RFR(S): 8210381: Obsolete EmitSync

[Daniel D. Daugherty]

On 9/4/18 3:36 PM, Mikael Vidstedt wrote:
> Please review this change which obsoletes the EmitSync flag. In particular, I could use some help from ppc, aarch64, and s390 maintainers to verify that the change actually builds and (still) works.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8210381 <https://bugs.openjdk.java.net/browse/JDK-8210381>
> Webrev: http://cr.openjdk.java.net/~mikael/webrevs/8210381/webrev.00/open/webrev/ <http://cr.openjdk.java.net/~mikael/webrevs/8210381/webrev.00/open/webrev/>

src/hotspot/cpu/aarch64/aarch64.ad
     No comments.

src/hotspot/cpu/ppc/macroAssembler_ppc.cpp
     No comments.

src/hotspot/cpu/s390/macroAssembler_s390.cpp
     No comments.

src/hotspot/cpu/sparc/macroAssembler_sparc.cpp
     No comments.

src/hotspot/cpu/x86/macroAssembler_x86.cpp
     No comments.

src/hotspot/share/runtime/arguments.cpp
     No comments.

src/hotspot/share/runtime/globals.hpp
     No comments.


General comments:

   - The 'if (EmitSync & 0x01) {' checks are how we disable the
     use of compiled code for monitors. We use -XX:EmitSync=1
     to diagnose things when we wonder if the compiled monitor
     code is broken.
   - The 'if (EmitSync & 4)' checks are how we disable the use
     of compiled code for unlocking of monitors; we still use
     compiled code for locking of monitors. We use -XX:EmitSync=4
     to diagnose things when we wonder if the compiled monitor
     unlock code is broken.
   - BTW, I think s390 is using (EmitSync & 0x01) differently
     than other platforms. No, I didn't try to figure out exactly
     how s390 was using that option.

This looks like a clean obsoletion of the '-XX:EmitSync=<val>'
option so thumbs up from that POV.

Use of '-XX:EmitSync=1' for diagnostic purposes is probably
limited to a handful of people including me. Use of the
'-XX:EmitSync=4' is probably even more rare; I can't remember
using it myself. So while I have used the '-XX:EmitSync=<val>'
option to debug Java monitor stuff, it has been a long time
time...

Dan

>
> * Background (from bug)
>
> The experimental EmitSync flag can in theory be used to select which implementation of the synchronization primitives to use. The flag was convenient when the various implementations were compared a long time ago.
>
> In practice the only implementation that is used and tested today is the default one. The EmitSync flag no longer serves the purpose it used to, and is "Unsafe, Unstable" (the documentation of the flag says so explicitly). It should be obsoleted and later removed.
>
> Cheers,
> Mikael
>
>