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diff --git a/src/include/port/atomics/fallback.h b/src/include/port/atomics/fallback.h
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+++ b/src/include/port/atomics/fallback.h
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+/*-------------------------------------------------------------------------
+ *
+ * fallback.h
+ *    Fallback for platforms without spinlock and/or atomics support. Slower
+ *    than native atomics support, but not unusably slow.
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/port/atomics/fallback.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+/* intentionally no include guards, should only be included by atomics.h */
+#ifndef INSIDE_ATOMICS_H
+#	error "should be included via atomics.h"
+#endif
+
+#ifndef pg_memory_barrier_impl
+/*
+ * If we have no memory barrier implementation for this architecture, we
+ * fall back to acquiring and releasing a spinlock.  This might, in turn,
+ * fall back to the semaphore-based spinlock implementation, which will be
+ * amazingly slow.
+ *
+ * It's not self-evident that every possible legal implementation of a
+ * spinlock acquire-and-release would be equivalent to a full memory barrier.
+ * For example, I'm not sure that Itanium's acq and rel add up to a full
+ * fence.  But all of our actual implementations seem OK in this regard.
+ */
+#define PG_HAVE_MEMORY_BARRIER_EMULATION
+
+extern void pg_spinlock_barrier(void);
+#define pg_memory_barrier_impl pg_spinlock_barrier
+#endif
+
+#ifndef pg_compiler_barrier_impl
+/*
+ * If the compiler/arch combination does not provide compiler barriers,
+ * provide a fallback.  The fallback simply consists of a function call into
+ * an externally defined function.  That should guarantee compiler barrier
+ * semantics except for compilers that do inter translation unit/global
+ * optimization - those better provide an actual compiler barrier.
+ *
+ * A native compiler barrier for sure is a lot faster than this...
+ */
+#define PG_HAVE_COMPILER_BARRIER_EMULATION
+extern void pg_extern_compiler_barrier(void);
+#define pg_compiler_barrier_impl pg_extern_compiler_barrier
+#endif
+
+
+/*
+ * If we have atomics implementation for this platform, fall back to providing
+ * the atomics API using a spinlock to protect the internal state. Possibly
+ * the spinlock implementation uses semaphores internally...
+ *
+ * We have to be a bit careful here, as it's not guaranteed that atomic
+ * variables are mapped to the same address in every process (e.g. dynamic
+ * shared memory segments). We can't just hash the address and use that to map
+ * to a spinlock. Instead assign a spinlock on initialization of the atomic
+ * variable.
+ */
+#if !defined(PG_HAVE_ATOMIC_FLAG_SUPPORT) && !defined(PG_HAVE_ATOMIC_U32_SUPPORT)
+
+#define PG_HAVE_ATOMIC_FLAG_SIMULATION
+#define PG_HAVE_ATOMIC_FLAG_SUPPORT
+
+typedef struct pg_atomic_flag
+{
+	/*
+	 * To avoid circular includes we can't use s_lock as a type here. Instead
+	 * just reserve enough space for all spinlock types. Some platforms would
+	 * be content with just one byte instead of 4, but that's not too much
+	 * waste.
+	 */
+#if defined(__hppa) || defined(__hppa__)	/* HP PA-RISC, GCC and HP compilers */
+	int			sema[4];
+#else
+	int			sema;
+#endif
+	volatile bool value;
+} pg_atomic_flag;
+
+#endif /* PG_HAVE_ATOMIC_FLAG_SUPPORT */
+
+#if !defined(PG_HAVE_ATOMIC_U32_SUPPORT)
+
+#define PG_HAVE_ATOMIC_U32_SIMULATION
+
+#define PG_HAVE_ATOMIC_U32_SUPPORT
+typedef struct pg_atomic_uint32
+{
+	/* Check pg_atomic_flag's definition above for an explanation */
+#if defined(__hppa) || defined(__hppa__)	/* HP PA-RISC, GCC and HP compilers */
+	int			sema[4];
+#else
+	int			sema;
+#endif
+	volatile uint32 value;
+} pg_atomic_uint32;
+
+#endif /* PG_HAVE_ATOMIC_U32_SUPPORT */
+
+#if !defined(PG_HAVE_ATOMIC_U64_SUPPORT)
+
+#define PG_HAVE_ATOMIC_U64_SIMULATION
+
+#define PG_HAVE_ATOMIC_U64_SUPPORT
+typedef struct pg_atomic_uint64
+{
+	/* Check pg_atomic_flag's definition above for an explanation */
+#if defined(__hppa) || defined(__hppa__)	/* HP PA-RISC, GCC and HP compilers */
+	int			sema[4];
+#else
+	int			sema;
+#endif
+	volatile uint64 value;
+} pg_atomic_uint64;
+
+#endif /* PG_HAVE_ATOMIC_U64_SUPPORT */
+
+#ifdef PG_HAVE_ATOMIC_FLAG_SIMULATION
+
+#define PG_HAVE_ATOMIC_INIT_FLAG
+extern void pg_atomic_init_flag_impl(volatile pg_atomic_flag *ptr);
+
+#define PG_HAVE_ATOMIC_TEST_SET_FLAG
+extern bool pg_atomic_test_set_flag_impl(volatile pg_atomic_flag *ptr);
+
+#define PG_HAVE_ATOMIC_CLEAR_FLAG
+extern void pg_atomic_clear_flag_impl(volatile pg_atomic_flag *ptr);
+
+#define PG_HAVE_ATOMIC_UNLOCKED_TEST_FLAG
+extern bool pg_atomic_unlocked_test_flag_impl(volatile pg_atomic_flag *ptr);
+
+#endif /* PG_HAVE_ATOMIC_FLAG_SIMULATION */
+
+#ifdef PG_HAVE_ATOMIC_U32_SIMULATION
+
+#define PG_HAVE_ATOMIC_INIT_U32
+extern void pg_atomic_init_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val_);
+
+#define PG_HAVE_ATOMIC_WRITE_U32
+extern void pg_atomic_write_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val);
+
+#define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
+extern bool pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
+												uint32 *expected, uint32 newval);
+
+#define PG_HAVE_ATOMIC_FETCH_ADD_U32
+extern uint32 pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_);
+
+#endif /* PG_HAVE_ATOMIC_U32_SIMULATION */
+
+
+#ifdef PG_HAVE_ATOMIC_U64_SIMULATION
+
+#define PG_HAVE_ATOMIC_INIT_U64
+extern void pg_atomic_init_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 val_);
+
+#define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U64
+extern bool pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
+												uint64 *expected, uint64 newval);
+
+#define PG_HAVE_ATOMIC_FETCH_ADD_U64
+extern uint64 pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_);
+
+#endif /* PG_HAVE_ATOMIC_U64_SIMULATION */