md (cortex_a8_vfp_muld): Reduce
>     reservation duration to 15 cycles.
>     (cortex_a8_vfp_macs): Likewise.
>     (cortex_a8_vfp_macd): Likewise.
>     (cortex_a8_vfp_divs): Likewise.
>     (cortex_a8_vfp_divd): Likewise.