From cf197cada461d1d442458cbebdd2bb8ba314692e Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Mon, 25 Nov 2024 15:24:16 -0500
Subject: ioq: Prefetch pointers before popping them

Also, cache-align struct ioq_ent to avoid false sharing when two workers
are handling neighbouring requests.
---
 src/ioq.c |  8 ++++++++
 src/ioq.h | 10 ++--------
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/src/ioq.c b/src/ioq.c
index 5668a83..017b6c1 100644
--- a/src/ioq.c
+++ b/src/ioq.c
@@ -356,6 +356,14 @@ static bool ioq_slot_push(struct ioqq *ioqq, ioq_slot *slot, struct ioq_ent *ent
 static struct ioq_ent *ioq_slot_pop(struct ioqq *ioqq, ioq_slot *slot, bool block) {
 	uintptr_t prev = load(slot, relaxed);
 	while (true) {
+#if __has_builtin(__builtin_prefetch)
+		// Optimistically prefetch the pointer in this slot.  If this
+		// slot is not full, this will prefetch an invalid address, but
+		// experimentally this is worth it on both Intel (Alder Lake)
+		// and AMD (Zen 2).
+		__builtin_prefetch((void *)(prev << 1));
+#endif
+
 		// empty     → skip(1)
 		// skip(n)   → skip(n + 1)
 		// full(ptr) → full(ptr - 1)
diff --git a/src/ioq.h b/src/ioq.h
index fce1d7f..da0a525 100644
--- a/src/ioq.h
+++ b/src/ioq.h
@@ -8,6 +8,7 @@
 #ifndef BFS_IOQ_H
 #define BFS_IOQ_H
 
+#include "bfs.h"
 #include "dir.h"
 #include "stat.h"
 
@@ -44,19 +45,12 @@ enum ioq_nop_type {
 	IOQ_NOP_HEAVY,
 };
 
-/**
- * The I/O queue implementation needs two tag bits in each pointer to a struct
- * ioq_ent, so we need to ensure at least 4-byte alignment.  The natural
- * alignment is enough on most architectures, but not m68k, so over-align it.
- */
-#define IOQ_ENT_ALIGN alignas(4)
-
 /**
  * An I/O queue entry.
  */
 struct ioq_ent {
 	/** The I/O operation. */
-	IOQ_ENT_ALIGN enum ioq_op op;
+	cache_align enum ioq_op op;
 
 	/** The return value (on success) or negative error code (on failure). */
 	int result;
-- 
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