atomics.h

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/*-------------------------------------------------------------------------
 *
 * atomics.h
 *    Atomic operations.
 *
 * Hardware and compiler dependent functions for manipulating memory
 * atomically and dealing with cache coherency. Used to implement locking
 * facilities and lockless algorithms/data structures.
 *
 * To bring up postgres on a platform/compiler at the very least
 * implementations for the following operations should be provided:
 * * pg_compiler_barrier(), pg_write_barrier(), pg_read_barrier()
 * * pg_atomic_compare_exchange_u32(), pg_atomic_fetch_add_u32()
 * * pg_atomic_test_set_flag(), pg_atomic_init_flag(), pg_atomic_clear_flag()
 * * PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY should be defined if appropriate.
 *
 * There exist generic, hardware independent, implementations for several
 * compilers which might be sufficient, although possibly not optimal, for a
 * new platform. If no such generic implementation is available spinlocks will
 * be used to implement the 64-bit parts of the API.
 *
 * Implement _u64 atomics if and only if your platform can use them
 * efficiently (and obviously correctly).
 *
 * Use higher level functionality (lwlocks, spinlocks, heavyweight locks)
 * whenever possible. Writing correct code using these facilities is hard.
 *
 * For an introduction to using memory barriers within the PostgreSQL backend,
 * see src/backend/storage/lmgr/README.barrier
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/port/atomics.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef ATOMICS_H
#define ATOMICS_H
 
#ifdef FRONTEND
#error "atomics.h may not be included from frontend code"
#endif
 
#define INSIDE_ATOMICS_H
 
#include <limits.h>
 
/*
 * First a set of architecture specific files is included.
 *
 * These files can provide the full set of atomics or can do pretty much
 * nothing if all the compilers commonly used on these platforms provide
 * usable generics.
 *
 * Don't add an inline assembly of the actual atomic operations if all the
 * common implementations of your platform provide intrinsics. Intrinsics are
 * much easier to understand and potentially support more architectures.
 *
 * It will often make sense to define memory barrier semantics here, since
 * e.g. generic compiler intrinsics for x86 memory barriers can't know that
 * postgres doesn't need x86 read/write barriers do anything more than a
 * compiler barrier.
 *
 */
#if defined(__arm__) || defined(__arm) || defined(__aarch64__)
#include "port/atomics/arch-arm.h"
#elif defined(__i386__) || defined(__i386) || defined(__x86_64__)
#include "port/atomics/arch-x86.h"
#elif defined(__ppc__) || defined(__powerpc__) || defined(__ppc64__) || defined(__powerpc64__)
#include "port/atomics/arch-ppc.h"
#endif
 
/*
 * Compiler specific, but architecture independent implementations.
 *
 * Provide architecture independent implementations of the atomic
 * facilities. At the very least compiler barriers should be provided, but a
 * full implementation of
 * * pg_compiler_barrier(), pg_write_barrier(), pg_read_barrier()
 * * pg_atomic_compare_exchange_u32(), pg_atomic_fetch_add_u32()
 * using compiler intrinsics are a good idea.
 */
/*
 * gcc or compatible, including clang and icc.
 */
#if defined(__GNUC__) || defined(__INTEL_COMPILER)
#include "port/atomics/generic-gcc.h"
#elif defined(_MSC_VER)
#include "port/atomics/generic-msvc.h"
#else
/* Unknown compiler. */
#endif
 
/* Fail if we couldn't find implementations of required facilities. */
#if !defined(PG_HAVE_ATOMIC_U32_SUPPORT)
#error "could not find an implementation of pg_atomic_uint32"
#endif
#if !defined(pg_compiler_barrier_impl)
#error "could not find an implementation of pg_compiler_barrier"
#endif
#if !defined(pg_memory_barrier_impl)
#error "could not find an implementation of pg_memory_barrier_impl"
#endif
 
 
/*
 * Provide a spinlock-based implementation of the 64 bit variants, if
 * necessary.
 */
#include "port/atomics/fallback.h"
 
/*
 * Provide additional operations using supported infrastructure. These are
 * expected to be efficient if the underlying atomic operations are efficient.
 */
#include "port/atomics/generic.h"
 
 
/*
 * pg_compiler_barrier - prevent the compiler from moving code across
 *
 * A compiler barrier need not (and preferably should not) emit any actual
 * machine code, but must act as an optimization fence: the compiler must not
 * reorder loads or stores to main memory around the barrier.  However, the
 * CPU may still reorder loads or stores at runtime, if the architecture's
 * memory model permits this.
 */
#define pg_compiler_barrier()   pg_compiler_barrier_impl()
 
