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Diffstat (limited to 'lib/atomic.c')
| -rw-r--r-- | lib/atomic.c | 315 |
1 files changed, 315 insertions, 0 deletions
diff --git a/lib/atomic.c b/lib/atomic.c new file mode 100644 index 000000000000..a291f0d051d1 --- /dev/null +++ b/lib/atomic.c @@ -0,0 +1,315 @@ +/*===-- atomic.c - Implement support functions for atomic operations.------=== + * + * The LLVM Compiler Infrastructure + * + * This file is dual licensed under the MIT and the University of Illinois Open + * Source Licenses. See LICENSE.TXT for details. + * + *===----------------------------------------------------------------------=== + * + * atomic.c defines a set of functions for performing atomic accesses on + * arbitrary-sized memory locations. This design uses locks that should + * be fast in the uncontended case, for two reasons: + * + * 1) This code must work with C programs that do not link to anything + * (including pthreads) and so it should not depend on any pthread + * functions. + * 2) Atomic operations, rather than explicit mutexes, are most commonly used + * on code where contended operations are rate. + * + * To avoid needing a per-object lock, this code allocates an array of + * locks and hashes the object pointers to find the one that it should use. + * For operations that must be atomic on two locations, the lower lock is + * always acquired first, to avoid deadlock. + * + *===----------------------------------------------------------------------=== + */ + +#include <stdint.h> +#include <string.h> + +// Clang objects if you redefine a builtin. This little hack allows us to +// define a function with the same name as an intrinsic. +#pragma redefine_extname __atomic_load_c __atomic_load +#pragma redefine_extname __atomic_store_c __atomic_store +#pragma redefine_extname __atomic_exchange_c __atomic_exchange +#pragma redefine_extname __atomic_compare_exchange_c __atomic_compare_exchange + +/// Number of locks. This allocates one page on 32-bit platforms, two on +/// 64-bit. This can be specified externally if a different trade between +/// memory usage and contention probability is required for a given platform. +#ifndef SPINLOCK_COUNT +#define SPINLOCK_COUNT (1<<10) +#endif +static const long SPINLOCK_MASK = SPINLOCK_COUNT - 1; + +//////////////////////////////////////////////////////////////////////////////// +// Platform-specific lock implementation. Falls back to spinlocks if none is +// defined. Each platform should define the Lock type, and corresponding +// lock() and unlock() functions. +//////////////////////////////////////////////////////////////////////////////// +#ifdef __FreeBSD__ +#include <errno.h> +#include <sys/types.h> +#include <machine/atomic.h> +#include <sys/umtx.h> +typedef struct _usem Lock; +inline static void unlock(Lock *l) { + __c11_atomic_store((_Atomic(uint32_t)*)&l->_count, 1, __ATOMIC_RELEASE); + __c11_atomic_thread_fence(__ATOMIC_SEQ_CST); + if (l->_has_waiters) + _umtx_op(l, UMTX_OP_SEM_WAKE, 1, 0, 0); +} +inline static void lock(Lock *l) { + uint32_t old = 1; + while (!__c11_atomic_compare_exchange_weak((_Atomic(uint32_t)*)&l->_count, &old, + 0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)) { + _umtx_op(l, UMTX_OP_SEM_WAIT, 0, 0, 0); + old = 1; + } +} +/// locks for atomic operations +static Lock locks[SPINLOCK_COUNT] = { [0 ... SPINLOCK_COUNT-1] = {0,1,0} }; +#else +typedef _Atomic(uintptr_t) Lock; +/// Unlock a lock. This is a release operation. +inline static void unlock(Lock *l) { + __c11_atomic_store(l, 0, __ATOMIC_RELEASE); +} +/// Locks a lock. In the current implementation, this is potentially +/// unbounded in the contended case. +inline static void lock(Lock *l) { + uintptr_t old = 0; + while (!