Here is another patch set to sprinkle some C11 features around the
code. My aim is to make a little bit of use of several C11 features
as examples and encouragement for future code, and to test compilers.
Here, I'm proposing to make some use of the alignas specifier. This
takes the place of compiler extensions such as
__attribute__((aligned(a))) and __declspec(align(a)), packaged up as
pg_attribute_aligned(a), which are used in a variety of places. Also,
we can simplify some places where unions are used to encourage
alignment, and remove a few workaround for lack of alignment attribute
support.
Some detail notes:
- Technically, compilers are only required to support alignas up to
(handwaving over some terminology) the largest alignment of a built-in
type, so maybe 8 or 16. Support for larger alignments like
alignas(PG_CACHE_LINE_SIZE) is implementation-defined. I have split up
my patches so that fundamental and extended alignments are in separate
patches, so this could be eased into, but I'm expecting that all
compilers in practical use support alignments up to PG_IO_ALIGN_SIZE.
(For MSVC, 4096 appears to be the actual limit by default, per [0], but
this is independent of using alignas or __declspec. I haven't found any
explicit documentation for clang or gcc.)
[0]:
https://learn.microsoft.com/en-us/cpp/build/reference/align-section-alignment?view=msvc-170
- You cannot use alignas on a typedef. So some uses of the attribute
pg_attribute_aligned() like for PgAioUringContext or the whole int128
business cannot be converted directly. The solution for cases like
PgAioUringContext could be to move the alignas into the struct, but I
haven't studied this code closely enough, so I'm leaving it. For
int128, there is no straightforward solution, so I'm also leaving that
as is.
(The reason for this restriction is that typedefs are supposed to be
type aliases that are interchangeable. But if you have two otherwise
compatible typedefs with different alignments, this kind of violates the
C type system and the compiler has to do some nonstandard magic to
handle this (or fail to, see "checking for __int128 alignment bug").)
- You cannot use alignas to underalign a type. So again, int128 cannot
be handled by this.
For at least these reasons, I'm leaving pg_attribute_aligned() and some
of its more tricky uses in place and unchanged.
From 228f3973b64c38c7e715eba5cfa069c7c5b4501b Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <[email protected]>
Date: Wed, 12 Nov 2025 11:55:29 +0100
Subject: [PATCH 1/4] Add <stdalign.h> to c.h
This allows using the C11 constructs alignas and alignof (not done in
this patch).
---
src/include/c.h | 1 +
1 file changed, 1 insertion(+)
diff --git a/src/include/c.h b/src/include/c.h
index 757dfff4782..b3bf1b412ee 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -60,6 +60,7 @@
/* System header files that should be available everywhere in Postgres */
#include <inttypes.h>
+#include <stdalign.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
base-commit: 2ddc8d9e9baae28ba63a9f1c85f0a36deb9112aa
--
2.51.0
From e31938294770cc997a309c97c38424a8fe31834c Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <[email protected]>
Date: Wed, 12 Nov 2025 11:55:29 +0100
Subject: [PATCH 2/4] C11 alignas instead of unions
This changes a few union members that only existed to ensure
alignments and replaces them with the C11 alignas specifier.
This change only uses fundamental alignments (meaning approximately
alignments of basic types), which all C11 compilers must support.
There are opportunities for similar changes using extended alignments,
for example in PGIOAlignedBlock, but these are not necessarily
supported by all compilers, so they are kept as a separate change.
