RFR (M): 8188764: Obsolete AssumeMP and then remove all support for non-MP builds

Doerr, Martin martin.doerr at sap.com
Tue Sep 18 06:11:44 UTC 2018

Hi David,

thanks for doing this change.
As written before, PPC and s390 parts look good.

Seems like os::is_MP() is unused with this change. But we're keeping it for any future use?
I'm ok with it, just asking.

Best regards,

-----Original Message-----
From: hotspot-dev <hotspot-dev-bounces at openjdk.java.net> On Behalf Of Mikael Vidstedt
Sent: Dienstag, 18. September 2018 08:07
To: David Holmes <david.holmes at oracle.com>
Cc: hotspot-dev developers <hotspot-dev at openjdk.java.net>
Subject: Re: RFR (M): 8188764: Obsolete AssumeMP and then remove all support for non-MP builds

Nice cleanup, looks good!

Minor nit:

sharedRuntime_x86_32.cpp: two right brackets on the same line (2102).


> On Sep 17, 2018, at 10:32 PM, David Holmes <david.holmes at oracle.com> wrote:
> Bug: https://bugs.openjdk.java.net/browse/JDK-8188764
> Webrev: http://cr.openjdk.java.net/~dholmes/8188764/webrev/
> As previously discussed in the RFC thread:
> http://mail.openjdk.java.net/pipermail/hotspot-dev/2018-September/034166.html
> this change obsoletes the AssumeMP flag and removes the bulk of the logic that is conditional on os::is_MP() as follows:
> 1. to gate the introduction of MP-specific features, notably memory barriers
> The is_MP check is removed and we will always issue memory barriers
> or pre-pend lock prefix etc.
> 2. to align certain patchable code sequences (method entry, call sites)
> The is_MP is removed and we always align patchable locations.
> 3. to gate certain optimizations which are questionable on uniprocessors
> (see quicken_jni_functions)
> These are psuedo-memory-barriers where we manufacture a data-dependency
> instead of inserting mfence/lfence (x86 example). These are treated as
> #1 and is_MP is removed.
> 4. to gate optimizations which are desirable on uniprocessors
> (mutex/monitor short circuits)
> These are spin controls and so is_MP remains.
> I have not updated the about-to-be-removed Oracle ARM port code.
> Testing:
>  - Tiers 1 -3 (mach5)
> Performance run TBD.
> Thanks,
> David

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