view hotspot/src/os_cpu/linux_aarch64/vm/os_linux_aarch64.cpp @ 42906:1a8db9cf1407

8170655: [posix] Fix minimum stack size computations Reviewed-by: dcubed, coleenp
author goetz
date Mon, 19 Dec 2016 15:48:57 -0800
parents 1af223983f82
children d1b25477073b
line wrap: on
line source
/*
 * Copyright (c) 1999, 2016, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2014, Red Hat Inc. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

// no precompiled headers
#include "asm/macroAssembler.hpp"
#include "classfile/classLoader.hpp"
#include "classfile/systemDictionary.hpp"
#include "classfile/vmSymbols.hpp"
#include "code/codeCache.hpp"
#include "code/icBuffer.hpp"
#include "code/vtableStubs.hpp"
#include "code/nativeInst.hpp"
#include "interpreter/interpreter.hpp"
#include "jvm_linux.h"
#include "memory/allocation.inline.hpp"
#include "os_share_linux.hpp"
#include "prims/jniFastGetField.hpp"
#include "prims/jvm.h"
#include "prims/jvm_misc.hpp"
#include "runtime/arguments.hpp"
#include "runtime/extendedPC.hpp"
#include "runtime/frame.inline.hpp"
#include "runtime/interfaceSupport.hpp"
#include "runtime/java.hpp"
#include "runtime/javaCalls.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/osThread.hpp"
#include "runtime/sharedRuntime.hpp"
#include "runtime/stubRoutines.hpp"
#include "runtime/thread.inline.hpp"
#include "runtime/timer.hpp"
#include "utilities/events.hpp"
#include "utilities/vmError.hpp"
#ifdef BUILTIN_SIM
#include "../../../../../../simulator/simulator.hpp"
#endif

// put OS-includes here
# include <sys/types.h>
# include <sys/mman.h>
# include <pthread.h>
# include <signal.h>
# include <errno.h>
# include <dlfcn.h>
# include <stdlib.h>
# include <stdio.h>
# include <unistd.h>
# include <sys/resource.h>
# include <pthread.h>
# include <sys/stat.h>
# include <sys/time.h>
# include <sys/utsname.h>
# include <sys/socket.h>
# include <sys/wait.h>
# include <pwd.h>
# include <poll.h>
# include <ucontext.h>
# include <fpu_control.h>

#ifdef BUILTIN_SIM
#define REG_SP REG_RSP
#define REG_PC REG_RIP
#define REG_FP REG_RBP
#define SPELL_REG_SP "rsp"
#define SPELL_REG_FP "rbp"
#else
#define REG_FP 29

#define SPELL_REG_SP "sp"
#define SPELL_REG_FP "x29"
#endif

address os::current_stack_pointer() {
  register void *esp __asm__ (SPELL_REG_SP);
  return (address) esp;
}

char* os::non_memory_address_word() {
  // Must never look like an address returned by reserve_memory,
  // even in its subfields (as defined by the CPU immediate fields,
  // if the CPU splits constants across multiple instructions).

  return (char*) 0xffffffffffff;
}

void os::initialize_thread(Thread *thr) {
}

address os::Linux::ucontext_get_pc(const ucontext_t * uc) {
#ifdef BUILTIN_SIM
  return (address)uc->uc_mcontext.gregs[REG_PC];
#else
  return (address)uc->uc_mcontext.pc;
#endif
}

void os::Linux::ucontext_set_pc(ucontext_t * uc, address pc) {
#ifdef BUILTIN_SIM
  uc->uc_mcontext.gregs[REG_PC] = (intptr_t)pc;
#else
  uc->uc_mcontext.pc = (intptr_t)pc;
#endif
}

intptr_t* os::Linux::ucontext_get_sp(const ucontext_t * uc) {
#ifdef BUILTIN_SIM
  return (intptr_t*)uc->uc_mcontext.gregs[REG_SP];
#else
  return (intptr_t*)uc->uc_mcontext.sp;
#endif
}

intptr_t* os::Linux::ucontext_get_fp(const ucontext_t * uc) {
#ifdef BUILTIN_SIM
  return (intptr_t*)uc->uc_mcontext.gregs[REG_FP];
#else
  return (intptr_t*)uc->uc_mcontext.regs[REG_FP];
#endif
}