---
src/backend/access/common/toast_internals.c | 4 +---
src/backend/commands/async.c | 12 ++++--------
src/backend/storage/large_object/inv_api.c | 8 ++------
src/include/c.h | 10 +++-------
4 files changed, 10 insertions(+), 24 deletions(-)
diff --git a/src/backend/access/common/toast_internals.c
b/src/backend/access/common/toast_internals.c
index 81dbd67c725..63b848473f8 100644
--- a/src/backend/access/common/toast_internals.c
+++ b/src/backend/access/common/toast_internals.c
@@ -287,11 +287,9 @@ toast_save_datum(Relation rel, Datum value,
bool t_isnull[3] = {0};
union
{
- struct varlena hdr;
+ alignas(int32) struct varlena hdr;
/* this is to make the union big enough for a chunk: */
char data[TOAST_MAX_CHUNK_SIZE + VARHDRSZ];
- /* ensure union is aligned well enough: */
- int32 align_it;
} chunk_data;
int32 chunk_size;
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 4bd37d5beb5..4bdf21c319c 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -1854,12 +1854,8 @@ asyncQueueReadAllNotifications(void)
QueuePosition head;
Snapshot snapshot;
- /* page_buffer must be adequately aligned, so use a union */
- union
- {
- char buf[QUEUE_PAGESIZE];
- AsyncQueueEntry align;
- } page_buffer;
+ /* page_buffer must be adequately aligned */
+ alignas(AsyncQueueEntry) char page_buffer[QUEUE_PAGESIZE];
/* Fetch current state */
LWLockAcquire(NotifyQueueLock, LW_SHARED);
@@ -1957,7 +1953,7 @@ asyncQueueReadAllNotifications(void)
/* fetch all the rest of the page */
copysize = QUEUE_PAGESIZE - curoffset;
}
- memcpy(page_buffer.buf + curoffset,
+ memcpy(page_buffer + curoffset,
NotifyCtl->shared->page_buffer[slotno] +
curoffset,
copysize);
/* Release lock that we got from
SimpleLruReadPage_ReadOnly() */
@@ -1979,7 +1975,7 @@ asyncQueueReadAllNotifications(void)
* while sending the notifications to the frontend.
*/
reachedStop = asyncQueueProcessPageEntries(&pos, head,
-
page_buffer.buf,
+
page_buffer,
snapshot);
} while (!reachedStop);
}
diff --git a/src/backend/storage/large_object/inv_api.c
b/src/backend/storage/large_object/inv_api.c
index f6d2f9dba13..2bd872d6581 100644
--- a/src/backend/storage/large_object/inv_api.c
+++ b/src/backend/storage/large_object/inv_api.c
@@ -556,11 +556,9 @@ inv_write(LargeObjectDesc *obj_desc, const char *buf, int
nbytes)
bool pfreeit;
union
{
- bytea hdr;
+ alignas(int32) bytea hdr;
/* this is to make the union big enough for a LO data chunk: */
char data[LOBLKSIZE + VARHDRSZ];
- /* ensure union is aligned well enough: */
- int32 align_it;
} workbuf = {0};
char *workb = VARDATA(&workbuf.hdr);
HeapTuple newtup;
@@ -747,11 +745,9 @@ inv_truncate(LargeObjectDesc *obj_desc, int64 len)
Form_pg_largeobject olddata;
union
{
- bytea hdr;
+ alignas(int32) bytea hdr;
/* this is to make the union big enough for a LO data chunk: */
char data[LOBLKSIZE + VARHDRSZ];
- /* ensure union is aligned well enough: */
- int32 align_it;
} workbuf = {0};
char *workb = VARDATA(&workbuf.hdr);
HeapTuple newtup;
diff --git a/src/include/c.h b/src/include/c.h
index b3bf1b412ee..250d9c1d40b 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -1117,15 +1117,11 @@ pg_noreturn extern void ExceptionalCondition(const char
*conditionName,
* Use this, not "char buf[BLCKSZ]", to declare a field or local variable
* holding a page buffer, if that page might be accessed as a page. Otherwise
* the variable might be under-aligned, causing problems on alignment-picky
- * hardware. We include both "double" and "int64" in the union to ensure that
- * the compiler knows the value must be MAXALIGN'ed (cf. configure's
- * computation of MAXIMUM_ALIGNOF).
+ * hardware.
*/
-typedef union PGAlignedBlock
+typedef struct PGAlignedBlock
{
- char data[BLCKSZ];
- double force_align_d;
- int64 force_align_i64;
+ alignas(MAXIMUM_ALIGNOF) char data[BLCKSZ];
} PGAlignedBlock;
/*
--
2.51.0
From 00afcc6891b29c7c91a4aec9e414773bbf9b2468 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <[email protected]>
Date: Wed, 12 Nov 2025 11:55:29 +0100
Subject: [PATCH 3/4] Use C11 alignas in pg_atomic_uint64 definitions
XXX This is just separate from the previous patch because it's harder
to test.
