RFR: 8287697: Limit auto vectorization to 32-byte vector on Cascade Lake

[Vladimir Kozlov]

On Wed, 25 May 2022 01:48:16 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

> We observe ~20% regression in SPECjvm2008 mpegaudio sub benchmark on Cascade Lake with Default vs -XX:UseAVX=2.
> The performance of all the other non-startup sub benchmarks of SPECjvm2008 is within +/- 5%. 
> The performance regression is due to auto-vectorization of small loops. 
> We don’t have AVX3Threshold consideration in auto-vectorization. 
> The performance regression in mpegaudio can be recovered by limiting auto-vectorization to 32-byte vectors.
> 
> This PR limits auto-vectorization to 32-byte vectors by default on Cascade Lake. Users can override this by either setting -XX:UseAVX=3 or -XX:SuperWordMaxVectorSize=64 on JVM command line.
> 
> Please review.
> 
> Best Regard,
> Sandhya

You have trailing white spaces.

src/hotspot/share/opto/vectornode.cpp line 1280:

> 1278:       (vlen > 1) && is_power_of_2(vlen) &&
> 1279:       Matcher::vector_size_supported(bt, vlen) &&
> 1280:       (vlen * type2aelembytes(bt) <= SuperWordMaxVectorSize)) {

Can you put this whole condition into separate `static bool VectorNode::vector_size_supported(vlen, bt)` and use in both cases?

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PR: https://git.openjdk.java.net/jdk/pull/8877