// For Forte Analyzer AsyncGetCallTrace profiling support - thread
// is currently interrupted by SIGPROF.
// os::Solaris::fetch_frame_from_ucontext() tries to skip nested signal
// frames. Currently we don't do that on Linux, so it's the same as
// os::fetch_frame_from_context().
ExtendedPC os::Linux::fetch_frame_from_ucontext(Thread* thread,
  const ucontext_t* uc, intptr_t** ret_sp, intptr_t** ret_fp) {

  assert(thread != NULL, "just checking");
  assert(ret_sp != NULL, "just checking");
  assert(ret_fp != NULL, "just checking");

  return os::fetch_frame_from_context(uc, ret_sp, ret_fp);
}

ExtendedPC os::fetch_frame_from_context(const void* ucVoid,
                    intptr_t** ret_sp, intptr_t** ret_fp) {

  ExtendedPC  epc;
  const ucontext_t* uc = (const ucontext_t*)ucVoid;

  if (uc != NULL) {
    epc = ExtendedPC(os::Linux::ucontext_get_pc(uc));
    if (ret_sp) *ret_sp = os::Linux::ucontext_get_sp(uc);
    if (ret_fp) *ret_fp = os::Linux::ucontext_get_fp(uc);
  } else {
    // construct empty ExtendedPC for return value checking
    epc = ExtendedPC(NULL);
    if (ret_sp) *ret_sp = (intptr_t *)NULL;
    if (ret_fp) *ret_fp = (intptr_t *)NULL;
  }

  return epc;
}

frame os::fetch_frame_from_context(const void* ucVoid) {
  intptr_t* sp;
  intptr_t* fp;
  ExtendedPC epc = fetch_frame_from_context(ucVoid, &sp, &fp);
  return frame(sp, fp, epc.pc());
}

// By default, gcc always saves frame pointer rfp on this stack. This
// may get turned off by -fomit-frame-pointer.
frame os::get_sender_for_C_frame(frame* fr) {
#ifdef BUILTIN_SIM
  return frame(fr->sender_sp(), fr->link(), fr->sender_pc());
#else
  return frame(fr->link(), fr->link(), fr->sender_pc());
#endif
}

intptr_t* _get_previous_fp() {
  register intptr_t **ebp __asm__ (SPELL_REG_FP);
  return (intptr_t*) *ebp;   // we want what it points to.
}


frame os::current_frame() {
  intptr_t* fp = _get_previous_fp();
  frame myframe((intptr_t*)os::current_stack_pointer(),
                (intptr_t*)fp,
                CAST_FROM_FN_PTR(address, os::current_frame));
  if (os::is_first_C_frame(&myframe)) {
    // stack is not walkable
    return frame();
  } else {
    return os::get_sender_for_C_frame(&myframe);
  }
}

// Utility functions

// From IA32 System Programming Guide
enum {
  trap_page_fault = 0xE
};

#ifdef BUILTIN_SIM
extern "C" void Fetch32PFI () ;
extern "C" void Fetch32Resume () ;
extern "C" void FetchNPFI () ;
extern "C" void FetchNResume () ;
#endif

extern "C" JNIEXPORT int
JVM_handle_linux_signal(int sig,
                        siginfo_t* info,
                        void* ucVoid,
                        int abort_if_unrecognized) {
  ucontext_t* uc = (ucontext_t*) ucVoid;

  Thread* t = Thread::current_or_null_safe();

  // Must do this before SignalHandlerMark, if crash protection installed we will longjmp away
  // (no destructors can be run)
  os::WatcherThreadCrashProtection::check_crash_protection(sig, t);

  SignalHandlerMark shm(t);

