RFR: 8243339: AArch64: Obsolete UseBarriersForVolatile option

[Derek White]

Hi Xiaohong,

This looks good to me (not a (R)eviewer). Thanks for including the patch for ThunderX !

This is a nice cleanup of the code and especially the volatile tests.

I did NOT check that the removal of the Compiler Interface declarations does or doesn't impact compilation of the Graal compiler. Did you check that side?

Thanks,
 - Derek

-----Original Message-----
From: hotspot-compiler-dev <hotspot-compiler-dev-bounces at openjdk.java.net> On Behalf Of Xiaohong Gong
Sent: Thursday, May 7, 2020 11:39 PM
To: aarch64-port-dev at openjdk.java.net; hotspot-compiler-dev at openjdk.java.net
Cc: nd <nd at arm.com>
Subject: [EXT] RFR: 8243339: AArch64: Obsolete UseBarriersForVolatile option

External Email

----------------------------------------------------------------------
Hi,

Please  help to review this patch which obsoletes the product flag "-XX:UseBarrierssForVolatile" and its related code:
Webrev:
https://urldefense.proofpoint.com/v2/url?u=http-3A__cr.openjdk.java.net_-7Exgong_rfr_8243339_webrev.00_&d=DwIFAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=gW0hANMfJfyELYt_X2mceubwzCNjT0vmaU97kngYUJk&m=OT2KWgejq1kc_YJt_NWWZBKwYqCqlRvzSlfRO04igpk&s=oI-OcgRUa25GiBZaU5V4OmuS8aewSxBaMLbKw3A7lnA&e=
JBS:
https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.openjdk.java.net_browse_JDK-2D8243339&d=DwIFAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=gW0hANMfJfyELYt_X2mceubwzCNjT0vmaU97kngYUJk&m=OT2KWgejq1kc_YJt_NWWZBKwYqCqlRvzSlfRO04igpk&s=T-oDfXrBvQorUBzFZq7Omb17P5yqQjg_q3dBo4EExCA&e=
https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.openjdk.java.net_browse_JDK-2D8243456&d=DwIFAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=gW0hANMfJfyELYt_X2mceubwzCNjT0vmaU97kngYUJk&m=OT2KWgejq1kc_YJt_NWWZBKwYqCqlRvzSlfRO04igpk&s=MDMAqUQRO_kmBtmTodGJ2wNuaVy-u_Y_jykpMmyMQwI&e=  (CSR)

As described in the CSR, using "-XX:+UseBarriersForVolatile" might have memory consistent issue like that mentioned in [1]. It needs more effort to fix the issue and maintain the memory consistency in future. Since "ldar/stlr" has worked well for a long time, and so does "ldaxr/stlxr" for unsafe atomics, we'd better simplify things by removing this option and the alternative implementation for the volatile access.

Since its only one signifcant usage on a kind of CPU would also like to be removed (See [2]), it can work well without this option. So we directly obsolete this option and remove the code, rather than deprecate it firstly.

Besides obsoleting this option, this patch also removes an AArch64 CPU feature "CPU_DMB_ATOMICS" together. It is a  workaround while not an AArch64 official feature, which is not required anymore (See [2]).

[1] https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.openjdk.java.net_browse_JDK-2D8241137&d=DwIFAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=gW0hANMfJfyELYt_X2mceubwzCNjT0vmaU97kngYUJk&m=OT2KWgejq1kc_YJt_NWWZBKwYqCqlRvzSlfRO04igpk&s=XQwo26nMgDENOKN5U4pW2EunOEt2UtLycucN1BCScaU&e=
[2] https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.openjdk.java.net_browse_JDK-2D8242469&d=DwIFAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=gW0hANMfJfyELYt_X2mceubwzCNjT0vmaU97kngYUJk&m=OT2KWgejq1kc_YJt_NWWZBKwYqCqlRvzSlfRO04igpk&s=ChT4b4Jj_TkXozuRs6HIqUVPn1iap0DzKvB-2dKYf0g&e= 

Testing:
Jtreg hotspot::hotspot_all_no_apps, jdk::jdk_core, langtools::tier1
JCStress: tests-all

Thanks,
Xiaohong Gong