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

[Kyrill Tkachov]

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?

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.
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