/*
 * pg_memory_barrier - prevent the CPU from reordering memory access
 *
 * A memory barrier must act as a compiler barrier, and in addition must
 * guarantee that all loads and stores issued prior to the barrier are
 * completed before any loads or stores issued after the barrier.  Unless
 * loads and stores are totally ordered (which is not the case on most
 * architectures) this requires issuing some sort of memory fencing
 * instruction.
 */
#define pg_memory_barrier() pg_memory_barrier_impl()
 
/*
 * pg_(read|write)_barrier - prevent the CPU from reordering memory access
 *
 * A read barrier must act as a compiler barrier, and in addition must
 * guarantee that any loads issued prior to the barrier are completed before
 * any loads issued after the barrier.  Similarly, a write barrier acts
 * as a compiler barrier, and also orders stores.  Read and write barriers
 * are thus weaker than a full memory barrier, but stronger than a compiler
 * barrier.  In practice, on machines with strong memory ordering, read and
 * write barriers may require nothing more than a compiler barrier.
 */
#define pg_read_barrier()   pg_read_barrier_impl()
#define pg_write_barrier()  pg_write_barrier_impl()
 
/*
 * Spinloop delay - Allow CPU to relax in busy loops
 */
#define pg_spin_delay() pg_spin_delay_impl()
 
/*
 * pg_atomic_init_flag - initialize atomic flag.
 *
 * No barrier semantics.
 */
static inline void
pg_atomic_init_flag(volatile pg_atomic_flag *ptr)
{
    pg_atomic_init_flag_impl(ptr);
}
 
/*
 * pg_atomic_test_set_flag - TAS()
 *
 * Returns true if the flag has successfully been set, false otherwise.
 *
 * Acquire (including read barrier) semantics.
 */
static inline bool
pg_atomic_test_set_flag(volatile pg_atomic_flag *ptr)
{
    return pg_atomic_test_set_flag_impl(ptr);
}
 
/*
 * pg_atomic_unlocked_test_flag - Check if the lock is free
 *
 * Returns true if the flag currently is not set, false otherwise.
 *
 * No barrier semantics.
 */
static inline bool
pg_atomic_unlocked_test_flag(volatile pg_atomic_flag *ptr)
{
    return pg_atomic_unlocked_test_flag_impl(ptr);
}
 
/*
 * pg_atomic_clear_flag - release lock set by TAS()
 *
 * Release (including write barrier) semantics.
 */
static inline void
pg_atomic_clear_flag(volatile pg_atomic_flag *ptr)
{
    pg_atomic_clear_flag_impl(ptr);
}
 
 
/*
 * pg_atomic_init_u32 - initialize atomic variable
 *
 * Has to be done before any concurrent usage..
 *
 * No barrier semantics.
 */
static inline void
pg_atomic_init_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
{
    AssertPointerAlignment(ptr, 4);
 
    pg_atomic_init_u32_impl(ptr, val);
}
 
/*
 * pg_atomic_read_u32 - unlocked read from atomic variable.
 *
 * The read is guaranteed to return a value as it has been written by this or
 * another process at some point in the past. There's however no cache
 * coherency interaction guaranteeing the value hasn't since been written to
 * again.
 *
 * No barrier semantics.
 */
static inline uint32
pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
{
    AssertPointerAlignment(ptr, 4);
    return pg_atomic_read_u32_impl(ptr);
}
 
/*
 * pg_atomic_read_membarrier_u32 - read with barrier semantics.
 *
 * This read is guaranteed to return the current value, provided that the value
 * is only ever updated via operations with barrier semantics, such as
 * pg_atomic_compare_exchange_u32() and pg_atomic_write_membarrier_u32().
 * While this may be less performant than pg_atomic_read_u32(), it may be
 * easier to reason about correctness with this function in less performance-
 * sensitive code.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_read_membarrier_u32(volatile pg_atomic_uint32 *ptr)
{
    AssertPointerAlignment(ptr, 4);
 
