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diff --git a/lib/libc/aarch64/string/Makefile.inc b/lib/libc/aarch64/string/Makefile.inc
--- a/lib/libc/aarch64/string/Makefile.inc
+++ b/lib/libc/aarch64/string/Makefile.inc
@@ -20,6 +20,8 @@
strnlen \
strrchr
+MDSRCS+= strcspn.S
+
#
# Add the above functions. Generate an asm file that includes the needed
# Arm Optimized Routines file defining the function name to the libc name.
diff --git a/lib/libc/aarch64/string/strcspn.S b/lib/libc/aarch64/string/strcspn.S
new file mode 100644
--- /dev/null
+++ b/lib/libc/aarch64/string/strcspn.S
@@ -0,0 +1,109 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Getz Mikalsen <getz@FreeBSD.org>
+*/
+
+#include <machine/asm.h>
+
+ .weak strcspn
+ .set strcspn, __strcspn
+ .text
+
+ENTRY(__strcspn)
+ stp x29, x30, [sp, #-16]!
+ mov x29, sp
+ mov x15, #1 // preload register with 1 for stores
+
+ /* check for special cases */
+ ldrb w4, [x1] // first character in the set
+ cbz w4, .Lstrlen
+
+ movi v0.16b, #0
+
+ ldrb w5, [x1, #1] // second character in the set
+ cbz w5, .Lstrchr
+
+ sub sp, sp, #256 // allocate 256 bytes on the stack
+
+ /* no special case matches -- prepare lookup table */
+ mov w3, #20
+ .p2align 4
+0: add x9, sp, x3, lsl #3
+ stp xzr, xzr, [x9]
+ stp xzr, xzr, [x9, #16]
+ subs w3, w3, #4
+ b.cs 0b
+
+ /* utilize SIMD stores to speed up zeroing the table */
+ stp q0, q0, [sp, #6*32]
+ stp q0, q0, [sp, #7*32]
+
+ add x1, x1, #2
+ strb w15, [sp, x4] // register first chars in the set
+ strb w15, [sp, x5]
+
+ mov x4, x0 // stash a copy of src
+
+ /* process remaining chars in set */
+ .p2align 4
+0: ldrb w5, [x1]
+ strb w15, [sp, x5]
+ cbz w5, 1f // end of set?
+
+ ldrb w5, [x1, #1]
+ strb w15, [sp, x5]
+ cbz w5, 1f
+
+ add x1, x1, #2
+ b 0b
+
+ /* find match */
+ .p2align 4
+1: ldrb w8, [x0]
+ ldrb w9, [sp, x8]
+ cbnz w9, 2f
+
+ ldrb w8, [x0, #1]
+ ldrb w9, [sp, x8]
+ cbnz w9, 3f
+
+ ldrb w8, [x0, #2]
+ ldrb w9, [sp, x8]
+ cbnz w9, 4f
+
+ ldrb w8, [x0, #3]
+ ldrb w9, [sp, x8]
+ add x0, x0, #4
+ cbz w9, 1b
+
+ sub x0, x0, #3 // fix up return value
+4: sub x4, x4, #1
+3: add x0, x0, #1
+2: sub x0, x0, x4
+ mov sp, x29
+ ldp x29, x30, [sp], #16 // restore sp and lr
+ ret
+
+ /* set is empty, degrades to strlen */
+ .p2align 4
+.Lstrlen:
+ mov sp, x29
+ ldp x29, x30, [sp], #16 // restore sp and lr
+ b strlen
+
+ /* just one character in set, degrades to strchrnul */
+ .p2align 4
+.Lstrchr:
+ stp x0, x1, [sp, #-16]!
+ mov x1, x4
+
+ bl strchrnul
+
+ ldp x18, x17, [sp], #16 // restore stashed src
+ sub x0, x0, x18
+
+ ldp x29, x30, [sp], #16 // Restore sp and lr
+ ret
+
+END(__strcspn)

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