[hjl@gnu-tools-1 tmp]$ cat x.c 
extern void xxx (void) __attribute__ ((__noreturn__));
void
foo (int len)
{
  if (len)
    xxx ();
}
[hjl@gnu-tools-1 tmp]$ gcc -S -O2 x.c
[hjl@gnu-tools-1 tmp]$ cat x.s
	.file	"x.c"
	.section	.text.unlikely,"ax",@progbits
.LCOLDB0:
	.text
.LHOTB0:
	.p2align 4,,15
	.globl	foo
	.type	foo, @function
foo:
.LFB0:
	.cfi_startproc
	testl	%edi, %edi
	jne	.L6
	rep ret
.L6:
	pushq	%rax
	.cfi_def_cfa_offset 16
	call	xxx
	.cfi_endproc
.LFE0:
	.size	foo, .-foo
	.section	.text.unlikely
.LCOLDE0:
	.text
.LHOTE0:
	.ident	"GCC: (GNU) 5.2.1 20150716 (Red Hat 5.2.1-1)"
	.section	.note.GNU-stack,"",@progbits
[hjl@gnu-tools-1 tmp]$