    return pg_atomic_read_membarrier_u32_impl(ptr);
}
 
/*
 * pg_atomic_write_u32 - write to atomic variable.
 *
 * The write is guaranteed to succeed as a whole, i.e. it's not possible to
 * observe a partial write for any reader.  Note that this correctly interacts
 * with pg_atomic_compare_exchange_u32, in contrast to
 * pg_atomic_unlocked_write_u32().
 *
 * No barrier semantics.
 */
static inline void
pg_atomic_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
{
    AssertPointerAlignment(ptr, 4);
 
    pg_atomic_write_u32_impl(ptr, val);
}
 
/*
 * pg_atomic_unlocked_write_u32 - unlocked write to atomic variable.
 *
 * Write to an atomic variable, without atomicity guarantees. I.e. it is not
 * guaranteed that a concurrent reader will not see a torn value, nor is this
 * guaranteed to correctly interact with concurrent read-modify-write
 * operations like pg_atomic_compare_exchange_u32.  This should only be used
 * in cases where minor performance regressions due to atomic operations are
 * unacceptable and where exclusive access is guaranteed via some external
 * means.
 *
 * No barrier semantics.
 */
static inline void
pg_atomic_unlocked_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
{
    AssertPointerAlignment(ptr, 4);
 
    pg_atomic_unlocked_write_u32_impl(ptr, val);
}
 
/*
 * pg_atomic_write_membarrier_u32 - write with barrier semantics.
 *
 * The write is guaranteed to succeed as a whole, i.e., it's not possible to
 * observe a partial write for any reader.  Note that this correctly interacts
 * with both pg_atomic_compare_exchange_u32() and
 * pg_atomic_read_membarrier_u32().  While this may be less performant than
 * pg_atomic_write_u32(), it may be easier to reason about correctness with
 * this function in less performance-sensitive code.
 *
 * Full barrier semantics.
 */
static inline void
pg_atomic_write_membarrier_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
{
    AssertPointerAlignment(ptr, 4);
 
    pg_atomic_write_membarrier_u32_impl(ptr, val);
}
 
/*
 * pg_atomic_exchange_u32 - exchange newval with current value
 *
 * Returns the old value of 'ptr' before the swap.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_exchange_u32(volatile pg_atomic_uint32 *ptr, uint32 newval)
{
    AssertPointerAlignment(ptr, 4);
 
    return pg_atomic_exchange_u32_impl(ptr, newval);
}
 
/*
 * pg_atomic_compare_exchange_u32 - CAS operation
 *
 * Atomically compare the current value of ptr with *expected and store newval
 * iff ptr and *expected have the same value. The current value of *ptr will
 * always be stored in *expected.
 *
 * Return true if values have been exchanged, false otherwise.
 *
 * Full barrier semantics.
 */
static inline bool
pg_atomic_compare_exchange_u32(volatile pg_atomic_uint32 *ptr,
                               uint32 *expected, uint32 newval)
{
    AssertPointerAlignment(ptr, 4);
    AssertPointerAlignment(expected, 4);
 
    return pg_atomic_compare_exchange_u32_impl(ptr, expected, newval);
}
 
/*
 * pg_atomic_fetch_add_u32 - atomically add to variable
 *
 * Returns the value of ptr before the arithmetic operation.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_fetch_add_u32(volatile pg_atomic_uint32 *ptr, int32 add_)
{
    AssertPointerAlignment(ptr, 4);
    return pg_atomic_fetch_add_u32_impl(ptr, add_);
}
 
/*
 * pg_atomic_fetch_sub_u32 - atomically subtract from variable
 *
 * Returns the value of ptr before the arithmetic operation. Note that sub_
 * may not be INT_MIN due to platform limitations.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_fetch_sub_u32(volatile pg_atomic_uint32 *ptr, int32 sub_)
{
    AssertPointerAlignment(ptr, 4);
    Assert(sub_ != INT_MIN);
    return pg_atomic_fetch_sub_u32_impl(ptr, sub_);
}
 
/*
 * pg_atomic_fetch_and_u32 - atomically bit-and and_ with variable
 *
 * Returns the value of ptr before the arithmetic operation.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_fetch_and_u32(volatile pg_atomic_uint32 *ptr, uint32 and_)
{
    AssertPointerAlignment(ptr, 4);
    return pg_atomic_fetch_and_u32_impl(ptr, and_);
}
 
/*
 * pg_atomic_fetch_or_u32 - atomically bit-or or_ with variable
 *
 * Returns the value of ptr before the arithmetic operation.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_fetch_or_u32(volatile pg_atomic_uint32 *ptr, uint32 or_)
{
    AssertPointerAlignment(ptr, 4);
    return pg_atomic_fetch_or_u32_impl(ptr, or_);
}
 
