RFR: 8265491: Math Signum optimization for x86 [v4]

[Marcus G K Williams]

> x86 Math.Signum() uses two floating point compares and a copy sign operation involving data movement to gpr and XMM.
> 
> We can optimize to one floating point compare and sign computation in XMM. We observe ~25% performance improvement with this optimization.
> 
> Base:
> Benchmark Mode Cnt Score Error Units
> Signum._1_signumFloatTest avgt 5 4.660 ? 0.040 ns/op
> Signum._2_overheadFloat avgt 5 3.314 ? 0.023 ns/op
> Signum._3_signumDoubleTest avgt 5 4.809 ? 0.043 ns/op
> Signum._4_overheadDouble avgt 5 3.313 ? 0.015 ns/op
>  
> Optimized:
> signum intrinsic patch
> Benchmark Mode Cnt Score Error Units
> Signum._1_signumFloatTest avgt 5 3.782 ? 0.019 ns/op
> Signum._2_overheadFloat avgt 5 3.309 ? 0.011 ns/op
> Signum._3_signumDoubleTest avgt 5 3.782 ? 0.017 ns/op
> Signum._4_overheadDouble avgt 5 3.310 ? 0.006 ns/op
> 
> Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>

Marcus G K Williams has updated the pull request incrementally with one additional commit since the last revision:

  Fix predicate
  
  Signed-off-by: Marcus G K Williams <marcus.williams at intel.com>

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3581/files
  - new: https://git.openjdk.java.net/jdk/pull/3581/files/a6d670be..0349468b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3581&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3581&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3581.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3581/head:pull/3581

PR: https://git.openjdk.java.net/jdk/pull/3581