Feedback-ID: ia2694551:Fastmail
Date: Mon, 1 Jul 2024 16:54:51 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Lakhin <exclusion@gmail.com>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
 Andres Freund <andres@anarazel.de>,
	Jeff Davis <pgsql@j-davis.com>, pgsql-hackers@lists.postgresql.org,
	Jaime Casanova <jcasanov@systemguards.com.ec>
Subject: Re: LogwrtResult contended spinlock
Message-ID: <202407011454.hni6ldpc3hoa@alvherre.pgsql>
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Maybe we can do something like this,

diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index 78987f3154..f6fa90bad8 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -580,7 +580,20 @@ pg_atomic_sub_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
 static inline uint64
 pg_atomic_monotonic_advance_u64(volatile pg_atomic_uint64 *ptr, uint64 target_)
 {
+	/*
+	 * When using actual (not simulated) atomics, the target variable for
+	 * pg_atomic_compare_exchange_u64 must have suitable alignment, which
+	 * is acquired naturally on most platforms, but not on 32-bit ones;
+	 * persuade the compiler in that case, but fail if we
+	 * cannot.
+	 */
+#if MAXIMUM_ALIGNOF >= 8
 	uint64		currval;
+#elif defined(pg_attribute_aligned) && !defined(PG_HAVE_ATOMIC_U64_SIMULATION)
+	pg_attribute_aligned(8)	uint64		currval;
+#else
+#error "Must have pg_attribute aligned or simulated atomics"
+#endif
 
 #ifndef PG_HAVE_ATOMIC_U64_SIMULATION
 	AssertPointerAlignment(ptr, 8);


-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Just treat us the way you want to be treated + some extra allowance
 for ignorance."                                    (Michael Brusser)