8263718: unused-result warning happens at os_linux.cpp

Reviewed-by: dholmes, stuefe

hotspot/src/os/bsd/vm/os_bsd.cpp

@@ -693,15 +693,6 @@ static uint64_t locate_unique_thread_id(mach_port_t mach_thread_port) {
 
 // Thread start routine for all newly created threads
 static void *java_start(Thread *thread) {
-  // Try to randomize the cache line index of hot stack frames.
-  // This helps when threads of the same stack traces evict each other's
-  // cache lines. The threads can be either from the same JVM instance, or
-  // from different JVM instances. The benefit is especially true for
-  // processors with hyperthreading technology.
-  static int counter = 0;
-  int pid = os::current_process_id();
-  alloca(((pid ^ counter++) & 7) * 128);
-
   ThreadLocalStorage::set_thread(thread);
 
   OSThread* osthread = thread->osthread();

hotspot/src/os/linux/vm/os_linux.cpp

@@ -812,14 +812,20 @@ static bool _thread_safety_check(Thread* thread) {
 
 // Thread start routine for all newly created threads
 static void *java_start(Thread *thread) {
+#ifndef __GLIBC__
   // Try to randomize the cache line index of hot stack frames.
   // This helps when threads of the same stack traces evict each other's
   // cache lines. The threads can be either from the same JVM instance, or
   // from different JVM instances. The benefit is especially true for
   // processors with hyperthreading technology.
+  // This code is not needed anymore in glibc because it has MULTI_PAGE_ALIASING
+  // and we did not see any degradation in performance without `alloca()`.
   static int counter = 0;
   int pid = os::current_process_id();
-  alloca(((pid ^ counter++) & 7) * 128);
+  void *stackmem = alloca(((pid ^ counter++) & 7) * 128);
+  // Ensure the alloca result is used in a way that prevents the compiler from eliding it.
+  *(char *)stackmem = 1;
+#endif
 
   ThreadLocalStorage::set_thread(thread);
 

hotspot/src/os/windows/vm/os_windows.cpp

@@ -413,15 +413,6 @@ LONG WINAPI topLevelExceptionFilter(struct _EXCEPTION_POINTERS* exceptionInfo);
 
 // Thread start routine for all new Java threads
 static unsigned __stdcall java_start(Thread* thread) {
-  // Try to randomize the cache line index of hot stack frames.
-  // This helps when threads of the same stack traces evict each other's
-  // cache lines. The threads can be either from the same JVM instance, or
-  // from different JVM instances. The benefit is especially true for
-  // processors with hyperthreading technology.
-  static int counter = 0;
-  int pid = os::current_process_id();
-  _alloca(((pid ^ counter++) & 7) * 128);
-
   OSThread* osthr = thread->osthread();
   assert(osthr->get_state() == RUNNABLE, "invalid os thread state");