__c11_atomic_compare_exchange_weak(l, &old, 1, __ATOMIC_ACQUIRE, + __ATOMIC_RELAXED)) + old = 0; +} +/// locks for atomic operations +static Lock locks[SPINLOCK_COUNT]; +#endif + + +/// Returns a lock to use for a given pointer. +static inline Lock *lock_for_pointer(void *ptr) { + intptr_t hash = (intptr_t)ptr; + // Disregard the lowest 4 bits. We want all values that may be part of the + // same memory operation to hash to the same value and therefore use the same + // lock. + hash >>= 4; + // Use the next bits as the basis for the hash + intptr_t low = hash & SPINLOCK_MASK; + // Now use the high(er) set of bits to perturb the hash, so that we don't + // get collisions from atomic fields in a single object + hash >>= 16; + hash ^= low; + // Return a pointer to the word to use + return locks + (hash & SPINLOCK_MASK); +} + +/// Macros for determining whether a size is lock free. Clang can not yet +/// codegen __atomic_is_lock_free(16), so for now we assume 16-byte values are +/// not lock free. +#define IS_LOCK_FREE_1 __c11_atomic_is_lock_free(1) +#define IS_LOCK_FREE_2 __c11_atomic_is_lock_free(2) +#define IS_LOCK_FREE_4 __c11_atomic_is_lock_free(4) +#define IS_LOCK_FREE_8 __c11_atomic_is_lock_free(8) +#define IS_LOCK_FREE_16 0 + +/// Macro that calls the compiler-generated lock-free versions of functions +/// when they exist. +#define LOCK_FREE_CASES() \ + do {\ + switch (size) {\ + case 2:\ + if (IS_LOCK_FREE_2) {\ + LOCK_FREE_ACTION(uint16_t);\ + }\ + case 4:\ + if (IS_LOCK_FREE_4) {\ + LOCK_FREE_ACTION(uint32_t);\ + }\ + case 8:\ + if (IS_LOCK_FREE_8) {\ + LOCK_FREE_ACTION(uint64_t);\ + }\ + case 16:\ + if (IS_LOCK_FREE_16) {\ + /* FIXME: __uint128_t isn't available on 32 bit platforms. + LOCK_FREE_ACTION(__uint128_t);*/\ + }\ + }\ + } while (0) + + +/// An atomic load operation. This is atomic with respect to the source +/// pointer only. +void __atomic_load_c(int size, void *src, void *dest, int model) { +#define LOCK_FREE_ACTION(type) \ + *((type*)dest) = __c11_atomic_load((_Atomic(type)*)src, model);\ + return; + LOCK_FREE_CASES(); +#undef LOCK_FREE_ACTION + Lock *l = lock_for_pointer(src); + lock(l); + memcpy(dest, src, size); + unlock(l); +} + +/// An atomic store operation. This is atomic with respect to the destination +/// pointer only. +void __atomic_store_c(int size, void *dest, void *src, int model) { +#define LOCK_FREE_ACTION(type) \ + __c11_atomic_store((_Atomic(type)*)dest, *(type*)dest, model);\ + return; + LOCK_FREE_CASES(); +#undef LOCK_FREE_ACTION + Lock *l = lock_for_pointer(dest); + lock(l); + memcpy(dest, src, size); + unlock(l); +} + +/// Atomic compare and exchange operation. If the value at *ptr is identical +/// to the value at *expected, then this copies value at *desired to *ptr. If +/// they are not, then this stores the current value from *ptr in *expected. +/// +/// This function returns 1 if the exchange takes place or 0 if it fails. +int __atomic_compare_exchange_c(int size, void *ptr, void *expected, + void *desired, int success, int failure) { +#define LOCK_FREE_ACTION(type) \ + return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, (type*)expected,\ + *(type*)desired, success, failure) + LOCK_FREE_CASES(); +#undef LOCK_FREE_ACTION + Lock *l = lock_for_pointer(ptr); + lock(l); + if (memcmp(ptr, expected, size) == 0) { + memcpy(ptr, desired, size); + unlock(l); + return 1; + } + memcpy(expected, ptr, size); + unlock(l); + return 0; +} + +/// Performs an atomic exchange operation between two pointers. This