On Tue, 6 Jun 2023 20:43:36 GMT, Martin Doerr <mdo...@openjdk.org> wrote:
>> 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) {` >> ` }` >> `}` > > test/hotspot/jtreg/vmTestbase/nsk/jvmti/RunAgentThread/agentthr001/agentthr001.cpp > line 26: > >> 24: #include <stdio.h> >> 25: #include <string.h> >> 26: #include <unistd.h> > > Breaks Windows build: agentthr001.cpp(26): fatal error C1083: Cannot open > include file: 'unistd.h': No such file or directory On Windows I think you need windows.h. I guess you can do it like here: https://github.com/openjdk/jdk/blob/4a75fd462c002a209201d8bfc8d6c9eb286a7444/src/java.base/share/native/libjli/wildcard.c#L99 ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/14330#discussion_r1220410431
