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

[David Holmes]

Hi Boris,

Thanks for jumping on this. I hope you didn't spend too much time on it 
as I had actually started on the ARM code then decided I didn't need to 
make the changes, so I'd already gone through pretty much everything 
that is needed. My concern is with things like the change in

src/hotspot/cpu/arm/assembler_arm_32.hpp

where you removed the is_MP() check but in fact that needs to remain 
because the instruction pldw is not present on variants of the v7 
architecture without the multi-processor extensions:

http://www.keil.com/support/man/docs/armclang_asm/armclang_asm_pge1425899018492.htm

That makes me wonder whether any of the memory barrier instructions may 
also be missing in some v6/v7 variants as well? Plus I have to account 
for uniprocessor ARMv5 so need to see whether MacroAssembler::membar, 
for example, must also retain the is_MP() check.

Also in:

src/hotspot/cpu/arm/stubGenerator_arm.cpp

you can't remove the !is_MP() check and related code as that is needed 
for ARMv5 (uniprocessor) support.

Similarly in

src/hotspot/cpu/arm/vm_version_arm_32.cpp

you'll probably still want to disable biased-locking for ARMv5.

Cheers,
David

On 20/09/2018 3:31 AM, Boris Ulasevich wrote:
> Hi David,
> 
>  > I have not updated the about-to-be-removed Oracle ARM port code.
> 
> Ok, but we can do the changes in ARM32 codes leaving ARM64 intact.
> I would propose the following change - I think we can either add it to 
> your update or to create a separate RFR after your patch and JEP-340 
> commit:
> 
> http://cr.openjdk.java.net/~bulasevich/8188764/webrev.00/
> 
> regards,
> Boris
> 
> On 18.09.2018 08:32, David Holmes 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