diff --git a/src/include/port/atomics/arch-x86.h b/src/include/port/atomics/arch-x86.h
index fb5623d..168a49c 100644
--- a/src/include/port/atomics/arch-x86.h
+++ b/src/include/port/atomics/arch-x86.h
@@ -73,6 +73,7 @@ typedef struct pg_atomic_uint32
 #define PG_HAVE_ATOMIC_U64_SUPPORT
 typedef struct pg_atomic_uint64
 {
+	/* alignment guaranteed due to being on a 64bit platform */
 	volatile uint64 value;
 } pg_atomic_uint64;
 #endif
diff --git a/src/include/port/atomics/generic-acc.h b/src/include/port/atomics/generic-acc.h
index e16e282..c5639aa 100644
--- a/src/include/port/atomics/generic-acc.h
+++ b/src/include/port/atomics/generic-acc.h
@@ -40,6 +40,12 @@ typedef struct pg_atomic_uint32
 #define PG_HAVE_ATOMIC_U64_SUPPORT
 typedef struct pg_atomic_uint64
 {
+	/*
+	 * Alignment is guaranteed to be 64bit. Search for "Well-behaved
+	 * application restrictions" => "Data alignment and data sharing" on HP's
+	 * website. Unfortunately the URL doesn't seem to stable enough to
+	 * include.
+	 */
 	volatile uint64 value;
 } pg_atomic_uint64;
 
diff --git a/src/include/port/atomics/generic-gcc.h b/src/include/port/atomics/generic-gcc.h
index f19ad34..fea1cb5 100644
--- a/src/include/port/atomics/generic-gcc.h
+++ b/src/include/port/atomics/generic-gcc.h
@@ -98,7 +98,7 @@ typedef struct pg_atomic_uint32
 
 typedef struct pg_atomic_uint64
 {
-	volatile uint64 value;
+	volatile uint64 value __attribute__((aligned(8)));
 } pg_atomic_uint64;
 
 #endif /* defined(HAVE_GCC__ATOMIC_INT64_CAS) || defined(HAVE_GCC__SYNC_INT64_CAS) */
diff --git a/src/include/port/atomics/generic-msvc.h b/src/include/port/atomics/generic-msvc.h
index 1d763ab..d259d6f 100644
--- a/src/include/port/atomics/generic-msvc.h
+++ b/src/include/port/atomics/generic-msvc.h
@@ -41,7 +41,7 @@ typedef struct pg_atomic_uint32
 } pg_atomic_uint32;
 
 #define PG_HAVE_ATOMIC_U64_SUPPORT
-typedef struct pg_atomic_uint64
+typedef struct __declspec(align(8)) pg_atomic_uint64
 {
 	volatile uint64 value;
 } pg_atomic_uint64;
diff --git a/src/include/port/atomics/generic-sunpro.h b/src/include/port/atomics/generic-sunpro.h
index b756fb9..7a3028e 100644
--- a/src/include/port/atomics/generic-sunpro.h
+++ b/src/include/port/atomics/generic-sunpro.h
@@ -55,7 +55,13 @@ typedef struct pg_atomic_uint32
 #define PG_HAVE_ATOMIC_U64_SUPPORT
 typedef struct pg_atomic_uint64
 {
-	volatile uint64 value;
+	/*
+	 * Syntax to enforce variable alignment should be supported by versions
+	 * supporting atomic.h, but it's hard to find accurate documentation. If
+	 * it proves to be a problem, we'll have to add more version checks for 64
+	 * bit support.
+	 */
+	volatile uint64 value __attribute__((__aligned__(8)));
 } pg_atomic_uint64;
 
 #endif /* HAVE_ATOMIC_H */
diff --git a/src/include/port/atomics/generic-xlc.h b/src/include/port/atomics/generic-xlc.h
index 92bba6f..7a4c12a 100644
--- a/src/include/port/atomics/generic-xlc.h
+++ b/src/include/port/atomics/generic-xlc.h
@@ -32,7 +32,7 @@ typedef struct pg_atomic_uint32
 #define PG_HAVE_ATOMIC_U64_SUPPORT
 typedef struct pg_atomic_uint64
 {
-	volatile uint64 value;
+	volatile uint64 value __attribute__((__aligned__(8)));
 } pg_atomic_uint64;
 
 #endif /* __64BIT__ */