linux-kernel - [PATCH] riscv: lib: optimize strlen loop efficiency

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Message-Id: <20251218032614.57356-1-jiangfeng@kylinos.cn>
Date: Thu, 18 Dec 2025 11:26:14 +0800
From: Feng Jiang <jiangfeng@...inos.cn>
To: pjw@...nel.org,
	palmer@...belt.com,
	aou@...s.berkeley.edu,
	alex@...ti.fr,
	samuel.holland@...ive.com,
	charlie@...osinc.com,
	conor.dooley@...rochip.com,
	jiangfeng@...inos.cn
Cc: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] 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@...inos.cn>
---
 arch/riscv/lib/strlen.S | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/lib/strlen.S b/arch/riscv/lib/strlen.S
index eb4d2b7ed22b..e7736ccda514 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
 
-- 
2.25.1