From 944a4f340a65c21ee311d2d3e617034bef9d0b25 Mon Sep 17 00:00:00 2001
From: Coly Li <colyli@suse.de>
Date: Fri, 28 Jun 2019 19:59:41 +0800
Subject: [PATCH] bcache: make bset_search_tree() be more understandable
Git-commit: 944a4f340a65c21ee311d2d3e617034bef9d0b25
References: bsc#1140652
Patch-mainline: v5.3-rc1

The purpose of following code in bset_search_tree() is to avoid a branch
instruction,
 994         if (likely(f->exponent != 127))
 995                 n = j * 2 + (((unsigned int)
 996                               (f->mantissa -
 997                                bfloat_mantissa(search, f))) >> 31);
 998         else
 999                 n = (bkey_cmp(tree_to_bkey(t, j), search) > 0)
1000                         ? j * 2
1001                         : j * 2 + 1;

This piece of code is not very clear to understand, even when I tried to
add code comment for it, I made mistake. This patch removes the implict
bit operation and uses explicit branch to calculate next location in
binary tree search.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---
 drivers/md/bcache/bset.c | 30 +++++++++++-------------------
 1 file changed, 11 insertions(+), 19 deletions(-)

diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index 8af9509e78bd..08768796b543 100644
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -975,25 +975,17 @@ static struct bset_search_iter bset_search_tree(struct bset_tree *t,
 		j = n;
 		f = &t->tree[j];
 
-		/*
-		 * Similar bit trick, use subtract operation to avoid a branch
-		 * instruction.
-		 *
-		 * n = (f->mantissa > bfloat_mantissa())
-		 *	? j * 2
-		 *	: j * 2 + 1;
-		 *
-		 * We need to subtract 1 from f->mantissa for the sign bit trick
-		 * to work  - that's done in make_bfloat()
-		 */
-		if (likely(f->exponent != 127))
-			n = j * 2 + (((unsigned int)
-				      (f->mantissa -
-				       bfloat_mantissa(search, f))) >> 31);
-		else
-			n = (bkey_cmp(tree_to_bkey(t, j), search) > 0)
-				? j * 2
-				: j * 2 + 1;
+		if (likely(f->exponent != 127)) {
+			if (f->mantissa >= bfloat_mantissa(search, f))
+				n = j * 2;
+			else
+				n = j * 2 + 1;
+		} else {
+			if (bkey_cmp(tree_to_bkey(t, j), search) > 0)
+				n = j * 2;
+			else
+				n = j * 2 + 1;
+		}
 	} while (n < t->size);
 
 	inorder = to_inorder(j, t);
-- 
2.16.4