RFR: JDK-8309462: vmTestbase/nsk/jvmti/RunAgentThread/agentthr001/TestDescription.java crashing due to empty while loop [v2]

[JoKern65]

> The sys_thread_3() function contains an empty while loop, which by the standard can be optimized away. Please refer to discussion in https://github.com/llvm/llvm-project/issues/60622
> The xlc17 compiler is doing so, and IBM claims that they are following the standard and will not fix this on compiler side.
> So we have (at least) 3 ways to circumvent this behavior.
> 
> 1. we can introduce the call of a nop library function, which will hinder the optimizer to throw away the loop (This is our proposed solution, but instead of a heavy looping thread, the result is a more or less idle thread):
> `#include <unistd.h>`
> `static void`
> `sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p)`
> `{`
> `                while (1) {`
> `                  sleep(1);`
> `                }`
> `}`
> 
> 2. We can make use of a volatile variable in the loop body which also hinders the optimizer to throw away the loop:
> `static void`
> `sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p)`
> `{`
> `                volatile int i = 1;`
> `                while (i) {`
> `                  i += 2;`
> `                }`
> `}`
> 
> 3. we can use the __attribute__ ((optnone)) modifier in the function declaration to suppress the optimization at all:
> `static void`
> `sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p) __attribute__ ((optnone))`
> `{`
> `                while (1) {`
> `                }`
> `}`
> 
> To make the third approach platform independent, we can implement it in the following way:
> In globalDefinitions.hpp
> `#ifndef OPTNONE`
> `#define OPTNONE`
> `#endif`
> 
> In globalDefinitions_xlc.hpp
> `// optnone support`
> `//`
> `// To use if a function should not be optimized`
> `// Usage:`
> `// void* func(size_t size) OPTNONE {...}`
> `#define OPTNONE __attribute__(( optnone))`
> 
> With this we can change libagentthr001.cpp in a platform independent way to
> `static void`
> `sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p) OPTNONE`
> `{`
> `     while (1) {`
> `     }`
> `}`

JoKern65 has updated the pull request incrementally with one additional commit since the last revision:

  switched to solution two

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14330/files
  - new: https://git.openjdk.org/jdk/pull/14330/files/ae0d67ae..e44edba6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14330&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14330&range=00-01

  Stats: 32 lines in 2 files changed: 29 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14330.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14330/head:pull/14330

PR: https://git.openjdk.org/jdk/pull/14330