  // Note: it's not uncommon that JNI code uses signal/sigset to install
  // then restore certain signal handler (e.g. to temporarily block SIGPIPE,
  // or have a SIGILL handler when detecting CPU type). When that happens,
  // JVM_handle_linux_signal() might be invoked with junk info/ucVoid. To
  // avoid unnecessary crash when libjsig is not preloaded, try handle signals
  // that do not require siginfo/ucontext first.

  if (sig == SIGPIPE || sig == SIGXFSZ) {
    // allow chained handler to go first
    if (os::Linux::chained_handler(sig, info, ucVoid)) {
      return true;
    } else {
      // Ignoring SIGPIPE/SIGXFSZ - see bugs 4229104 or 6499219
      return true;
    }
  }

  JavaThread* thread = NULL;
  VMThread* vmthread = NULL;
  if (os::Linux::signal_handlers_are_installed) {
    if (t != NULL ){
      if(t->is_Java_thread()) {
        thread = (JavaThread*)t;
      }
      else if(t->is_VM_thread()){
        vmthread = (VMThread *)t;
      }
    }
  }
/*
  NOTE: does not seem to work on linux.
  if (info == NULL || info->si_code <= 0 || info->si_code == SI_NOINFO) {
    // can't decode this kind of signal
    info = NULL;
  } else {
    assert(sig == info->si_signo, "bad siginfo");
  }
*/
  // decide if this trap can be handled by a stub
  address stub = NULL;

  address pc          = NULL;

  //%note os_trap_1
  if (info != NULL && uc != NULL && thread != NULL) {
    pc = (address) os::Linux::ucontext_get_pc(uc);

#ifdef BUILTIN_SIM
    if (pc == (address) Fetch32PFI) {
       uc->uc_mcontext.gregs[REG_PC] = intptr_t(Fetch32Resume) ;
       return 1 ;
    }
    if (pc == (address) FetchNPFI) {
       uc->uc_mcontext.gregs[REG_PC] = intptr_t (FetchNResume) ;
       return 1 ;
    }
#else
    if (StubRoutines::is_safefetch_fault(pc)) {
      os::Linux::ucontext_set_pc(uc, StubRoutines::continuation_for_safefetch_fault(pc));
      return 1;
    }
#endif

    // Handle ALL stack overflow variations here
    if (sig == SIGSEGV) {
      address addr = (address) info->si_addr;

      // check if fault address is within thread stack
      if (thread->on_local_stack(addr)) {
        // stack overflow
        if (thread->in_stack_yellow_reserved_zone(addr)) {
          thread->disable_stack_yellow_reserved_zone();
          if (thread->thread_state() == _thread_in_Java) {
            // Throw a stack overflow exception.  Guard pages will be reenabled
            // while unwinding the stack.
            stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::STACK_OVERFLOW);
          } else {
            // Thread was in the vm or native code.  Return and try to finish.
            return 1;
          }
        } else if (thread->in_stack_red_zone(addr)) {
          // Fatal red zone violation.  Disable the guard pages and fall through
          // to handle_unexpected_exception way down below.
          thread->disable_stack_red_zone();
          tty->print_raw_cr("An irrecoverable stack overflow has occurred.");

          // This is a likely cause, but hard to verify. Let's just print
          // it as a hint.
          tty->print_raw_cr("Please check if any of your loaded .so files has "
                            "enabled executable stack (see man page execstack(8))");
        } else {
          // Accessing stack address below sp may cause SEGV if current
          // thread has MAP_GROWSDOWN stack. This should only happen when
          // current thread was created by user code with MAP_GROWSDOWN flag
          // and then attached to VM. See notes in os_linux.cpp.
          if (thread->osthread()->expanding_stack() == 0) {
             thread->osthread()->set_expanding_stack();
             if (os::Linux::manually_expand_stack(thread, addr)) {
               thread->osthread()->clear_expanding_stack();
               return 1;
             }
             thread->osthread()->clear_expanding_stack();
          } else {
             fatal("recursive segv. expanding stack.");
          }
        }
      }
    }

    if (thread->thread_state() == _thread_in_Java) {
      // Java thread running in Java code => find exception handler if any
      // a fault inside compiled code, the interpreter, or a stub