is atomic +/// with respect to the target address. +void __atomic_exchange_c(int size, void *ptr, void *val, void *old, int model) { +#define LOCK_FREE_ACTION(type) \ + *(type*)old = __c11_atomic_exchange((_Atomic(type)*)ptr, *(type*)val,\ + model);\ + return; + LOCK_FREE_CASES(); +#undef LOCK_FREE_ACTION + Lock *l = lock_for_pointer(ptr); + lock(l); + memcpy(old, ptr, size); + memcpy(ptr, val, size); + unlock(l); +} + +//////////////////////////////////////////////////////////////////////////////// +// Where the size is known at compile time, the compiler may emit calls to +// specialised versions of the above functions. +//////////////////////////////////////////////////////////////////////////////// +#define OPTIMISED_CASES\ + OPTIMISED_CASE(1, IS_LOCK_FREE_1, uint8_t)\ + OPTIMISED_CASE(2, IS_LOCK_FREE_2, uint16_t)\ + OPTIMISED_CASE(4, IS_LOCK_FREE_4, uint32_t)\ + OPTIMISED_CASE(8, IS_LOCK_FREE_8, uint64_t)\ + /* FIXME: __uint128_t isn't available on 32 bit platforms. + OPTIMISED_CASE(16, IS_LOCK_FREE_16, __uint128_t)*/\ + +#define OPTIMISED_CASE(n, lockfree, type)\ +type __atomic_load_##n(type *src, int model) {\ + if (lockfree)\ + return __c11_atomic_load((_Atomic(type)*)src, model);\ + Lock *l = lock_for_pointer(src);\ + lock(l);\ + type val = *src;\ + unlock(l);\ + return val;\ +} +OPTIMISED_CASES +#undef OPTIMISED_CASE + +#define OPTIMISED_CASE(n, lockfree, type)\ +void __atomic_store_##n(type *dest, type val, int model) {\ + if (lockfree) {\ + __c11_atomic_store((_Atomic(type)*)dest, val, model);\ + return;\ + }\ + Lock *l = lock_for_pointer(dest);\ + lock(l);\ + *dest = val;\ + unlock(l);\ + return;\ +} +OPTIMISED_CASES +#undef OPTIMISED_CASE + +#define OPTIMISED_CASE(n, lockfree, type)\ +type __atomic_exchange_##n(type *dest, type val, int model) {\ + if (lockfree)\ + return __c11_atomic_exchange((_Atomic(type)*)dest, val, model);\ + Lock *l = lock_for_pointer(dest);\ + lock(l);\ + type tmp = *dest;\ + *dest = val;\ + unlock(l);\ + return tmp;\ +} +OPTIMISED_CASES +#undef OPTIMISED_CASE + +#define OPTIMISED_CASE(n, lockfree, type)\ +int __atomic_compare_exchange_##n(type *ptr, type *expected, type desired,\ + int success, int failure) {\ + if (lockfree)\ + return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, expected, desired,\ + success, failure);\ + Lock *l = lock_for_pointer(ptr);\ + lock(l);\ + if (*ptr == *expected) {\ + *ptr = desired;\ + unlock(l);\ + return 1;\ + }\ + *expected = *ptr;\ + unlock(l);\ + return 0;\ +} +OPTIMISED_CASES +#undef OPTIMISED_CASE + +//////////////////////////////////////////////////////////////////////////////// +// Atomic read-modify-write operations for integers of various sizes. +//////////////////////////////////////////////////////////////////////////////// +#define ATOMIC_RMW(n, lockfree, type, opname, op) \ +type __atomic_fetch_##opname##_##n(type *ptr, type val, int model) {\ + if (lockfree) \ + return __c11_atomic_fetch_##opname((_Atomic(type)*)ptr, val, model);\ + Lock *l = lock_for_pointer(ptr);\ + lock(l);\ + type tmp = *ptr;\ + *ptr = tmp op val;\ + unlock(l);\ + return tmp;\ +} + +#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, add, +) +OPTIMISED_CASES +#undef OPTIMISED_CASE +#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, sub, -) +OPTIMISED_CASES +#undef OPTIMISED_CASE +#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, and, &) +OPTIMISED_CASES +#undef OPTIMISED_CASE +#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, or, |) +OPTIMISED_CASES +#undef OPTIMISED_CASE +#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, xor, ^) +OPTIMISED_CASES +#undef OPTIMISED_CASE |