RFR(m) 8170094: PPC64: Keep immediate value 0 cached into a register to improve performance

[Gustavo Serra Scalet]

Alright, thanks for your reply Martin. I didn't know there were experiments related to that. Thanks for sharing your results also.
 
> -----Original Message-----
> From: Doerr, Martin [mailto:martin.doerr at sap.com]
> Sent: quarta-feira, 23 de novembro de 2016 09:34
> To: Gustavo Serra Scalet <gustavo.scalet at eldorado.org.br>; hotspot-
> compiler-dev at openjdk.java.net
> Cc: ppc-aix-port-dev at openjdk.java.net
> Subject: RE: RFR(m) 8170094: PPC64: Keep immediate value 0 cached into a
> register to improve performance
> 
> Hi Gustavo,
> 
> thank you very much for making these experiments.
> 
> We already had the same idea a long time ago and we had run a lot of
> benchmarks. Our conclusion was that not spending a dedicated register
> for the constant 0 was slightly better.
> The key difference between our old implementation and your one seems to
> be that we also modified the INTPRESSURE.
> 
> Giving C2's register allocator less registers without decreasing
> INTPRESSURE may impact register allocation time and increase the risk of
> getting non-compilable methods because the register allocator is giving
> up.
> 
> We had compared "zero_reg with INTPRESSURE-1" against "no zero_reg with
> INTPRESSURE".
> 
> In general, I agree with you that there is opportunity to reduce
> reloading of 0. Unfortunately, I can't see how the load zero nodes were
> generated which have led to the code which you had posted in
> http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-
> October/024664.html
> 
> Maybe C2 could be improved for that. Or a peephole optimizer could help.
> 
> Thanks and best regards,
> Martin
> 
> 
> -----Original Message-----
> From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-
> bounces at openjdk.java.net] On Behalf Of Gustavo Serra Scalet
> Sent: Dienstag, 22. November 2016 20:23
> To: hotspot-compiler-dev at openjdk.java.net
> Cc: ppc-aix-port-dev at openjdk.java.net
> Subject: RE: RFR(m) 8170094: PPC64: Keep immediate value 0 cached into a
> register to improve performance
> 
> CC'ing the ppc-aix-port-dev list.
> 
> > -----Original Message-----
> > From: hotspot-compiler-dev [mailto:hotspot-compiler-dev-
> > bounces at openjdk.java.net] On Behalf Of Gustavo Serra Scalet
> > Sent: terça-feira, 22 de novembro de 2016 15:11
> > To: hotspot-compiler-dev at openjdk.java.net
> > Subject: RFR(m) 8170094: PPC64: Keep immediate value 0 cached into a
> > register to improve performance
> >
> > Hi,
> >
> > I was concerned with an optimization marked as "Hoist CSE of constant
> > load"[1] as I found zeroes being stored in the memory by using a new
> > reg every time.
> >
> > To address this issue, I reserved one of the many registers PPC64 has
> > (on C1 and C2 compilers) to cache zero and then the zero can be stored
> > at once with a single store. (I didn't notice a performance drop
> > simply by removing 1 register from the register bank)
> >
> > I notice small performance gains (on average around 1%) for some
> > microbenchmarks but no noticeable change on SPECjvm2008 score. Maybe
> > there is still some room for improvement by using this approach?
> >
> > Please review if I built a correct solution:
> >
> > Bug and webrev for this change:
> > https://bugs.openjdk.java.net/browse/JDK-8170094
> > https://gut.github.io/openjdk/webrev/JDK-8170094/webrev/index.html
> >
> > Thanks in advance,
> > Gustavo Serra Scalet
> >
> > References:
> > [1]
> > http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-
> > October/024664.html