[AArch64] Change IssueWidth to 6 in AArch64SchedNeoverseV2.td (#142565)
I think that the issue width for neoverse-v2 CPUs is set too
high and does not properly reflect the dispatch constraints.
I tested various values of IssueWidth (16, 8 and 6) with runs
of SPEC2017 on a neoverse-v2 machine and I got the highest
overall geomean score with an issue width of 6, although it's
only a marginal 0.14% improvement. I also observed a 1-2%
improvement when testing the Gromacs application with some
workloads. Here are some notable changes in SPEC2017 ref
runtimes, i.e. has a ~0.5% change or greater ('-' means
faster):
548.exchange2: -1.7%
510.parest: -0.78%
538.imagick: -0.73%
500.perlbench: -0.57%
525.x264: -0.55%
507.cactuBSSN: -0.5%
520.omnetpp: -0.48%
511.povray: +0.57%
544.nab: +0.65%
503.bwaves: +0.68%
526.blender: +0.75%
If this patch causes any major regressions post-commit it can
be easily reverted, but I think it should be an overall
improvement.
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