From 535192530c4360a808c97398eadfd5a7456f79a9 Mon Sep 17 00:00:00 2001
From: Doug Moore <dougm@FreeBSD.org>
Date: Sat, 11 May 2019 09:09:10 +0000
Subject: [PATCH] When bitpos can't be implemented with an inline ffs*
 instruction, change the binary search so that it does not depend on a single
 bit only being set in the bitmask. Use bitpos more generally, and avoid some
 clearing of bits to accommodate its current behavior.

Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20237
---
 sys/kern/subr_blist.c | 61 +++++++++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 26 deletions(-)

diff --git a/sys/kern/subr_blist.c b/sys/kern/subr_blist.c
index d4d5493bf841..a72bc842c7a7 100644
--- a/sys/kern/subr_blist.c
+++ b/sys/kern/subr_blist.c
@@ -192,30 +192,40 @@ bitrange(int n, int count)
 
 
 /*
- * Use binary search, or a faster method, to find the 1 bit in a u_daddr_t.
- * Assumes that the argument has only one bit set.
+ * Find the first bit set in a u_daddr_t.
  */
+static inline int
+generic_bitpos(u_daddr_t mask)
+{
+	int hi, lo, mid;
+
+	lo = 0;
+	hi = BLIST_BMAP_RADIX;
+	while (lo + 1 < hi) {
+		mid = (lo + hi) >> 1;
+		if (mask & bitrange(0, mid))
+			hi = mid;
+		else
+			lo = mid;
+	}
+	return (lo);
+}
+
 static inline int
 bitpos(u_daddr_t mask)
 {
-	int hi, lo, mid;
 
 	switch (sizeof(mask)) {
 #ifdef HAVE_INLINE_FFSLL
 	case sizeof(long long):
 		return (ffsll(mask) - 1);
+#endif
+#ifdef HAVE_INLINE_FFS
+	case sizeof(int):
+		return (ffs(mask) - 1);
 #endif
 	default:
-		lo = 0;
-		hi = BLIST_BMAP_RADIX;
-		while (lo + 1 < hi) {
-			mid = (lo + hi) >> 1;
-			if ((mask >> mid) != 0)
-				lo = mid;
-			else
-				hi = mid;
-		}
-		return (lo);
+		return (generic_bitpos(mask));
 	}
 }
 
@@ -532,7 +542,8 @@ blist_stats(blist_t bl, struct sbuf *s)
 	struct gap_stats gstats;
 	struct gap_stats *stats = &gstats;
 	daddr_t i, nodes, radix;
-	u_daddr_t bit, diff, mask;
+	u_daddr_t diff, mask;
+	int digit;
 
 	init_gap_stats(stats);
 	nodes = 0;
@@ -570,9 +581,9 @@ blist_stats(blist_t bl, struct sbuf *s)
 			if (gap_stats_counting(stats))
 				diff ^= 1;
 			while (diff != 0) {
-				bit = diff & -diff;
-				update_gap_stats(stats, i + bitpos(bit));
-				diff ^= bit;
+				digit = bitpos(diff);
+				update_gap_stats(stats, i + digit);
+				diff ^= bitrange(digit, 1);
 			}
 		}
 		nodes += radix_to_skip(radix);
@@ -776,7 +787,7 @@ static daddr_t
 blst_meta_alloc(blmeta_t *scan, daddr_t cursor, daddr_t count, u_daddr_t radix)
 {
 	daddr_t blk, i, r, skip;
-	u_daddr_t bit, mask;
+	u_daddr_t mask;
 	bool scan_from_start;
 	int digit;
 
@@ -808,8 +819,7 @@ blst_meta_alloc(blmeta_t *scan, daddr_t cursor, daddr_t count, u_daddr_t radix)
 	 * Examine the nonempty subtree associated with each bit set in mask.
 	 */
 	do {
-		bit = mask & -mask;
-		digit = bitpos(bit);
+		digit = bitpos(mask);
 		i = 1 + digit * skip;
 		if (count <= scan[i].bm_bighint) {
 			/*
@@ -819,12 +829,12 @@ blst_meta_alloc(blmeta_t *scan, daddr_t cursor, daddr_t count, u_daddr_t radix)
 			    count, radix);
 			if (r != SWAPBLK_NONE) {
 				if (scan[i].bm_bitmap == 0)
-					scan->bm_bitmap ^= bit;
+					scan->bm_bitmap ^= bitrange(digit, 1);
 				return (r);
 			}
 		}
 		cursor = blk;
-	} while ((mask ^= bit) != 0);
+	} while ((mask ^= bitrange(digit, 1)) != 0);
 
 	/*
 	 * We couldn't allocate count in this subtree.  If the whole tree was
@@ -1019,7 +1029,7 @@ static void
 blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix, int tab)
 {
 	daddr_t skip;
-	u_daddr_t bit, mask;
+	u_daddr_t mask;
 	int digit;
 
 	if (radix == BLIST_BMAP_RADIX) {
@@ -1051,11 +1061,10 @@ blst_radix_print(blmeta_t *scan, daddr_t blk, daddr_t radix, int tab)
 	mask = scan->bm_bitmap;
 	/* Examine the nonempty subtree associated with each bit set in mask */
 	do {
-		bit = mask & -mask;
-		digit = bitpos(bit);
+		digit = bitpos(mask);
 		blst_radix_print(&scan[1 + digit * skip], blk + digit * radix,
 		    radix, tab);
-	} while ((mask ^= bit) != 0);
+	} while ((mask ^= bitrange(digit, 1)) != 0);
 	tab -= 4;
 
 	printf(