---
src/include/port/atomics/arch-ppc.h | 2 +-
src/include/port/atomics/generic-gcc.h | 2 +-
src/include/port/atomics/generic-msvc.h | 4 ++--
3 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/src/include/port/atomics/arch-ppc.h
b/src/include/port/atomics/arch-ppc.h
index b93f6766d29..2b3c3283469 100644
--- a/src/include/port/atomics/arch-ppc.h
+++ b/src/include/port/atomics/arch-ppc.h
@@ -36,7 +36,7 @@ typedef struct pg_atomic_uint32
#define PG_HAVE_ATOMIC_U64_SUPPORT
typedef struct pg_atomic_uint64
{
- volatile uint64 value pg_attribute_aligned(8);
+ alignas(8) volatile uint64 value;
} pg_atomic_uint64;
#endif
diff --git a/src/include/port/atomics/generic-gcc.h
b/src/include/port/atomics/generic-gcc.h
index a0751f2286a..18ea8787199 100644
--- a/src/include/port/atomics/generic-gcc.h
+++ b/src/include/port/atomics/generic-gcc.h
@@ -103,7 +103,7 @@ typedef struct pg_atomic_uint32
typedef struct pg_atomic_uint64
{
- volatile uint64 value pg_attribute_aligned(8);
+ alignas(8) volatile uint64 value;
} 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 a6ea5f1c2e7..3d3c5363446 100644
--- a/src/include/port/atomics/generic-msvc.h
+++ b/src/include/port/atomics/generic-msvc.h
@@ -37,9 +37,9 @@ typedef struct pg_atomic_uint32
} pg_atomic_uint32;
#define PG_HAVE_ATOMIC_U64_SUPPORT
-typedef struct pg_attribute_aligned(8) pg_atomic_uint64
+typedef struct pg_atomic_uint64
{
- volatile uint64 value;
+ alignas(8) volatile uint64 value;
} pg_atomic_uint64;
--
2.51.0
From 0093ad4b8981b9b225fb517b1f9d9a35bc76b588 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <[email protected]>
Date: Wed, 12 Nov 2025 11:55:29 +0100
Subject: [PATCH 4/4] C11 alignas instead of unions -- extended alignments
This replaces some uses of pg_attribute_aligned() with the standard
alignas() for cases where extended alignment (larger than max_align_t)
is required.
This patch stipulates that all supported compilers must support
alignments up to PG_IO_ALIGN_SIZE, but that seems pretty likely.
We can then also desupport the case where direct I/O is disabled
because pg_attribute_aligned is not supported.
---
src/include/c.h | 18 ++++--------------
src/include/storage/fd.h | 5 ++---
2 files changed, 6 insertions(+), 17 deletions(-)
diff --git a/src/include/c.h b/src/include/c.h
index 250d9c1d40b..61af8487a03 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -1132,25 +1132,15 @@ typedef struct PGAlignedBlock
* for I/O in general, but may be strictly required on some platforms when
* using direct I/O.
*/
-typedef union PGIOAlignedBlock
+typedef struct PGIOAlignedBlock
{
-#ifdef pg_attribute_aligned
- pg_attribute_aligned(PG_IO_ALIGN_SIZE)
-#endif
- char data[BLCKSZ];
- double force_align_d;
- int64 force_align_i64;
+ alignas(PG_IO_ALIGN_SIZE) char data[BLCKSZ];
} PGIOAlignedBlock;
/* Same, but for an XLOG_BLCKSZ-sized buffer */
-typedef union PGAlignedXLogBlock
+typedef struct PGAlignedXLogBlock
{
-#ifdef pg_attribute_aligned
- pg_attribute_aligned(PG_IO_ALIGN_SIZE)
-#endif
- char data[XLOG_BLCKSZ];
- double force_align_d;
- int64 force_align_i64;
+ alignas(PG_IO_ALIGN_SIZE) char data[XLOG_BLCKSZ];
} PGAlignedXLogBlock;
/* msb for char */
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index b77d8e5e30e..2e2091a9159 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -85,10 +85,9 @@ extern PGDLLIMPORT int max_safe_fds;
* to the appropriate Windows flag in src/port/open.c. We simulate it with
* fcntl(F_NOCACHE) on macOS inside fd.c's open() wrapper. We use the name
* PG_O_DIRECT rather than defining O_DIRECT in that case (probably not a good
- * idea on a Unix). We can only use it if the compiler will correctly align
- * PGIOAlignedBlock for us, though.
+ * idea on a Unix).
*/
-#if defined(O_DIRECT) && defined(pg_attribute_aligned)
+#if defined(O_DIRECT)
#define PG_O_DIRECT O_DIRECT
#elif defined(F_NOCACHE)
#define PG_O_DIRECT 0x80000000
--
2.51.0
