Merge copies of converting an XID to a FullTransactionId.

Assume twophase.c is the performance-sensitive caller, and preserve its
choice of unlikely() branch hint.  Add some retrospective rationale for
that choice.  Back-patch to v17, for the next commit to use it.

Reviewed (in earlier versions) by Michael Paquier.

Discussion: https://postgr.es/m/17821-dd8c334263399284@postgresql.org
Discussion: https://postgr.es/m/20250116010051.f3.nmisch@google.com

1 files changed, 43 insertions, 0 deletions

diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index 0cab8653f1b..7d82cd2eb56 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -370,6 +370,49 @@ FullTransactionIdNewer(FullTransactionId a, FullTransactionId b)
 	return b;
 }
 
+/*
+ * Compute FullTransactionId for the given TransactionId, assuming xid was
+ * between [oldestXid, nextXid] at the time when TransamVariables->nextXid was
+ * nextFullXid.  When adding calls, evaluate what prevents xid from preceding
+ * oldestXid if SetTransactionIdLimit() runs between the collection of xid and
+ * the collection of nextFullXid.
+ */
+static inline FullTransactionId
+FullTransactionIdFromAllowableAt(FullTransactionId nextFullXid,
+								 TransactionId xid)
+{
+	uint32		epoch;
+
+	/* Special transaction ID. */
+	if (!TransactionIdIsNormal(xid))
+		return FullTransactionIdFromEpochAndXid(0, xid);
+
+	Assert(TransactionIdPrecedesOrEquals(xid,
+										 XidFromFullTransactionId(nextFullXid)));
+
+	/*
+	 * The 64 bit result must be <= nextFullXid, since nextFullXid hadn't been
+	 * issued yet when xid was in the past.  The xid must therefore be from
+	 * the epoch of nextFullXid or the epoch before.  We know this because we
+	 * must remove (by freezing) an XID before assigning the XID half an epoch
+	 * ahead of it.
+	 *
+	 * The unlikely() branch hint is dubious.  It's perfect for the first 2^32
+	 * XIDs of a cluster's life.  Right at 2^32 XIDs, misprediction shoots to
+	 * 100%, then improves until perfection returns 2^31 XIDs later.  Since
+	 * current callers pass relatively-recent XIDs, expect >90% prediction
+	 * accuracy overall.  This favors average latency over tail latency.
+	 */
+	epoch = EpochFromFullTransactionId(nextFullXid);
+	if (unlikely(xid > XidFromFullTransactionId(nextFullXid)))
+	{
+		Assert(epoch != 0);
+		epoch--;
+	}
+
+	return FullTransactionIdFromEpochAndXid(epoch, xid);
+}
+
 #endif							/* FRONTEND */
 
 #endif							/* TRANSAM_H */