path: root/includes/stg/SMP.h

/* ----------------------------------------------------------------------------
 *
 * (c) The GHC Team, 2005-2011
 *
 * Macros for multi-CPU support
 *
 * Do not #include this file directly: #include "Rts.h" instead.
 *
 * To understand the structure of the RTS headers, see the wiki:
 *   http://ghc.haskell.org/trac/ghc/wiki/Commentary/SourceTree/Includes
 *
 * -------------------------------------------------------------------------- */

#ifndef SMP_H
#define SMP_H

#if arm_HOST_ARCH && defined(arm_HOST_ARCH_PRE_ARMv6)
void arm_atomic_spin_lock(void);
void arm_atomic_spin_unlock(void);
#endif

#if defined(THREADED_RTS)

/* ----------------------------------------------------------------------------
   Atomic operations
   ------------------------------------------------------------------------- */

#if !IN_STG_CODE || IN_STGCRUN
// We only want the barriers, e.g. write_barrier(), declared in .hc
// files.  Defining the other inline functions here causes type
// mismatch errors from gcc, because the generated C code is assuming
// that there are no prototypes in scope.

/*
 * The atomic exchange operation: xchg(p,w) exchanges the value
 * pointed to by p with the value w, returning the old value.
 *
 * Used for locking closures during updates (see lockClosure()
 * in includes/rts/storage/SMPClosureOps.h) and the MVar primops.
 */
EXTERN_INLINE StgWord xchg(StgPtr p, StgWord w);

/*
 * Compare-and-swap.  Atomically does this:
 *
 * cas(p,o,n) {
 *    r = *p;
 *    if (r == o) { *p = n };
 *    return r;
 * }
 */
EXTERN_INLINE StgWord cas(StgVolatilePtr p, StgWord o, StgWord n);

/*
 * Atomic addition by the provided quantity
 *
 * atomic_inc(p, n) {
 *   return ((*p) += n);
 * }
 */
EXTERN_INLINE StgWord atomic_inc(StgVolatilePtr p, StgWord n);


/*
 * Atomic decrement
 *
 * atomic_dec(p) {
 *   return --(*p);
 * }
 */
EXTERN_INLINE StgWord atomic_dec(StgVolatilePtr p);

/*
 * Busy-wait nop: this is a hint to the CPU that we are currently in a
 * busy-wait loop waiting for another CPU to change something.  On a
 * hypertreaded CPU it should yield to another thread, for example.
 */
EXTERN_INLINE void busy_wait_nop(void);

#endif // !IN_STG_CODE

/*
 * Various kinds of memory barrier.
 *  write_barrier: prevents future stores occurring before prededing stores.
 *  store_load_barrier: prevents future loads occurring before preceding stores.
 *  load_load_barrier: prevents future loads occurring before earlier stores.
 *
 * Reference for these: "The JSR-133 Cookbook for Compiler Writers"
 * http://gee.cs.oswego.edu/dl/jmm/cookbook.html
 *
 * To check whether you got these right, try the test in
 *   testsuite/tests/rts/testwsdeque.c
 * This tests the work-stealing deque implementation, which relies on
 * properly working store_load and load_load memory barriers.
 */
EXTERN_INLINE void write_barrier(void);
EXTERN_INLINE void store_load_barrier(void);
EXTERN_INLINE void load_load_barrier(void);

/* ----------------------------------------------------------------------------
   Implementations
   ------------------------------------------------------------------------- */

#if !IN_STG_CODE || IN_STGCRUN

/*
 * Exchange the value pointed to by p with w and return the former. This
 * function is used to acquire a lock. An acquire memory barrier is sufficient
 * for a lock operation because corresponding unlock operation issues a
 * store-store barrier (write_barrier()) immediately before releasing the lock.
 */
EXTERN_INLINE StgWord
xchg(StgPtr p, StgWord w)
{
    // When porting GHC to a new platform check that
    // __sync_lock_test_and_set() actually stores w in *p.
    // Use test rts/atomicxchg to verify that the correct value is stored.
    // From the gcc manual:
    // (https://gcc.gnu.org/onlinedocs/gcc-4.4.3/gcc/Atomic-Builtins.html)
    // This built-in function, as described by Intel, is not
    // a traditional test-and-set operation, but rather an atomic
    // exchange operation.
    // [...]
    // Many targets have only minimal support for such locks,
    // and do not support a full exchange operation. In this case,
    // a target may support reduced functionality here by which the
    // only valid value to store is the immediate constant 1. The
    // exact value actually stored in *ptr is implementation defined.
    return __sync_lock_test_and_set(p, w);
}

/*
 * CMPXCHG - the single-word atomic compare-and-exchange instruction.  Used
 * in the STM implementation.
 */
EXTERN_INLINE StgWord
cas(StgVolatilePtr p, StgWord o, StgWord n)
{
    return __sync_val_compare_and_swap(p, o, n);
}

// RRN: Generalized to arbitrary increments to enable fetch-and-add in
// Haskell code (fetchAddIntArray#).
// PT: add-and-fetch, returns new value
EXTERN_INLINE StgWord
atomic_inc(StgVolatilePtr p, StgWord incr)
{
    return __sync_add_and_fetch(p, incr);
}

EXTERN_INLINE StgWord
atomic_dec(StgVolatilePtr p)
{
    return __sync_sub_and_fetch(p, (StgWord) 1);
}

