RFR: 8260899: ARM32: SyncOnValueBasedClassTest fails with assert(is_valid()) failed: invalid register

[Aleksey Shipilev]

$ CONF=linux-arm-server-fastdebug make run-test TEST=runtime/Monitor/SyncOnValueBasedClassTest.java
...

#  Internal Error (/home/pi/jdk/src/hotspot/cpu/arm/register_arm.hpp:155), pid=3793, tid=3808
#  assert(is_valid()) failed: invalid register
#
# JRE version: OpenJDK Runtime Environment (17.0) (fastdebug build 17-internal+0-adhoc.pi.jdk)
# Java VM: OpenJDK Server VM (fastdebug 17-internal+0-adhoc.pi.jdk, compiled mode, emulated-client, g1 gc, linux-arm)
# Problematic frame:
# V  [libjvm.so+0xe1a6a8]  MacroAssembler::load_klass(RegisterImpl*, RegisterImpl*, AsmCondition)+0xa4

Current CompileTask:
C1:    318    2   !b        java.lang.Class::desiredAssertionStatus (54 bytes)

Stack: [0x72580000,0x72600000],  sp=0x725fe170,  free space=504k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0xe1a6a8]  MacroAssembler::load_klass(RegisterImpl*, RegisterImpl*, AsmCondition)+0xa4
V  [libjvm.so+0x43b6b4]  C1_MacroAssembler::lock_object(RegisterImpl*, RegisterImpl*, RegisterImpl*, RegisterImpl*, Label&)+0xcf8
V  [libjvm.so+0x3d731c]  LIR_Assembler::emit_lock(LIR_OpLock*)+0x160


The problem is in this code:

  if (DiagnoseSyncOnValueBasedClasses != 0) {
    load_klass(tmp1, obj); <--- asserts
    ldr_u32(tmp1, Address(tmp1, Klass::access_flags_offset()));
    tst(tmp1, JVM_ACC_IS_VALUE_BASED_CLASS);
    b(slow_case, ne);
  }

`tmp1` is `noreg` when `!BiasedLocking`, because `c1_LIRGenerator_arm.cpp` provides it only when `UseBiasedLocking` is enabled:

void LIRGenerator::do_MonitorEnter(MonitorEnter* x) {
  ...
  // Need a scratch register for biased locking on arm
  LIR_Opr scratch = LIR_OprFact::illegalOpr;
  if(UseBiasedLocking) {
    scratch = new_pointer_register();
  } else {
    scratch = atomicLockOpr(); // <--- actually illegalOpr
  }

  ...

  monitor_enter(obj.result(), lock, hdr, scratch,
                x->monitor_no(), info_for_exception, info);
}

The way out is to use `tmp2`, which is the alias for `Rtemp` and always available.

Additional testing:
 - [x] Linux ARM32 `SyncOnValueBasedClassTest`, `-XX:+UseBiasedLocking`
 - [x] Linux ARM32 `SyncOnValueBasedClassTest`, `-XX:-UseBiasedLocking`

-------------

Commit messages:
 - 8260899: ARM32: SyncOnValueBasedClassTest fails with assert(is_valid()) failed: invalid register

Changes: https://git.openjdk.java.net/jdk/pull/2349/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2349&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260899
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2349.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2349/head:pull/2349

PR: https://git.openjdk.java.net/jdk/pull/2349