Add minor improvements and cleanup

libc/sysv/vfork.S

@@ -0,0 +1,48 @@
+/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8     -*-│
+│vi: set et ft=asm ts=8 tw=8 fenc=utf-8                                     :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ This program is free software; you can redistribute it and/or modify         │
+│ it under the terms of the GNU General Public License as published by         │
+│ the Free Software Foundation; version 2 of the License.                      │
+│                                                                              │
+│ This program is distributed in the hope that it will be useful, but          │
+│ WITHOUT ANY WARRANTY; without even the implied warranty of                   │
+│ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU             │
+│ General Public License for more details.                                     │
+│                                                                              │
+│ You should have received a copy of the GNU General Public License            │
+│ along with this program; if not, write to the Free Software                  │
+│ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA                │
+│ 02110-1301 USA                                                               │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/dce.h"
+#include "libc/macros.h"
+
+/	Forks process without copying page tables.
+/
+/	This is the same as fork() except it's optimized for the case
+/	where the caller invokes exec() immediately afterwards.
+/
+/	@return	pid of child process or 0 if forked process
+/	@returnstwice
+vfork:	mov	__NR_vfork(%rip),%eax
+	cmp	$-1,%eax
+	je	systemfive.enosys
+	pop	%rsi
+	testb	IsBsd()
+	jnz	vfork.bsd
+	syscall
+	push	%rsi
+	cmp	$-4095,%rax
+	jae	systemfive.error
+	ret
+	.endfn	vfork,globl
+
+vfork.bsd:
+	syscall
+	push	%rsi
+	jc	systemfive.errno
+	ret
+	.endfn	vfork.bsd