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

[Mikael Vidstedt]

Nice cleanup, looks good!

Minor nit:

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

Cheers,
Mikael

> 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