Remove bogus assertion in pg_atomic_monotonic_advance_u64

This code wanted to ensure that the 'exchange' variable passed to
pg_atomic_compare_exchange_u64 has correct alignment, but apparently
platforms don't actually require anything that doesn't come naturally.

While messing with pg_atomic_monotonic_advance_u64: instead of using
Max() to determine the value to return, just use
pg_atomic_compare_exchange_u64()'s return value to decide; also, use
pg_atomic_compare_exchange_u64 instead of the _impl version; also remove
the unnecessary underscore at the end of variable name "target".

Backpatch to 17, where this code was introduced by commit bf3ff7bf83bc.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/36796438-a718-cf9b-2071-b2c1b947c1b5@gmail.com

src/include/port/atomics.h

@@ -509,7 +509,6 @@ pg_atomic_compare_exchange_u64(volatile pg_atomic_uint64 *ptr,
 {
 #ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);
-	AssertPointerAlignment(expected, 8);
 #endif
 	return pg_atomic_compare_exchange_u64_impl(ptr, expected, newval);
 }
@@ -578,7 +577,7 @@ pg_atomic_sub_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
  * Full barrier semantics (even when value is unchanged).
  */
 static inline uint64
-pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target_)
+pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target)
 {
 	uint64		currval;
 
@@ -587,23 +586,19 @@ pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target_)
 #endif
 
 	currval = pg_atomic_read_u64_impl(ptr);
-	if (currval >= target_)
+	if (currval >= target)
 	{
 		pg_memory_barrier();
 		return currval;
 	}
 
-#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
+	while (currval < target)
-	AssertPointerAlignment(&currval, 8);
-#endif
-
-	while (currval < target_)
 	{
-		if (pg_atomic_compare_exchange_u64_impl(ptr, &currval, target_))
+		if (pg_atomic_compare_exchange_u64(ptr, &currval, target))
-			break;
+			return target;
 	}
 
-	return Max(target_, currval);
+	return currval;
 }
 
 #undef INSIDE_ATOMICS_H

src/include/port/atomics/arch-ppc.h

@@ -173,6 +173,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 	uint32 condition_register;
 	bool ret;
 
+	AssertPointerAlignment(expected, 8);
+
 	/* Like u32, but s/lwarx/ldarx/; s/stwcx/stdcx/; s/cmpw/cmpd/ */
 #ifdef HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
 	if (__builtin_constant_p(*expected) &&

src/include/port/atomics/arch-x86.h

@@ -207,6 +207,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 {
 	char	ret;
 
+	AssertPointerAlignment(expected, 8);
+
 	/*
 	 * Perform cmpxchg and use the zero flag which it implicitly sets when
 	 * equal to measure the success.

src/include/port/atomics/generic-gcc.h

@@ -240,6 +240,7 @@ static inline bool
 pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 									uint64 *expected, uint64 newval)
 {
+	AssertPointerAlignment(expected, 8);
 	return __atomic_compare_exchange_n(&ptr->value, expected, newval, false,
 									   __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
 }
@@ -253,6 +254,8 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 {
 	bool	ret;
 	uint64	current;
+
+	AssertPointerAlignment(expected, 8);
 	current = __sync_val_compare_and_swap(&ptr->value, *expected, newval);
 	ret = current == *expected;
 	*expected = current;

src/include/port/atomics/generic-sunpro.h

@@ -102,6 +102,7 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 	bool	ret;
 	uint64	current;
 
+	AssertPointerAlignment(expected, 8);
 	current = atomic_cas_64(&ptr->value, *expected, newval);
 	ret = current == *expected;
 	*expected = current;