diff --git a/lib/csu/Makefile.inc b/lib/csu/Makefile.inc
--- a/lib/csu/Makefile.inc
+++ b/lib/csu/Makefile.inc
@@ -20,7 +20,6 @@
 ACFLAGS+=	-DLOCORE
 
 CFLAGS+=	-DSTRIP_FBSDID
-CFLAGS+=	-fno-asynchronous-unwind-tables
 CFLAGS+=	-fno-omit-frame-pointer
 CFLAGS+=	-I${.CURDIR:H}/common \
 		-I${SRCTOP}/lib/libc/include
@@ -35,25 +34,28 @@
 # These FILES qualify as libraries for the purpose of LIBRARIES_ONLY.
 .undef LIBRARIES_ONLY
 
+CRT1SRC?=	crt1_c.c
+CRT1OBJ?=
+
 CLEANFILES+=	${OBJS} ${CRT1OBJS} crt1_c.o gcrt1_c.o Scrt1_c.o
-CLEANFILES+=	crti_s.o
+CLEANFILES+=	crti_s.o ${CRT1SRC:C/.[S|c]$/.o/}
 
-crt1.o:	crt1_c.o ${CRT1OBJS}
+crt1.o:	${CRT1SRC:C/.[S|c]$/.o/} ${CRT1OBJS} ${CRT1OBJ}
 	${LD} ${_LDFLAGS} -o ${.TARGET} -r ${.ALLSRC:M*.o}
 .if ${MACHINE_ARCH} == "i386"
 	${OBJCOPY} --localize-symbol _start1 ${.TARGET}
 .endif
 
-gcrt1_c.o: crt1_c.c
-	${CC} ${CFLAGS} -DGCRT -c -o ${.TARGET} ${.CURDIR}/crt1_c.c
+gcrt1_c.o: ${CRT1SRC}
+	${CC} ${CFLAGS} -DGCRT -c -o ${.TARGET} ${.CURDIR}/${CRT1SRC}
 
-gcrt1.o: gcrt1_c.o ${CRT1OBJS}
+gcrt1.o: gcrt1_c.o ${CRT1OBJS} ${CRT1OBJ}
 	${LD} ${_LDFLAGS} -o ${.TARGET} -r ${.ALLSRC:M*.o}
 
-Scrt1_c.o: crt1_c.c
-	${CC} ${CFLAGS} -fPIC -DPIC -c -o ${.TARGET} ${.CURDIR}/crt1_c.c
+Scrt1_c.o: ${CRT1SRC}
+	${CC} ${CFLAGS} -fPIC -DPIC -c -o ${.TARGET} ${.CURDIR}/${CRT1SRC}
 
-Scrt1.o: Scrt1_c.o ${CRT1OBJS}
+Scrt1.o: Scrt1_c.o ${CRT1OBJS} ${CRT1OBJ}
 	${LD} ${_LDFLAGS} -o ${.TARGET} -r ${.ALLSRC:M*.o}
 .if ${MACHINE_ARCH} == "i386"
 	${OBJCOPY} --localize-symbol _start1 ${.TARGET}
diff --git a/lib/csu/amd64/Makefile b/lib/csu/amd64/Makefile
--- a/lib/csu/amd64/Makefile
+++ b/lib/csu/amd64/Makefile
@@ -5,4 +5,7 @@
 CFLAGS+=	-I${.CURDIR}
 CFLAGS+=	-fno-omit-frame-pointer
 
+CRT1SRC=	crt1_s.S
+CRT1OBJ=	crt1_c.o
+
 .include <bsd.lib.mk>
diff --git a/lib/csu/amd64/crt1_c.c b/lib/csu/amd64/crt1_c.c
--- a/lib/csu/amd64/crt1_c.c
+++ b/lib/csu/amd64/crt1_c.c
@@ -29,26 +29,4 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include "libc_private.h"
 #include "csu_common.h"
-
-void _start(char **, void (*)(void)) __dead2;
-
-/* The entry function. */
-void
-_start(char **ap, void (*cleanup)(void))
-{
-	int argc;
-	char **argv;
-	char **env;
-
-	argc = *(long *)(void *)ap;
-	argv = ap + 1;
-	env = ap + 2 + argc;
-#ifdef GCRT
-	__libc_start1_gcrt(argc, argv, env, cleanup, main, &eprol, &etext);
-__asm__("eprol:");
-#else
-	__libc_start1(argc, argv, env, cleanup, main);
-#endif
-}
diff --git a/lib/csu/amd64/crt1_s.S b/lib/csu/amd64/crt1_s.S
new file mode 100644
--- /dev/null
+++ b/lib/csu/amd64/crt1_s.S
@@ -0,0 +1,88 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (C) 2023 Dmitry Chagin <dchagin@FreeBSD.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <machine/asm.h>
+
+
+	.text
+	.align 8
+
+	/*
+	 * The program entry point
+	 *		  %rdi		%rsi
+	 * void _start(char **ap, void (*cleanup)(void)) __dead2
+	 */
+	.globl	_start
+	.type	_start, @function
+_start:
+	.cfi_startproc
+	.cfi_undefined %rip		/* Terminate call chain. */
+	pushq	%rbp			/* Align stack, terminate call chain. */
+	.cfi_def_cfa_offset 8
+	movq	%rsp, %rbp
+	.cfi_offset %rbp, -16
+	.cfi_def_cfa_register %rbp
+#ifdef GCRT
+	subq	$16, %rsp
+#endif
+	movq	%rsi, %rcx
+	movq	%rdi, %rsi		/* argv = ap */
+	addq	$8, %rsi		/* argv += 1 */
+	movq	%rdi, %rdx		/* env = ap */
+	addq	$16, %rdx		/* env += 2 */
+	movslq	(%rdi), %rax
+	movl	%eax, %edi		/* argc = *(long *)(void *)ap */
+	shlq	$3, %rax
+	addq	%rax, %rdx		/* env += argc */
+#ifdef PIC
+	/*
+	 * XXX. %rip relative addressing does not intended to use in the
+	 * large memory model due to offset from %rip is limited to 32 bits.
+	 */
+	leaq	main(%rip), %r8
+#else
+	movabsq	$main, %r8
+#endif
+#ifdef GCRT
+	movabsq	$eprol, %r9
+	movabsq	$etext, %rax
+	movq    %rax, (%rsp)
+	/*
+	 *		      %edi  %rsi  %rdx  %rcx    %r8    %r9    (%rsp)
+	 * __libc_start1_gcrt(argc, argv, env, cleanup, main, &eprol, &etext)
+	 */
+	callq	__libc_start1_gcrt
+eprol:
+#else
+	/* __libc_start1(argc, argv, env, cleanup, main) */
+	callq	__libc_start1
+#endif
+	int3
+	.cfi_endproc
+	.size   _start, . - _start
+
+	.section .note.GNU-stack,"",%progbits