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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
@@ -33,6 +33,7 @@
strlcat.c \
strlen.S \
timingsafe_bcmp.S \
+ timingsafe_memcmp.S \
bcopy.c \
bzero.c
diff --git a/lib/libc/aarch64/string/timingsafe_memcmp.S b/lib/libc/aarch64/string/timingsafe_memcmp.S
new file mode 100644
--- /dev/null
+++ b/lib/libc/aarch64/string/timingsafe_memcmp.S
@@ -0,0 +1,117 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Robert Clausecker
+ */
+
+#include <machine/asm.h>
+
+ENTRY(timingsafe_memcmp)
+ cmp x2, #16 // at least 17 bytes to process?
+ bhi .Lgt16
+
+ cmp x2, #8 // at least 9 bytes to process?
+ bhi .L0916
+
+ cmp x2, #4 // at least 5 bytes to process?
+ bhi .L0508
+
+ cmp x2, #2 // at least 3 bytes to process?
+ bhi .L0304
+
+ cbnz x2, .L0102 // buffer empty?
+
+ mov w0, #0 // empty buffer always matches
+ ret
+
+.L0102: ldrb w3, [x0] // load first bytes
+ ldrb w4, [x1]
+ sub x2, x2, #1
+ ldrb w5, [x0, x2] // load last bytes
+ ldrb w6, [x1, x2]
+ bfi w5, w3, #8, #8 // join bytes in big endian
+ bfi w6, w4, #8, #8
+ sub w0, w5, w6
+ ret
+
+
+.L0304: ldrh w3, [x0] // load first halfwords
+ ldrh w4, [x1]
+ sub x2, x2, #2
+ ldrh w5, [x0, x2] // load last halfwords
+ ldrh w6, [x1, x2]
+ bfi w3, w5, #16, #16 // join halfwords in little endian
+ bfi w4, w6, #16, #16
+ rev w3, w3 // swap word order
+ rev w4, w4
+ cmp w3, w4
+ csetm w0, lo // w0 = w3 >= w4 ? 0 : -1
+ csinc w0, w0, wzr, ls // w0 = w3 <=> w4 ? 1 : 0 : -1
+ ret
+
+.L0508: ldr w3, [x0] // load first words
+ ldr w4, [x1]
+ sub x2, x2, #4
+ ldr w5, [x0, x2] // load last words
+ ldr w6, [x1, x2]
+ bfi x3, x5, #32, #32 // join words in little endian
+ bfi x4, x6, #32, #32
+ rev x3, x3 // swap word order
+ rev x4, x4
+ cmp x3, x4
+ csetm w0, lo // x0 = x3 >= w4 ? 0 : -1
+ csinc w0, w0, wzr, ls // x0 = x3 <=> w4 ? 1 : 0 : -1
+ ret
+
+.L0916: ldr x3, [x0]
+ ldr x4, [x1]
+ sub x2, x2, #8
+ ldr x5, [x0, x2]
+ ldr x6, [x1, x2]
+ cmp x3, x4 // mismatch in first pair?
+ csel x3, x3, x5, ne // use second pair if first pair equal
+ csel x4, x4, x6, ne
+ rev x3, x3
+ rev x4, x4
+ cmp x3, x4
+ csetm w0, lo
+ csinc w0, w0, wzr, ls
+ ret
+
+ /* more than 16 bytes: process buffer in a loop */
+.Lgt16: ldp x3, x4, [x0], #16
+ ldp x5, x6, [x1], #16
+ cmp x3, x5 // mismatch in first pair?
+ csel x3, x3, x4, ne // use second pair if first pair equal
+ csel x5, x5, x6, ne
+ subs x2, x2, #32
+ bls .Ltail
+
+0: ldp x4, x7, [x0], #16
+ ldp x6, x8, [x1], #16
+ cmp x4, x6 // mismatch in first pair?
+ csel x4, x4, x7, ne // if not, try second pair
+ csel x6, x6, x8, ne
+ cmp x3, x5 // was there a mismatch previously?
+ csel x3, x3, x4, ne // apply new pair if there was not
+ csel x5, x5, x6, ne
+ subs x2, x2, #16
+ bhi 0b
+
+.Ltail: add x0, x0, x2
+ add x1, x1, x2
+ ldp x4, x7, [x0]
+ ldp x6, x8, [x1]
+ cmp x4, x6 // mismatch in first pair?
+ csel x4, x4, x7, ne // if not, try second pair
+ csel x6, x6, x8, ne
+ cmp x3, x5 // was there a mismatch previously?
+ csel x3, x3, x4, ne // apply new pair if there was not
+ csel x5, x5, x6, ne
+ rev x3, x3
+ rev x5, x5
+ cmp x3, x5
+ csetm w0, lo
+ csinc w0, w0, wzr, ls
+ ret
+END(timingsafe_bcmp)

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