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diff --git a/lib/libc/riscv/string/Makefile.inc b/lib/libc/riscv/string/Makefile.inc
--- a/lib/libc/riscv/string/Makefile.inc
+++ b/lib/libc/riscv/string/Makefile.inc
@@ -3,4 +3,5 @@
memcpy.S \
memset.S \
strlen.S \
+ strnlen.S \
strrchr.S
diff --git a/lib/libc/riscv/string/strnlen.S b/lib/libc/riscv/string/strnlen.S
new file mode 100644
--- /dev/null
+++ b/lib/libc/riscv/string/strnlen.S
@@ -0,0 +1,143 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Strahinja Stanisic <strajabot@FreeBSD.org>
+ */
+
+#include <machine/asm.h>
+
+/*
+ * a0 - const char *s
+ * a1 - size_t maxlen;
+ */
+ENTRY(strnlen)
+ /*
+ * a0 - const char *s;
+ * a1 - size_t maxlen;
+ * a2 - uint64_t *ptr;
+ * a3 - char iter[8];
+ * a4 - uint64_t *end_align;
+ * a5 - uint64_t *end_unroll;
+ */
+
+ beqz a1, .Lnot_found
+
+ /* ptr = s & ~0b111 */
+ /* t0 = 0x0101010101010101 */
+ /* t1 = 0x8080808080808080 */
+ /* end_align = (s + maxlen + 7) & ~0b111 */
+ /* mask_start = t0 >> ((-s.value) << 3) */
+ add a4, a0, a1
+ li t0, 0x01010101
+ addi a4, a4, 7
+ slli t1, t0, 32
+ neg t2, a0
+ andi a4, a4, ~0b111
+ or t0, t0, t1
+ slli t2, t2, 3
+ andi a2, a0, ~0b111
+ slli t1, t0, 7
+ srl t2, t0, t2
+
+ /* if pointer is aligned skip to loop */
+ beq a0, a2, .Lskip_start
+
+ /* iter = *ptr */
+ ld a3, (a2)
+
+ /* iter = iter | mask_start */
+ or a3, a3, t2
+
+ /* has_zero */
+ not t2, a3
+ sub a3, a3, t0
+ and t2, t2, t1
+ and a3, a3, t2
+
+ addi a2, a2, 8
+ bnez a3, .Lfind_zero
+
+.Lskip_start:
+ /* end_unroll */
+ sub t2, a4, a2
+ andi t2, t2, ~0b1111
+ add a5, a2, t2
+
+ /* while (ptr != end_unroll) */
+ beq a2, a5, .Lskip_loop
+.Lloop:
+ ld a3, (a2)
+ ld a6, 8(a2)
+
+ /* has_zero */
+ not t2, a3
+ not t3, a6
+ sub a3, a3, t0
+ sub a6, a6, t0
+ and t2, t2, t1
+ and t3, t3, t1
+ and a3, a3, t2
+ and a6, a6, t3
+
+ addi a2, a2, 8
+ bnez a3, .Lfind_zero
+
+ mv a3, a6
+
+ addi a2, a2, 8
+ bnez a3, .Lfind_zero
+
+ bne a2, a5, .Lloop
+
+.Lskip_loop:
+
+ beq a2, a4, .Lnot_found
+
+ ld a3, (a2)
+
+ /* has_zero */
+ not t2, a3
+ sub a3, a3, t0
+ and t2, t2, t1
+ and a3, a3, t2
+
+
+ addi a2, a2, 8
+ beqz a3, .Lnot_found
+
+.Lfind_zero:
+
+ /* move ptr back */
+ addi a2, a2, -8
+
+ /* isolate lowest set bit */
+ neg t0, a3
+ and a3, a3, t0
+
+ li t0, 0x0001020304050607
+ srli a3, a3, 7
+
+ /* lowest set bit is 2^(8*k)
+ * multiplying by it shifts the idx array in t0 by k bytes to the left */
+ mul a3, a3, t0
+
+ /* highest byte contains idx of first zero */
+ srli a3, a3, 56
+
+ /* zero_idx */
+ sub a2, a2, a0
+ add a2, a2, a3
+
+ /* min(zero_idx, maxlen) */
+ sub a2, a2, a1
+ srai t1, a2, 63
+ and a2, a2, t1
+ add a0, a1, a2
+
+ ret
+
+.Lnot_found:
+ mv a0, a1
+ ret
+
+END(strnlen)

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