This is xnu-8019. See this file in:
/*
 * Copyright (c) 2003 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */

/* Get the cpu_capabilities bit vector out of the comm page */

#define	__APPLE_API_PRIVATE
#include <machine/cpu_capabilities.h>
#undef	__APPLE_API_PRIVATE

#if defined(__x86_64__)

	.text
	.align 2, 0x90
	.globl __get_cpu_capabilities
__get_cpu_capabilities:
	movq	$(_COMM_PAGE_CPU_CAPABILITIES64), %rax
	movq	(%rax), %rax
	ret

#elif defined(__i386__)

	.text
	.align 2, 0x90
	.globl __get_cpu_capabilities
__get_cpu_capabilities:
	movl	_COMM_PAGE_CPU_CAPABILITIES64, %eax
	movl	_COMM_PAGE_CPU_CAPABILITIES64+4, %edx
	ret

#elif defined(__arm__)

	.text
	.align 2
	.globl __get_cpu_capabilities
__get_cpu_capabilities:
	mov	r0, #(_COMM_PAGE_CPU_CAPABILITIES & 0x000000ff)
	orr	r0, r0, #(_COMM_PAGE_CPU_CAPABILITIES & 0x0000ff00)
	orr	r0, r0, #(_COMM_PAGE_CPU_CAPABILITIES & 0x00ff0000)
	orr	r0, r0, #(_COMM_PAGE_CPU_CAPABILITIES & 0xff000000)
	ldr	r0, [r0]
	bx	lr

#elif defined(__arm64__)

	.text
	.align 2
	.globl __get_cpu_capabilities
__get_cpu_capabilities:
	ldr x0, Lcommpage_cc_addr
	ldr	x0, [x0]
	ret
Lcommpage_cc_addr:
.quad _COMM_PAGE_CPU_CAPABILITIES64

#else
#error Unsupported architecture
#endif