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riscv: lib: optimize strlen loop efficiency

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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>
This commit is contained in:
Feng Jiang 2026-01-25 21:09:58 -07:00 • committed by Paul Walmsley
parent 098921ec68
commit 18be4ca5cb
1 changed files with 3 additions and 5 deletions
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8
arch/riscv/lib/strlen.S
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@ -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