hotspot/src/share/vm/runtime/atomic.hpp

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#ifndef SHARE_VM_RUNTIME_ATOMIC_HPP
#define SHARE_VM_RUNTIME_ATOMIC_HPP

#include "memory/allocation.hpp"
#include "utilities/macros.hpp"

enum cmpxchg_memory_order {
  memory_order_relaxed,
  // Use value which doesn't interfere with C++2011. We need to be more conservative.
  memory_order_conservative = 8
};

class Atomic : AllStatic {
 public:
  // Atomic operations on jlong types are not available on all 32-bit
  // platforms. If atomic ops on jlongs are defined here they must only
  // be used from code that verifies they are available at runtime and
  // can provide an alternative action if not - see supports_cx8() for
  // a means to test availability.

  // The memory operations that are mentioned with each of the atomic
  // function families come from src/share/vm/runtime/orderAccess.hpp,
  // e.g., <fence> is described in that file and is implemented by the
  // OrderAccess::fence() function. See that file for the gory details
  // on the Memory Access Ordering Model.

  // All of the atomic operations that imply a read-modify-write action
  // guarantee a two-way memory barrier across that operation. Historically
  // these semantics reflect the strength of atomic operations that are
  // provided on SPARC/X86. We assume that strength is necessary unless
  // we can prove that a weaker form is sufficiently safe.

  // Atomically store to a location
  inline static void store    (jbyte    store_value, jbyte*    dest);
  inline static void store    (jshort   store_value, jshort*   dest);
  inline static void store    (jint     store_value, jint*     dest);
  // See comment above about using jlong atomics on 32-bit platforms
  inline static void store    (jlong    store_value, jlong*    dest);
  inline static void store_ptr(intptr_t store_value, intptr_t* dest);
  inline static void store_ptr(void*    store_value, void*     dest);

  inline static void store    (jbyte    store_value, volatile jbyte*    dest);
  inline static void store    (jshort   store_value, volatile jshort*   dest);
  inline static void store    (jint     store_value, volatile jint*     dest);
  // See comment above about using jlong atomics on 32-bit platforms
  inline static void store    (jlong    store_value, volatile jlong*    dest);
  inline static void store_ptr(intptr_t store_value, volatile intptr_t* dest);
  inline static void store_ptr(void*    store_value, volatile void*     dest);

  // See comment above about using jlong atomics on 32-bit platforms
  inline static jlong load(volatile jlong* src);

  // Atomically add to a location. Returns updated value. add*() provide:
  // <fence> add-value-to-dest <membar StoreLoad|StoreStore>
  inline static jshort   add    (jshort   add_value, volatile jshort*   dest);
  inline static jint     add    (jint     add_value, volatile jint*     dest);
  inline static size_t   add    (size_t   add_value, volatile size_t*   dest);
  inline static intptr_t add_ptr(intptr_t add_value, volatile intptr_t* dest);
  inline static void*    add_ptr(intptr_t add_value, volatile void*     dest);
  // See comment above about using jlong atomics on 32-bit platforms
  inline static jlong    add    (jlong    add_value, volatile jlong*    dest);

  // Atomically increment location. inc*() provide:
  // <fence> increment-dest <membar StoreLoad|StoreStore>
  inline static void inc    (volatile jint*     dest);
  inline static void inc    (volatile jshort*   dest);
  inline static void inc    (volatile size_t*   dest);
  inline static void inc_ptr(volatile intptr_t* dest);
  inline static void inc_ptr(volatile void*     dest);

  // Atomically decrement a location. dec*() provide:
  // <fence> decrement-dest <membar StoreLoad|StoreStore>
  inline static void dec    (volatile jint*     dest);
  inline static void dec    (volatile jshort*   dest);
  inline static void dec    (volatile size_t*   dest);
  inline static void dec_ptr(volatile intptr_t* dest);
  inline static void dec_ptr(volatile void*     dest);

  // Performs atomic exchange of *dest with exchange_value. Returns old
  // prior value of *dest. xchg*() provide:
  // <fence> exchange-value-with-dest <membar StoreLoad|StoreStore>
  inline static jint         xchg    (jint         exchange_value, volatile jint*         dest);
  inline static unsigned int xchg    (unsigned int exchange_value, volatile unsigned int* dest);
  inline static intptr_t     xchg_ptr(intptr_t     exchange_value, volatile intptr_t*     dest);
  inline static void*        xchg_ptr(void*        exchange_value, volatile void*         dest);

  // Performs atomic compare of *dest and compare_value, and exchanges
  // *dest with exchange_value if the comparison succeeded. Returns prior
  // value of *dest. cmpxchg*() provide:
  // <fence> compare-and-exchange <membar StoreLoad|StoreStore>
  inline static jbyte        cmpxchg    (jbyte        exchange_value, volatile jbyte*        dest, jbyte        compare_value, cmpxchg_memory_order order = memory_order_conservative);
  inline static jint         cmpxchg    (jint         exchange_value, volatile jint*         dest, jint         compare_value, cmpxchg_memory_order order = memory_order_conservative);
  // See comment above about using jlong atomics on 32-bit platforms
  inline static jlong        cmpxchg    (jlong        exchange_value, volatile jlong*        dest, jlong        compare_value, cmpxchg_memory_order order = memory_order_conservative);
  inline static unsigned int cmpxchg    (unsigned int exchange_value, volatile unsigned int* dest, unsigned int compare_value, cmpxchg_memory_order order = memory_order_conservative);
  inline static intptr_t     cmpxchg_ptr(intptr_t     exchange_value, volatile intptr_t*     dest, intptr_t     compare_value, cmpxchg_memory_order order = memory_order_conservative);
  inline static void*        cmpxchg_ptr(void*        exchange_value, volatile void*         dest, void*        compare_value, cmpxchg_memory_order order = memory_order_conservative);
};

