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	"os"
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	// For Go 1.21+, get auxv from the Go runtime.
28	if a := getAuxv(); len(a) > 0 {
29		for len(a) >= 2 {
30			tag, val := a[0], uint(a[1])
31			a = a[2:]
32			switch tag {
33			case _AT_HWCAP:
34				hwCap = val
35			case _AT_HWCAP2:
36				hwCap2 = val
37			}
38		}
39		return nil
40	}
41
42	buf, err := os.ReadFile(procAuxv)
43	if err != nil {
44		// e.g. on android /proc/self/auxv is not accessible, so silently
45		// ignore the error and leave Initialized = false. On some
46		// architectures (e.g. arm64) doinit() implements a fallback
47		// readout and will set Initialized = true again.
48		return err
49	}
50	bo := hostByteOrder()
51	for len(buf) >= 2*(uintSize/8) {
52		var tag, val uint
53		switch uintSize {
54		case 32:
55			tag = uint(bo.Uint32(buf[0:]))
56			val = uint(bo.Uint32(buf[4:]))
57			buf = buf[8:]
58		case 64:
59			tag = uint(bo.Uint64(buf[0:]))
60			val = uint(bo.Uint64(buf[8:]))
61			buf = buf[16:]
62		}
63		switch tag {
64		case _AT_HWCAP:
65			hwCap = val
66		case _AT_HWCAP2:
67			hwCap2 = val
68		}
69	}
70	return nil
71}
72