      // Handle signal from NativeJump::patch_verified_entry().
      if ((sig == SIGILL || sig == SIGTRAP)
          && nativeInstruction_at(pc)->is_sigill_zombie_not_entrant()) {
        if (TraceTraps) {
          tty->print_cr("trap: zombie_not_entrant (%s)", (sig == SIGTRAP) ? "SIGTRAP" : "SIGILL");
        }
        stub = SharedRuntime::get_handle_wrong_method_stub();
      } else if (sig == SIGSEGV && os::is_poll_address((address)info->si_addr)) {
        stub = SharedRuntime::get_poll_stub(pc);
      } else if (sig == SIGBUS /* && info->si_code == BUS_OBJERR */) {
        // BugId 4454115: A read from a MappedByteBuffer can fault
        // here if the underlying file has been truncated.
        // Do not crash the VM in such a case.
        CodeBlob* cb = CodeCache::find_blob_unsafe(pc);
        CompiledMethod* nm = (cb != NULL) ? cb->as_compiled_method_or_null() : NULL;
        if (nm != NULL && nm->has_unsafe_access()) {
          address next_pc = pc + NativeCall::instruction_size;
          stub = SharedRuntime::handle_unsafe_access(thread, next_pc);
        }
      }
      else

      if (sig == SIGFPE  &&
          (info->si_code == FPE_INTDIV || info->si_code == FPE_FLTDIV)) {
        stub =
          SharedRuntime::
          continuation_for_implicit_exception(thread,
                                              pc,
                                              SharedRuntime::
                                              IMPLICIT_DIVIDE_BY_ZERO);
      } else if (sig == SIGSEGV &&
               !MacroAssembler::needs_explicit_null_check((intptr_t)info->si_addr)) {
          // Determination of interpreter/vtable stub/compiled code null exception
          stub = SharedRuntime::continuation_for_implicit_exception(thread, pc, SharedRuntime::IMPLICIT_NULL);
      }
    } else if (thread->thread_state() == _thread_in_vm &&
               sig == SIGBUS && /* info->si_code == BUS_OBJERR && */
               thread->doing_unsafe_access()) {
      address next_pc = pc + NativeCall::instruction_size;
      stub = SharedRuntime::handle_unsafe_access(thread, next_pc);
    }

    // jni_fast_Get<Primitive>Field can trap at certain pc's if a GC kicks in
    // and the heap gets shrunk before the field access.
    if ((sig == SIGSEGV) || (sig == SIGBUS)) {
      address addr = JNI_FastGetField::find_slowcase_pc(pc);
      if (addr != (address)-1) {
        stub = addr;
      }
    }

    // Check to see if we caught the safepoint code in the
    // process of write protecting the memory serialization page.
    // It write enables the page immediately after protecting it
    // so we can just return to retry the write.
    if ((sig == SIGSEGV) &&
        os::is_memory_serialize_page(thread, (address) info->si_addr)) {
      // Block current thread until the memory serialize page permission restored.
      os::block_on_serialize_page_trap();
      return true;
    }
  }

  if (stub != NULL) {
    // save all thread context in case we need to restore it
    if (thread != NULL) thread->set_saved_exception_pc(pc);

    os::Linux::ucontext_set_pc(uc, stub);
    return true;
  }

  // signal-chaining
  if (os::Linux::chained_handler(sig, info, ucVoid)) {
     return true;
  }

  if (!abort_if_unrecognized) {
    // caller wants another chance, so give it to him
    return false;
  }

  if (pc == NULL && uc != NULL) {
    pc = os::Linux::ucontext_get_pc(uc);
  }

  // unmask current signal
  sigset_t newset;
  sigemptyset(&newset);
  sigaddset(&newset, sig);
  sigprocmask(SIG_UNBLOCK, &newset, NULL);

  VMError::report_and_die(t, sig, pc, info, ucVoid);