// platform specific in-line definitions - must come before shared definitions

#include OS_CPU_HEADER(atomic)

// shared in-line definitions

// size_t casts...
#if (SIZE_MAX != UINTPTR_MAX)
#error size_t is not WORD_SIZE, interesting platform, but missing implementation here
#endif

inline size_t Atomic::add(size_t add_value, volatile size_t* dest) {
  return (size_t) add_ptr((intptr_t) add_value, (volatile intptr_t*) dest);
}

inline void Atomic::inc(volatile size_t* dest) {
  inc_ptr((volatile intptr_t*) dest);
}

inline void Atomic::dec(volatile size_t* dest) {
  dec_ptr((volatile intptr_t*) dest);
}

#ifndef VM_HAS_SPECIALIZED_CMPXCHG_BYTE
/*
 * This is the default implementation of byte-sized cmpxchg. It emulates jbyte-sized cmpxchg
 * in terms of jint-sized cmpxchg. Platforms may override this by defining their own inline definition
 * as well as defining VM_HAS_SPECIALIZED_CMPXCHG_BYTE. This will cause the platform specific
 * implementation to be used instead.
 */
inline jbyte Atomic::cmpxchg(jbyte exchange_value, volatile jbyte* dest,
                             jbyte compare_value, cmpxchg_memory_order order) {
  STATIC_ASSERT(sizeof(jbyte) == 1);
  volatile jint* dest_int =
      static_cast<volatile jint*>(align_ptr_down(dest, sizeof(jint)));
  size_t offset = pointer_delta(dest, dest_int, 1);
  jint cur = *dest_int;
  jbyte* cur_as_bytes = reinterpret_cast<jbyte*>(&cur);

  // current value may not be what we are looking for, so force it
  // to that value so the initial cmpxchg will fail if it is different
  cur_as_bytes[offset] = compare_value;

  // always execute a real cmpxchg so that we get the required memory
  // barriers even on initial failure
  do {
    // value to swap in matches current value ...
    jint new_value = cur;
    // ... except for the one jbyte we want to update
    reinterpret_cast<jbyte*>(&new_value)[offset] = exchange_value;

    jint res = cmpxchg(new_value, dest_int, cur, order);
    if (res == cur) break; // success

    // at least one jbyte in the jint changed value, so update
    // our view of the current jint
    cur = res;
    // if our jbyte is still as cur we loop and try again
  } while (cur_as_bytes[offset] == compare_value);

  return cur_as_bytes[offset];
}

#endif // VM_HAS_SPECIALIZED_CMPXCHG_BYTE

inline unsigned Atomic::xchg(unsigned int exchange_value, volatile unsigned int* dest) {
  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
  return (unsigned int)Atomic::xchg((jint)exchange_value, (volatile jint*)dest);
}

inline unsigned Atomic::cmpxchg(unsigned int exchange_value,
                         volatile unsigned int* dest, unsigned int compare_value,
                         cmpxchg_memory_order order) {
  assert(sizeof(unsigned int) == sizeof(jint), "more work to do");
  return (unsigned int)Atomic::cmpxchg((jint)exchange_value, (volatile jint*)dest,
                                       (jint)compare_value, order);
}

inline jlong Atomic::add(jlong    add_value, volatile jlong*    dest) {
  jlong old = load(dest);
  jlong new_value = old + add_value;
  while (old != cmpxchg(new_value, dest, old)) {
    old = load(dest);
    new_value = old + add_value;
  }
  return old;
}

inline jshort Atomic::add(jshort add_value, volatile jshort* dest) {
  // Most platforms do not support atomic add on a 2-byte value. However,
  // if the value occupies the most significant 16 bits of an aligned 32-bit
  // word, then we can do this with an atomic add of (add_value << 16)
  // to the 32-bit word.
  //
  // The least significant parts of this 32-bit word will never be affected, even
  // in case of overflow/underflow.
  //
  // Use the ATOMIC_SHORT_PAIR macro (see macros.hpp) to get the desired alignment.
#ifdef VM_LITTLE_ENDIAN
  assert((intx(dest) & 0x03) == 0x02, "wrong alignment");
  jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest-1));
#else
  assert((intx(dest) & 0x03) == 0x00, "wrong alignment");
  jint new_value = Atomic::add(add_value << 16, (volatile jint*)(dest));
#endif
  return (jshort)(new_value >> 16); // preserves sign
}

inline void Atomic::inc(volatile jshort* dest) {
  (void)add(1, dest);
}

inline void Atomic::dec(volatile jshort* dest) {
  (void)add(-1, dest);
}

#endif // SHARE_VM_RUNTIME_ATOMIC_HPP