diff --git a/sys/amd64/amd64/support.S b/sys/amd64/amd64/support.S
index b623fba277d..994c5f15e24 100644
--- a/sys/amd64/amd64/support.S
+++ b/sys/amd64/amd64/support.S
@@ -697,6 +697,72 @@ ENTRY(fillw)
 	ret
 END(fillw)
 
+/*
+ * strlen(string)
+ *	  %rdi
+ *
+ * Uses the ((x - 0x01....01) & ~x & 0x80....80) trick.
+ *
+ * 0x80....80 is replaced with 0 - 0x80....80 so that it can be added
+ * with leaq.
+ *
+ * For a description see either:
+ * - "Hacker's Delight" by Henry S. Warren, Jr.
+ * - "Optimizing subroutines in assembly language: An optimization guide for x86 platforms"
+ *   by Agner Fog
+ *
+ * The latter contains a 32-bit variant of the same algorithm coded in assembly for i386.
+ */
+ENTRY(strlen)
+	PUSH_FRAME_POINTER
+	movabsq	$0xfefefefefefefeff,%r8
+	movabsq	$0x8080808080808080,%r9
+
+	movq	%rdi,%r10
+	movq	%rdi,%rcx
+	testb	$7,%dil
+	jz	2f
+
+	/*
+	 * Handle misaligned reads: align to 8 and fill
+	 * the spurious bytes.
+	 */
+	andq	$~7,%rdi
+	movq	(%rdi),%r11
+	shlq	$3,%rcx
+	movq	$-1,%rdx
+	shlq	%cl,%rdx
+	notq	%rdx
+	orq	%rdx,%r11
+
+	leaq	(%r11,%r8),%rcx
+	notq	%r11
+	andq	%r11,%rcx
+	andq	%r9,%rcx
+	jnz	3f
+
+	/*
+	 * Main loop.
+	 */
+	ALIGN_TEXT
+1:
+	leaq	8(%rdi),%rdi
+2:
+	movq	(%rdi),%r11
+	leaq	(%r11,%r8),%rcx
+	notq	%r11
+	andq	%rcx,%r11
+	andq	%r9,%rcx
+	jz	1b
+3:
+	bsfq	%rcx,%rcx
+	shrq	$3,%rcx
+	leaq	(%rcx,%rdi),%rax
+	subq	%r10,%rax
+	POP_FRAME_POINTER
+	ret
+END(strlen)
+
 /*****************************************************************************/
 /* copyout and fubyte family                                                 */
 /*****************************************************************************/
diff --git a/sys/conf/files b/sys/conf/files
index edca1003e90..1abfadb1e8d 100644
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -4085,7 +4085,6 @@ libkern/strdup.c		standard
 libkern/strndup.c		standard
 libkern/strlcat.c		standard
 libkern/strlcpy.c		standard
-libkern/strlen.c		standard
 libkern/strncat.c		standard
 libkern/strncmp.c		standard
 libkern/strncpy.c		standard
diff --git a/sys/conf/files.arm b/sys/conf/files.arm
index eb3a23b5fc2..69986585bdf 100644
--- a/sys/conf/files.arm
+++ b/sys/conf/files.arm
@@ -127,6 +127,7 @@ libkern/lshrdi3.c		standard
 libkern/memcmp.c		standard
 libkern/moddi3.c		standard
 libkern/qdivrem.c		standard
+libkern/strlen.c		standard
 libkern/ucmpdi2.c		standard
 libkern/udivdi3.c		standard
 libkern/umoddi3.c		standard
diff --git a/sys/conf/files.arm64 b/sys/conf/files.arm64
index f7003b1048c..42ec3b2787b 100644
--- a/sys/conf/files.arm64
+++ b/sys/conf/files.arm64
@@ -432,6 +432,7 @@ libkern/memcmp.c		standard				\
 	compile-with "${NORMAL_C:N-fsanitize*}"
 libkern/memset.c		standard				\
 	compile-with "${NORMAL_C:N-fsanitize*}"
+libkern/strlen.c		standard
 libkern/arm64/crc32c_armv8.S	standard
 cddl/dev/dtrace/aarch64/dtrace_asm.S			optional dtrace compile-with "${DTRACE_S}"
 cddl/dev/dtrace/aarch64/dtrace_subr.c			optional dtrace compile-with "${DTRACE_C}"
diff --git a/sys/conf/files.i386 b/sys/conf/files.i386
index 6560ab217d9..1e2ab5f8c52 100644
--- a/sys/conf/files.i386
+++ b/sys/conf/files.i386
@@ -218,6 +218,7 @@ libkern/memcmp.c		standard
 libkern/memset.c		standard
 libkern/moddi3.c		standard
 libkern/qdivrem.c		standard
+libkern/strlen.c		standard
 libkern/ucmpdi2.c		standard
 libkern/udivdi3.c		standard
 libkern/umoddi3.c		standard
diff --git a/sys/conf/files.mips b/sys/conf/files.mips
index c18f0a5c69b..7ee5b0019bd 100644
--- a/sys/conf/files.mips
+++ b/sys/conf/files.mips
@@ -66,6 +66,7 @@ libkern/ucmpdi2.c			optional	mips | mipshf | mipsel | mipselhf
 libkern/ashldi3.c			standard
 libkern/ashrdi3.c			standard
 libkern/memcmp.c			standard
+libkern/strlen.c			standard
 
 # cfe support
 dev/cfe/cfe_api.c			optional	cfe
diff --git a/sys/conf/files.powerpc b/sys/conf/files.powerpc
index 3022fd6f6e3..347abee153d 100644
--- a/sys/conf/files.powerpc
+++ b/sys/conf/files.powerpc
@@ -129,6 +129,7 @@ libkern/memcmp.c		standard
 libkern/memset.c		standard
 libkern/moddi3.c		optional	powerpc | powerpcspe
 libkern/qdivrem.c		optional	powerpc | powerpcspe
+libkern/strlen.c		standard
 libkern/ucmpdi2.c		optional	powerpc | powerpcspe
 libkern/udivdi3.c		optional	powerpc | powerpcspe
 libkern/umoddi3.c		optional	powerpc | powerpcspe
diff --git a/sys/conf/files.riscv b/sys/conf/files.riscv
index 3969528db07..7ecea016b9a 100644
--- a/sys/conf/files.riscv
+++ b/sys/conf/files.riscv
@@ -29,6 +29,7 @@ libkern/flsl.c			standard
 libkern/flsll.c			standard
 libkern/memcmp.c		standard
 libkern/memset.c		standard
+libkern/strlen.c		standard
 riscv/riscv/autoconf.c		standard
 riscv/riscv/bus_machdep.c	standard
 riscv/riscv/bus_space_asm.S	standard