1// Copyright 2019 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package cpu
6
7import (
8	"io/ioutil"
9)
10
11const (
12	_AT_HWCAP  = 16
13	_AT_HWCAP2 = 26
14
15	procAuxv = "/proc/self/auxv"
16
17	uintSize = int(32 << (^uint(0) >> 63))
18)
19
20// For those platforms don't have a 'cpuid' equivalent we use HWCAP/HWCAP2
21// These are initialized in cpu_$GOARCH.go
22// and should not be changed after they are initialized.
23var hwCap uint
24var hwCap2 uint
25
26func readHWCAP() error {
27	buf, err := ioutil.ReadFile(procAuxv)
28	if err != nil {
29		// e.g. on android /proc/self/auxv is not accessible, so silently
30		// ignore the error and leave Initialized = false. On some
31		// architectures (e.g. arm64) doinit() implements a fallback
32		// readout and will set Initialized = true again.
33		return err
34	}
35	bo := hostByteOrder()
36	for len(buf) >= 2*(uintSize/8) {
37		var tag, val uint
38		switch uintSize {
39		case 32:
40			tag = uint(bo.Uint32(buf[0:]))
41			val = uint(bo.Uint32(buf[4:]))
42			buf = buf[8:]
43		case 64:
44			tag = uint(bo.Uint64(buf[0:]))
45			val = uint(bo.Uint64(buf[8:]))
46			buf = buf[16:]
47		}
48		switch tag {
49		case _AT_HWCAP:
50			hwCap = val
51		case _AT_HWCAP2:
52			hwCap2 = val
53		}
54	}
55	return nil
56}
57