From: Feng Jiang <jiangfeng@kylinos.cn>
Date: Mon, 26 Jan 2026 04:09:58 +0000 (-0700)
Subject: riscv: lib: optimize strlen loop efficiency
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=18be4ca5cb4e5a86833de97d331f5bc14a6c5a6d;p=thirdparty%2Flinux.git

riscv: lib: optimize strlen loop efficiency

Optimize the generic strlen implementation by using a pre-decrement
pointer. This reduces the loop body from 4 instructions to 3 and
eliminates the unconditional jump ('j').

Old loop (4 instructions, 2 branches):
  1: lbu t0, 0(t1); beqz t0, 2f; addi t1, t1, 1; j 1b

New loop (3 instructions, 1 branch):
  1: addi t1, t1, 1; lbu t0, 0(t1); bnez t0, 1b

This change improves execution efficiency and reduces branch pressure
for systems without the Zbb extension.

Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Link: https://patch.msgid.link/20251218032614.57356-1-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>
---

diff --git a/arch/riscv/lib/strlen.S b/arch/riscv/lib/strlen.S
index eb4d2b7ed22b6..e7736ccda5141 100644
--- a/arch/riscv/lib/strlen.S
+++ b/arch/riscv/lib/strlen.S
@@ -21,13 +21,11 @@ SYM_FUNC_START(strlen)
 	 * Clobbers:
 	 *   t0, t1
 	 */
-	mv	t1, a0
+	addi	t1, a0, -1
 1:
-	lbu	t0, 0(t1)
-	beqz	t0, 2f
 	addi	t1, t1, 1
-	j	1b
-2:
+	lbu	t0, 0(t1)
+	bnez	t0, 1b
 	sub	a0, t1, a0
 	ret