RFR: 8283091: Support type conversion between different data sizes in SLP [v3]

[Fei Gao]

On Fri, 8 Apr 2022 02:51:51 GMT, Jie Fu <jiefu at openjdk.org> wrote:

> Can you explain why `convertD2I` becomes much slower on NEON after this patch? Thanks.

Thanks for your review.

On NEON, there are no real vector instructions to do the conversion from Double to Int, which is implemented in 5 instructions https://github.com/openjdk/jdk/blob/8c1870521815a24fd12480e73450c2201542a442/src/hotspot/cpu/aarch64/aarch64_neon.ad#L512, costing more than scalar instructions, as we know that there are only two elements for VectorCastD2I on 128-bit NEON machine.

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PR: https://git.openjdk.java.net/jdk/pull/7806