RFR: 8255401: Shenandoah: Allow oldval and newval registers to overlap in cmpxchg_oop()

[Roman Kennke]

We encountered a failure in testing:

Internal Error (/home/jenkins/workspace/nightly/jdk-jdk/src/hotspot/share/asm/register.hpp:141), pid=15470, tid=15611
assert(a != b && a != c && a != d && b != c && b != d && c != d) failed: registers must be different: a=0x0000000000000000, b=0x0000000000000000, c=0x000000000000000b, d=0x000000000000000a

in:

Stack: [0x00007fb8fa2e3000,0x00007fb8fa3e4000], sp=0x00007fb8fa3deca0, free space=1007k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x156890e] ShenandoahBarrierSetAssembler::cmpxchg_oop(MacroAssembler*, RegisterImpl*, Address, RegisterImpl*, RegisterImpl*, bool, RegisterImpl*, RegisterImpl*)+0xde
V [libjvm.so+0x3ec1d1] compareAndSwapN_shenandoahNode::emit(CodeBuffer&, PhaseRegAlloc*) const+0x571 

It seems to appear very rarely.

The failure is that both newval and oldval are the same register (rax). I believe it is ok for the two registers to overlap:
- It is not expected that newval is preserved across the cmpxchg
- The CAS will override newval, but:
  - The first CAS is unaffected by the overlap
  - The retry-loop is only entered when previous-value == old-value, and thus newval will still hold the same value

For aarch64 it matters even less, because newval is never overridden.

Testing: hotspot_gc_shenandoah (x86 & aarch64).

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Commit messages:
 - 8255401: Shenandoah: Allow oldval and newval registers to overlap in cmpxchg_oop()

Changes: https://git.openjdk.java.net/jdk/pull/871/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=871&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255401
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/871.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/871/head:pull/871

PR: https://git.openjdk.java.net/jdk/pull/871