RFR: 8269598: Regressions up to 5% on aarch64 seems due to JDK-8268858

[Joshua Zhu]

Hi,

Please review this change.
It fixes the SPECjvm2008 regression on aarch64 caused by JDK-8268858.

Compressor::compress() in case "compress" of SPECjvm2008 performed
degraded by 5% due to extra spillings. There existed an LRG that covered
almost the entire function, hence it interfered with more live ranges.
These interferences caused the LRG no freedom and lastly RA had to spill
it to stack. After the fix, the LRG is split into two shorter live
ranges. Both LRGs are assigned to registers due to fewer interferences.

See details at: http://cr.openjdk.java.net/~jzhu/8269598/analysis.pdf
OptoAssembly: http://cr.openjdk.java.net/~jzhu/8269598/

The root cause is:
When PreserveFramePointer is off, frame pointer is allocatable, but
different from other SOC registers, it is excluded from fatproj's mask
because its save type is No-Save. Therefore fatproj was not treated as
high-pressure transition point in the previous int_pressure_limit()
computation. This fix ensures high pressure at fatproj when
PreserveFramePointer is off. See check_pressure_at_fatproj() [1].

After this fix, the regression disappeared.
The result of regression cases in SPECjvm2008:
    http://cr.openjdk.java.net/~jzhu/8269598/Book1.pdf

[1] https://github.com/openjdk/jdk/blob/375fc2a2b29c454b36d3ae068a080b28f6ec04e9/src/hotspot/share/opto/chaitin.hpp#L620

Best Regards,
Joshua

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

Commit messages:
 - 8269598: Regressions up to 5% on aarch64 seems due to JDK-8268858

Changes: https://git.openjdk.java.net/jdk/pull/4766/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4766&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269598
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4766.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4766/head:pull/4766

PR: https://git.openjdk.java.net/jdk/pull/4766