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C11 alignas instead of unions -- extended alignments

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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.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/46f05236-d4d4-4b4e-84d4-faa500f14691%40eisentraut.org
This commit is contained in:
Peter Eisentraut
2025-11-24 07:36:49 +01:00
parent 4b203d499c
commit d4c0f91f7d
2 changed files with 6 additions and 17 deletions
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18
src/include/c.h
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@@ -1132,25 +1132,15 @@ typedef struct PGAlignedBlock
* for I/O in general, but may be strictly required on some platforms when * for I/O in general, but may be strictly required on some platforms when
* using direct I/O. * using direct I/O.
*/ */
typedef union PGIOAlignedBlock typedef struct PGIOAlignedBlock
{ {
#ifdef pg_attribute_aligned alignas(PG_IO_ALIGN_SIZE) char data[BLCKSZ];
pg_attribute_aligned(PG_IO_ALIGN_SIZE)
#endif
char data[BLCKSZ];
double force_align_d;
int64 force_align_i64;
} PGIOAlignedBlock; } PGIOAlignedBlock;
/* Same, but for an XLOG_BLCKSZ-sized buffer */ /* Same, but for an XLOG_BLCKSZ-sized buffer */
typedef union PGAlignedXLogBlock typedef struct PGAlignedXLogBlock
{ {
#ifdef pg_attribute_aligned alignas(PG_IO_ALIGN_SIZE) char data[XLOG_BLCKSZ];
pg_attribute_aligned(PG_IO_ALIGN_SIZE)
#endif
char data[XLOG_BLCKSZ];
double force_align_d;
int64 force_align_i64;
} PGAlignedXLogBlock; } PGAlignedXLogBlock;
/* msb for char */ /* msb for char */

5
src/include/storage/fd.h
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@@ -85,10 +85,9 @@ extern PGDLLIMPORT int max_safe_fds;
* to the appropriate Windows flag in src/port/open.c. We simulate it with * 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 * 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 * 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 * idea on a Unix).
* PGIOAlignedBlock for us, though.
*/ */
#if defined(O_DIRECT) && defined(pg_attribute_aligned) #if defined(O_DIRECT)
#define PG_O_DIRECT O_DIRECT #define PG_O_DIRECT O_DIRECT
#elif defined(F_NOCACHE) #elif defined(F_NOCACHE)
#define PG_O_DIRECT 0x80000000 #define PG_O_DIRECT 0x80000000