RFR: 8365290: [perf] x86 ArrayFill intrinsic generates SPLIT_STORE for unaligned arrays

[Vladimir Ivanov]

On the SRF platform for runs with intrinsic scores for the ArrayFill test reports ~2x drop for several sizes due to a lot of the 'MEM_UOPS_RETIRED.SPLIT_STORES' events. The 'good' case for the ArraysFill.testCharFill with size=8195 reports numbers like
MEM_UOPS_RETIRED.SPLIT_LOADS | 22.6711
MEM_UOPS_RETIRED.SPLIT_STORES | 4.0859
while for 'bad' case these metrics are
MEM_UOPS_RETIRED.SPLIT_LOADS | 69.1785
MEM_UOPS_RETIRED.SPLIT_STORES | 259200.3659

With alignment on the cache size no score drops due to split_stores but small reduction may be reported due to extra 
SRF 6740E | Size | orig | pathed | pO/orig
-- | -- | -- | -- | --
ArraysFill.testByteFill | 16 | 152031.2 | 157001.2 | 1.03
ArraysFill.testByteFill | 31 | 125795.9 | 177399.2 | 1.41
ArraysFill.testByteFill | 250 | 57961.69 | 120981.9 | 2.09
ArraysFill.testByteFill | 266 | 44900.15 | 147893.8 | 3.29
ArraysFill.testByteFill | 511 | 61908.17 | 129830.1 | 2.10
ArraysFill.testByteFill | 2047 | 32255.51 | 41986.6 | 1.30
ArraysFill.testByteFill | 2048 | 31928.97 | 42154.3 | 1.32
ArraysFill.testByteFill | 8195 | 10690.15 | 11036.3 | 1.03
ArraysFill.testIntFill | 16 | 145030.7 | 318796.9 | 2.20
ArraysFill.testIntFill | 31 | 134138.4 | 212487 | 1.58
ArraysFill.testIntFill | 250 | 74179.23 | 79522.66 | 1.07
ArraysFill.testIntFill | 266 | 68112.72 | 60116.49 | 0.88
ArraysFill.testIntFill | 511 | 39693.28 | 36225.09 | 0.91
ArraysFill.testIntFill | 2047 | 11504.14 | 10616.91 | 0.92
ArraysFill.testIntFill | 2048 | 11244.71 | 10969.14 | 0.98
ArraysFill.testIntFill | 8195 | 2751.289 | 2692.216 | 0.98
ArraysFill.testLongFill | 16 | 212532.5 | 212526 | 1.00
ArraysFill.testLongFill | 31 | 137432.4 | 137283.3 | 1.00
ArraysFill.testLongFill | 250 | 43185 | 43159.78 | 1.00
ArraysFill.testLongFill | 266 | 42172.22 | 42170.5 | 1.00
ArraysFill.testLongFill | 511 | 23370.15 | 23370.86 | 1.00
ArraysFill.testLongFill | 2047 | 6123.008 | 6122.73 | 1.00
ArraysFill.testLongFill | 2048 | 5793.722 | 5792.855 | 1.00
ArraysFill.testLongFill | 8195 | 616.552 | 616.585 | 1.00
ArraysFill.testShortFill | 16 | 152088.6 | 265646.1 | 1.75
ArraysFill.testShortFill | 31 | 137369.8 | 185596.4 | 1.35
ArraysFill.testShortFill | 250 | 58872.03 | 99621.15 | 1.69
ArraysFill.testShortFill | 266 | 91085.31 | 93746.62 | 1.03
ArraysFill.testShortFill | 511 | 65331.96 | 78003.83 | 1.19
ArraysFill.testShortFill | 2047 | 21716.32 | 21216.81 | 0.98
ArraysFill.testShortFill | 2048 | 21664.91 | 21328.72 | 0.98
ArraysFill.testShortFill | 8195 | 5922.547 | 5799.964 | 0.98

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Commit messages:
 - JDK-8365290 [perf] x86 ArrayFill intrinsic generates SPLIT_STORE for unaligned arrays

Changes: https://git.openjdk.org/jdk/pull/26747/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26747&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365290
  Stats: 17 lines in 1 file changed: 16 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26747.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26747/head:pull/26747

PR: https://git.openjdk.org/jdk/pull/26747