Hello Thomas,
23.10.2025 00:51, Thomas Munro wrote:
On a more practical note, Alexander, does this work?
#if !defined(pg_read_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT32_CAS) /* acquire semantics include read barrier semantics */
-# define pg_read_barrier_impl()
__atomic_thread_fence(__ATOMIC_ACQUIRE)
+# define pg_read_barrier_impl() do {
__atomic_signal_fence(__ATOMIC_ACQUIRE);
__atomic_thread_fence(__ATOMIC_ACQUIRE); } while (0) #endif
Someone might want to write an arch-riscv.h that spits out the asm
instead (should be just FENCE R, R and FENCE W, W as far as I can see,
which might be slightly better than what acquire/release generate, on
paper at least), but apparently this problem is wider than RISCV so we
need a general solution anyway.
Thank you for working on this!
Unfortunately, this change doesn't lead to change in disassembly of
pgaio_io_wait(), produced with clang-19 -O1. That is, I'm getting the same
disassembly as the one I sent you before (error/pgaio_io_wait.asm).
I tried:
#if !defined(pg_read_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT32_CAS)
/* acquire semantics include read barrier semantics */
-# define pg_read_barrier_impl() __atomic_thread_fence(__ATOMIC_ACQUIRE)
+# define pg_read_barrier_impl() do { } while (0)
#endif
to make sure that the change of pg_read_barrier_impl() affects
pgaio_io_wait() and really saw a difference in the disassembly.
I haven't run tests with the modified version yet, going to test later
just to be sure, but I suppose we want to see the binary code changed.
Best regards,
Alexander