/*
 * pg_atomic_add_fetch_u32 - atomically add to variable
 *
 * Returns the value of ptr after the arithmetic operation.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_add_fetch_u32(volatile pg_atomic_uint32 *ptr, int32 add_)
{
    AssertPointerAlignment(ptr, 4);
    return pg_atomic_add_fetch_u32_impl(ptr, add_);
}
 
/*
 * pg_atomic_sub_fetch_u32 - atomically subtract from variable
 *
 * Returns the value of ptr after the arithmetic operation. Note that sub_ may
 * not be INT_MIN due to platform limitations.
 *
 * Full barrier semantics.
 */
static inline uint32
pg_atomic_sub_fetch_u32(volatile pg_atomic_uint32 *ptr, int32 sub_)
{
    AssertPointerAlignment(ptr, 4);
    Assert(sub_ != INT_MIN);
    return pg_atomic_sub_fetch_u32_impl(ptr, sub_);
}
 
/* ----
 * The 64 bit operations have the same semantics as their 32bit counterparts
 * if they are available. Check the corresponding 32bit function for
 * documentation.
 * ----
 */
static inline void
pg_atomic_init_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
{
    /*
     * Can't necessarily enforce alignment - and don't need it - when using
     * the spinlock based fallback implementation. Therefore only assert when
     * not using it.
     */
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    pg_atomic_init_u64_impl(ptr, val);
}
 
static inline uint64
pg_atomic_read_u64(volatile pg_atomic_uint64 *ptr)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_read_u64_impl(ptr);
}
 
static inline uint64
pg_atomic_read_membarrier_u64(volatile pg_atomic_uint64 *ptr)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_read_membarrier_u64_impl(ptr);
}
 
static inline void
pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    pg_atomic_write_u64_impl(ptr, val);
}
 
static inline void
pg_atomic_unlocked_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
 
    pg_atomic_unlocked_write_u64_impl(ptr, val);
}
 
static inline void
pg_atomic_write_membarrier_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    pg_atomic_write_membarrier_u64_impl(ptr, val);
}
 
static inline uint64
pg_atomic_exchange_u64(volatile pg_atomic_uint64 *ptr, uint64 newval)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_exchange_u64_impl(ptr, newval);
}
 
static inline bool
pg_atomic_compare_exchange_u64(volatile pg_atomic_uint64 *ptr,
                               uint64 *expected, uint64 newval)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_compare_exchange_u64_impl(ptr, expected, newval);
}
 
static inline uint64
pg_atomic_fetch_add_u64(volatile pg_atomic_uint64 *ptr, int64 add_)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_fetch_add_u64_impl(ptr, add_);
}
 
static inline uint64
pg_atomic_fetch_sub_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    Assert(sub_ != PG_INT64_MIN);
    return pg_atomic_fetch_sub_u64_impl(ptr, sub_);
}
 
static inline uint64
pg_atomic_fetch_and_u64(volatile pg_atomic_uint64 *ptr, uint64 and_)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_fetch_and_u64_impl(ptr, and_);
}
 
static inline uint64
pg_atomic_fetch_or_u64(volatile pg_atomic_uint64 *ptr, uint64 or_)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_fetch_or_u64_impl(ptr, or_);
}
 
static inline uint64
pg_atomic_add_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 add_)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    return pg_atomic_add_fetch_u64_impl(ptr, add_);
}
 
static inline uint64
pg_atomic_sub_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
{
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
    Assert(sub_ != PG_INT64_MIN);
    return pg_atomic_sub_fetch_u64_impl(ptr, sub_);
}
 
/*
 * Monotonically advance the given variable using only atomic operations until
 * it's at least the target value.  Returns the latest value observed, which
 * may or may not be the target value.
 *
 * Full barrier semantics (even when value is unchanged).
 */
static inline uint64
pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target)
{
    uint64      currval;
 
#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
    AssertPointerAlignment(ptr, 8);
#endif
 
    currval = pg_atomic_read_u64_impl(ptr);
    if (currval >= target)
    {
        pg_memory_barrier();
        return currval;
    }
 
    while (currval < target)
    {
        if (pg_atomic_compare_exchange_u64(ptr, &currval, target))
            return target;
    }
 
    return currval;
}
 
#undef INSIDE_ATOMICS_H
 
#endif                          /* ATOMICS_H */