  ShouldNotReachHere();
  return true; // Mute compiler
}

void os::Linux::init_thread_fpu_state(void) {
}

int os::Linux::get_fpu_control_word(void) {
  return 0;
}

void os::Linux::set_fpu_control_word(int fpu_control) {
}

// Check that the linux kernel version is 2.4 or higher since earlier
// versions do not support SSE without patches.
bool os::supports_sse() {
  return true;
}

bool os::is_allocatable(size_t bytes) {
  return true;
}

////////////////////////////////////////////////////////////////////////////////
// thread stack

// Minimum usable stack sizes required to get to user code. Space for
// HotSpot guard pages is added later.
size_t os::Posix::_compiler_thread_min_stack_allowed = 32 * K;
size_t os::Posix::_java_thread_min_stack_allowed = 32 * K;
size_t os::Posix::_vm_internal_thread_min_stack_allowed = 64 * K;

// return default stack size for thr_type
size_t os::Posix::default_stack_size(os::ThreadType thr_type) {
  // default stack size (compiler thread needs larger stack)
  size_t s = (thr_type == os::compiler_thread ? 4 * M : 1 * M);
  return s;
}

/////////////////////////////////////////////////////////////////////////////
// helper functions for fatal error handler

void os::print_context(outputStream *st, const void *context) {
  if (context == NULL) return;

  const ucontext_t *uc = (const ucontext_t*)context;
  st->print_cr("Registers:");
#ifdef BUILTIN_SIM
  st->print(  "RAX=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RAX]);
  st->print(", RBX=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RBX]);
  st->print(", RCX=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RCX]);
  st->print(", RDX=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RDX]);
  st->cr();
  st->print(  "RSP=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RSP]);
  st->print(", RBP=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RBP]);
  st->print(", RSI=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RSI]);
  st->print(", RDI=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RDI]);
  st->cr();
  st->print(  "R8 =" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R8]);
  st->print(", R9 =" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R9]);
  st->print(", R10=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R10]);
  st->print(", R11=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R11]);
  st->cr();
  st->print(  "R12=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R12]);
  st->print(", R13=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R13]);
  st->print(", R14=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R14]);
  st->print(", R15=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_R15]);
  st->cr();
  st->print(  "RIP=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_RIP]);
  st->print(", EFLAGS=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_EFL]);
  st->print(", CSGSFS=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_CSGSFS]);
  st->print(", ERR=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_ERR]);
  st->cr();
  st->print("  TRAPNO=" INTPTR_FORMAT, uc->uc_mcontext.gregs[REG_TRAPNO]);
  st->cr();
#else
  for (int r = 0; r < 31; r++)
    st->print_cr(  "R%d=" INTPTR_FORMAT, r, (size_t)uc->uc_mcontext.regs[r]);
#endif
  st->cr();

  intptr_t *sp = (intptr_t *)os::Linux::ucontext_get_sp(uc);
  st->print_cr("Top of Stack: (sp=" PTR_FORMAT ")", p2i(sp));
  print_hex_dump(st, (address)sp, (address)(sp + 8*sizeof(intptr_t)), sizeof(intptr_t));
  st->cr();

  // Note: it may be unsafe to inspect memory near pc. For example, pc may
  // point to garbage if entry point in an nmethod is corrupted. Leave
  // this at the end, and hope for the best.
  address pc = os::Linux::ucontext_get_pc(uc);
  st->print_cr("Instructions: (pc=" PTR_FORMAT ")", p2i(pc));
  print_hex_dump(st, pc - 32, pc + 32, sizeof(char));
}

void os::print_register_info(outputStream *st, const void *context) {
  if (context == NULL) return;

  const ucontext_t *uc = (const ucontext_t*)context;

  st->print_cr("Register to memory mapping:");
  st->cr();

  // this is horrendously verbose but the layout of the registers in the
  // context does not match how we defined our abstract Register set, so
  // we can't just iterate through the gregs area

