Declare PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY for aarch64

From b61b065acba570f1f935ff6ca17dc687c45db2f2 Mon Sep 17 00:00:00 2001
From: Yura Sokolov <y.sokolov@postgrespro.ru>
Date: Wed, 16 Mar 2022 15:12:07 +0300
Subject: [PATCH v0] Declare aarch64 has single copy atomicity for 8 byte
 values.

Architecture Reference Manual for ARMv8 B2.2.1 [1]

For explicit memory effects generated from an Exception level the
following rules apply:
- A read that is generated by a load instruction that loads a single
general-purpose register and is aligned to the size of the read in the
instruction is single-copy atomic.
- A write that is generated by a store instruction that stores a single
general-purpose register and is aligned to the size of the write in the
instruction is single-copy atomic.

[1] https://documentation-service.arm.com/static/61fbe8f4fa8173727a1b734e
https://developer.arm.com/documentation/ddi0487/latest
---
 src/include/port/atomics/arch-arm.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/include/port/atomics/arch-arm.h b/src/include/port/atomics/arch-arm.h
index 2083e3230db..9fe8f1b95f7 100644
--- a/src/include/port/atomics/arch-arm.h
+++ b/src/include/port/atomics/arch-arm.h
@@ -23,4 +23,10 @@
  */
 #if !defined(__aarch64__) && !defined(__aarch64)
 #define PG_DISABLE_64_BIT_ATOMICS
+#else
+/*
+ * Architecture Reference Manual for ARMv8 states aligned read/write to/from
+ * general purpose register is atomic.
+ */
+#define PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY
 #endif  /* __aarch64__ || __aarch64 */
-- 
2.35.1