extract from https://github.com/lattera/glibc/blob/master/sysdeps/x86_64/start.S#L33

start.S

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

/* This is the canonical entry point, usually the first thing in the text
   segment.  The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
   point runs, most registers' values are unspecified, except for:
   %rdx		Contains a function pointer to be registered with `atexit'.
		This is how the dynamic linker arranges to have DT_FINI
		functions called for shared libraries that have been loaded
		before this code runs.
   %rsp		The stack contains the arguments and environment:
		0(%rsp)				argc
		LP_SIZE(%rsp)			argv[0]
		...
		(LP_SIZE*argc)(%rsp)		NULL
		(LP_SIZE*(argc+1))(%rsp)	envp[0]
		...
						NULL
*/

#include <sysdep.h>

ENTRY (_start)
	/* Clearing frame pointer is insufficient, use CFI.  */
	cfi_undefined (rip)
	/* Clear the frame pointer.  The ABI suggests this be done, to mark
	   the outermost frame obviously.  */
	xorl %ebp, %ebp

	/* Extract the arguments as encoded on the stack and set up
	   the arguments for __libc_start_main (int (*main) (int, char **, char **),
		   int argc, char *argv,
		   void (*init) (void), void (*fini) (void),
		   void (*rtld_fini) (void), void *stack_end).
	The arguments are passed via registers and on the stack:
	main:		%rdi
	argc:		%rsi
	argv:		%rdx
	init:		%rcx
	fini:		%r8
	rtld_fini:	%r9
	stack_end:	stack.	*/