/*
 * Some architectures have a way to tell the CPU that we're in a
 * busy-wait loop, and the processor should look for something else to
 * do (such as run another hardware thread).
 */
EXTERN_INLINE void
busy_wait_nop(void)
{
#if defined(i386_HOST_ARCH) || defined(x86_64_HOST_ARCH)
    // On Intel, the busy-wait-nop instruction is called "pause",
    // which is actually represented as a nop with the rep prefix.
    // On processors before the P4 this behaves as a nop; on P4 and
    // later it might do something clever like yield to another
    // hyperthread.  In any case, Intel recommends putting one
    // of these in a spin lock loop.
    __asm__ __volatile__ ("rep; nop");
#else
    // nothing
#endif
}

#endif // !IN_STG_CODE

/*
 * We need to tell both the compiler AND the CPU about the barriers.
 * It's no good preventing the CPU from reordering the operations if
 * the compiler has already done so - hence the "memory" restriction
 * on each of the barriers below.
 */
EXTERN_INLINE void
write_barrier(void) {
#if defined(NOSMP)
    return;
#elif i386_HOST_ARCH || x86_64_HOST_ARCH
    __asm__ __volatile__ ("" : : : "memory");
#elif powerpc_HOST_ARCH || powerpc64_HOST_ARCH || powerpc64le_HOST_ARCH
    __asm__ __volatile__ ("lwsync" : : : "memory");
#elif sparc_HOST_ARCH
    /* Sparc in TSO mode does not require store/store barriers. */
    __asm__ __volatile__ ("" : : : "memory");
#elif (arm_HOST_ARCH) || aarch64_HOST_ARCH
    __asm__ __volatile__ ("dmb  st" : : : "memory");
#else
#error memory barriers unimplemented on this architecture
#endif
}

EXTERN_INLINE void
store_load_barrier(void) {
#if defined(NOSMP)
    return;
#elif i386_HOST_ARCH
    __asm__ __volatile__ ("lock; addl $0,0(%%esp)" : : : "memory");
#elif x86_64_HOST_ARCH
    __asm__ __volatile__ ("lock; addq $0,0(%%rsp)" : : : "memory");
#elif powerpc_HOST_ARCH || powerpc64_HOST_ARCH || powerpc64le_HOST_ARCH
    __asm__ __volatile__ ("sync" : : : "memory");
#elif sparc_HOST_ARCH
    __asm__ __volatile__ ("membar #StoreLoad" : : : "memory");
#elif arm_HOST_ARCH
    __asm__ __volatile__ ("dmb" : : : "memory");
#elif aarch64_HOST_ARCH
    __asm__ __volatile__ ("dmb sy" : : : "memory");
#else
#error memory barriers unimplemented on this architecture
#endif
}

EXTERN_INLINE void
load_load_barrier(void) {
#if defined(NOSMP)
    return;
#elif i386_HOST_ARCH
    __asm__ __volatile__ ("" : : : "memory");
#elif x86_64_HOST_ARCH
    __asm__ __volatile__ ("" : : : "memory");
#elif powerpc_HOST_ARCH || powerpc64_HOST_ARCH || powerpc64le_HOST_ARCH
    __asm__ __volatile__ ("lwsync" : : : "memory");
#elif sparc_HOST_ARCH
    /* Sparc in TSO mode does not require load/load barriers. */
    __asm__ __volatile__ ("" : : : "memory");
#elif arm_HOST_ARCH
    __asm__ __volatile__ ("dmb" : : : "memory");
#elif aarch64_HOST_ARCH
    __asm__ __volatile__ ("dmb sy" : : : "memory");
#else
#error memory barriers unimplemented on this architecture
#endif
}

// Load a pointer from a memory location that might be being modified
// concurrently.  This prevents the compiler from optimising away
// multiple loads of the memory location, as it might otherwise do in
// a busy wait loop for example.
#define VOLATILE_LOAD(p) (*((StgVolatilePtr)(p)))

/* ---------------------------------------------------------------------- */
#else /* !THREADED_RTS */

EXTERN_INLINE void write_barrier(void);
EXTERN_INLINE void store_load_barrier(void);
EXTERN_INLINE void load_load_barrier(void);
EXTERN_INLINE void write_barrier     () {} /* nothing */
EXTERN_INLINE void store_load_barrier() {} /* nothing */
EXTERN_INLINE void load_load_barrier () {} /* nothing */

#if !IN_STG_CODE || IN_STGCRUN
INLINE_HEADER StgWord
xchg(StgPtr p, StgWord w)
{
    StgWord old = *p;
    *p = w;
    return old;
}

EXTERN_INLINE StgWord cas(StgVolatilePtr p, StgWord o, StgWord n);
EXTERN_INLINE StgWord
cas(StgVolatilePtr p, StgWord o, StgWord n)
{
    StgWord result;
    result = *p;
    if (result == o) {
        *p = n;
    }
    return result;
}

EXTERN_INLINE StgWord atomic_inc(StgVolatilePtr p, StgWord incr);
EXTERN_INLINE StgWord
atomic_inc(StgVolatilePtr p, StgWord incr)
{
    return ((*p) += incr);
}


INLINE_HEADER StgWord
atomic_dec(StgVolatilePtr p)
{
    return --(*p);
}
#endif

#define VOLATILE_LOAD(p) ((StgWord)*((StgWord*)(p)))

#endif /* !THREADED_RTS */

#endif /* SMP_H */