RFR (XS): 8220388: Increase -inlinehint-threshold for Clang to avoid G1 pause time regression

[Man Cao]

Hi all,

I propose to increase the -inlinehint-threshold for Clang build:
Webrev: https://cr.openjdk.java.net/~manc/8220388/webrev.00/
RFE: https://bugs.openjdk.java.net/browse/JDK-8220388

The background and performance impact is described in the RFE.
In summary, increasing the -inlinehint-threshold could noticeably reduce GC
pause time for G1 when building the JVM with Clang.

I have a couple of questions though:
(1) I agree that a better long-term solution might be to identify those
performance-critical functions and mark them with "ALWAYSINLINE" instead of
"inline".
However, it might be difficult to identify all those functions, and similar
issue might arise in collectors other than G1, or in other places in
HotSpot.
Currently ALWAYSINLINE is rarely used across HotSpot. I'm not sure if there
is a convention to prefer "inline" over "ALWAYSINLINE".
If we go down this path, we might end up changing many "inline" to
"ALWAYSINLINE" all over the places, due to the cascading effect of inlining.

(2) Since this change only affects Clang build, and Clang is only
officially supported for Mac OS (
https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms),
should I send this patch to broader mailing lists such
as build-dev, hotspot-dev?

Thanks,
Man
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