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

[Joshua Zhu]

On Tue, 13 Jul 2021 12:55:00 GMT, Joshua Zhu <jzhu at openjdk.org> wrote:

> 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

This pull request has now been integrated.

Changeset: 7c234919
Author:    Joshua Zhu <jzhu at openjdk.org>
Committer: Yi Yang <yyang at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/7c2349191893f07d133ff94a0be4472948f7920b
Stats:     8 lines in 1 file changed: 8 ins; 0 del; 0 mod

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

Reviewed-by: kvn, dlong

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