  // this is only for the "general purpose" registers

#ifdef BUILTIN_SIM
  st->print("RAX="); print_location(st, uc->uc_mcontext.gregs[REG_RAX]);
  st->print("RBX="); print_location(st, uc->uc_mcontext.gregs[REG_RBX]);
  st->print("RCX="); print_location(st, uc->uc_mcontext.gregs[REG_RCX]);
  st->print("RDX="); print_location(st, uc->uc_mcontext.gregs[REG_RDX]);
  st->print("RSP="); print_location(st, uc->uc_mcontext.gregs[REG_RSP]);
  st->print("RBP="); print_location(st, uc->uc_mcontext.gregs[REG_RBP]);
  st->print("RSI="); print_location(st, uc->uc_mcontext.gregs[REG_RSI]);
  st->print("RDI="); print_location(st, uc->uc_mcontext.gregs[REG_RDI]);
  st->print("R8 ="); print_location(st, uc->uc_mcontext.gregs[REG_R8]);
  st->print("R9 ="); print_location(st, uc->uc_mcontext.gregs[REG_R9]);
  st->print("R10="); print_location(st, uc->uc_mcontext.gregs[REG_R10]);
  st->print("R11="); print_location(st, uc->uc_mcontext.gregs[REG_R11]);
  st->print("R12="); print_location(st, uc->uc_mcontext.gregs[REG_R12]);
  st->print("R13="); print_location(st, uc->uc_mcontext.gregs[REG_R13]);
  st->print("R14="); print_location(st, uc->uc_mcontext.gregs[REG_R14]);
  st->print("R15="); print_location(st, uc->uc_mcontext.gregs[REG_R15]);
#else
  for (int r = 0; r < 31; r++)
    st->print_cr(  "R%d=" INTPTR_FORMAT, r, (uintptr_t)uc->uc_mcontext.regs[r]);
#endif
  st->cr();
}

void os::setup_fpu() {
}

#ifndef PRODUCT
void os::verify_stack_alignment() {
  assert(((intptr_t)os::current_stack_pointer() & (StackAlignmentInBytes-1)) == 0, "incorrect stack alignment");
}
#endif

int os::extra_bang_size_in_bytes() {
  // AArch64 does not require the additional stack bang.
  return 0;
}

extern "C" {
  int SpinPause() {
    return 0;
  }

  void _Copy_conjoint_jshorts_atomic(jshort* from, jshort* to, size_t count) {
    if (from > to) {
      jshort *end = from + count;
      while (from < end)
        *(to++) = *(from++);
    }
    else if (from < to) {
      jshort *end = from;
      from += count - 1;
      to   += count - 1;
      while (from >= end)
        *(to--) = *(from--);
    }
  }
  void _Copy_conjoint_jints_atomic(jint* from, jint* to, size_t count) {
    if (from > to) {
      jint *end = from + count;
      while (from < end)
        *(to++) = *(from++);
    }
    else if (from < to) {
      jint *end = from;
      from += count - 1;
      to   += count - 1;
      while (from >= end)
        *(to--) = *(from--);
    }
  }
  void _Copy_conjoint_jlongs_atomic(jlong* from, jlong* to, size_t count) {
    if (from > to) {
      jlong *end = from + count;
      while (from < end)
        os::atomic_copy64(from++, to++);
    }
    else if (from < to) {
      jlong *end = from;
      from += count - 1;
      to   += count - 1;
      while (from >= end)
        os::atomic_copy64(from--, to--);
    }
  }

  void _Copy_arrayof_conjoint_bytes(HeapWord* from,
                                    HeapWord* to,
                                    size_t    count) {
    memmove(to, from, count);
  }
  void _Copy_arrayof_conjoint_jshorts(HeapWord* from,
                                      HeapWord* to,
                                      size_t    count) {
    memmove(to, from, count * 2);
  }
  void _Copy_arrayof_conjoint_jints(HeapWord* from,
                                    HeapWord* to,
                                    size_t    count) {
    memmove(to, from, count * 4);
  }
  void _Copy_arrayof_conjoint_jlongs(HeapWord* from,
                                     HeapWord* to,
                                     size_t    count) {
    memmove(to, from, count * 8);
  }
};