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// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package cpu implements processor feature detection
// used by the Go standard library.
package cpu
// DebugOptions is set to true by the runtime if the OS supports reading
// GODEBUG early in runtime startup.
// This should not be changed after it is initialized.
var DebugOptions bool
// CacheLinePad is used to pad structs to avoid false sharing.
type CacheLinePad struct{ _ [CacheLinePadSize]byte }
// CacheLineSize is the CPU's assumed cache line size.
// There is currently no runtime detection of the real cache line size
// so we use the constant per GOARCH CacheLinePadSize as an approximation.
var CacheLineSize uintptr = CacheLinePadSize
// The booleans in X86 contain the correspondingly named cpuid feature bit.
// HasAVX and HasAVX2 are only set if the OS does support XMM and YMM registers
// in addition to the cpuid feature bit being set.
// The struct is padded to avoid false sharing.
var X86 struct {
_ CacheLinePad
HasAES bool
HasADX bool
HasAVX bool
HasAVX2 bool
HasBMI1 bool
HasBMI2 bool
HasERMS bool
HasFMA bool
HasPOPCNT bool
HasRDTSCP bool
HasSSE3 bool
HasSSSE3 bool
HasSSE41 bool
HasSSE42 bool
_ CacheLinePad
// The booleans in ARM contain the correspondingly named cpu feature bit.
// The struct is padded to avoid false sharing.
var ARM struct {
_ CacheLinePad
HasVFPv4 bool
HasIDIVA bool
_ CacheLinePad
// The booleans in ARM64 contain the correspondingly named cpu feature bit.
// The struct is padded to avoid false sharing.
var ARM64 struct {
_ CacheLinePad
HasAES bool
HasPMULL bool
HasSHA1 bool
HasSHA2 bool
HasCRC32 bool
HasCPUID bool
IsNeoverseN1 bool
IsZeus bool
_ CacheLinePad
var MIPS64X struct {
_ CacheLinePad
HasMSA bool // MIPS SIMD architecture
_ CacheLinePad
// For ppc64(le), it is safe to check only for ISA level starting on ISA v3.00,
// since there are no optional categories. There are some exceptions that also
// require kernel support to work (darn, scv), so there are feature bits for
// those as well. The minimum processor requirement is POWER8 (ISA 2.07).
// The struct is padded to avoid false sharing.
var PPC64 struct {
_ CacheLinePad
HasDARN bool // Hardware random number generator (requires kernel enablement)
HasSCV bool // Syscall vectored (requires kernel enablement)
IsPOWER8 bool // ISA v2.07 (POWER8)
IsPOWER9 bool // ISA v3.00 (POWER9)
_ CacheLinePad
var S390X struct {
_ CacheLinePad
HasZARCH bool // z architecture mode is active [mandatory]
HasSTFLE bool // store facility list extended [mandatory]
HasLDISP bool // long (20-bit) displacements [mandatory]
HasEIMM bool // 32-bit immediates [mandatory]
HasDFP bool // decimal floating point
HasETF3EH bool // ETF-3 enhanced
HasMSA bool // message security assist (CPACF)
HasAES bool // KM-AES{128,192,256} functions
HasAESCBC bool // KMC-AES{128,192,256} functions
HasAESCTR bool // KMCTR-AES{128,192,256} functions
HasAESGCM bool // KMA-GCM-AES{128,192,256} functions
HasGHASH bool // KIMD-GHASH function
HasSHA1 bool // K{I,L}MD-SHA-1 functions
HasSHA256 bool // K{I,L}MD-SHA-256 functions
HasSHA512 bool // K{I,L}MD-SHA-512 functions
HasSHA3 bool // K{I,L}MD-SHA3-{224,256,384,512} and K{I,L}MD-SHAKE-{128,256} functions
HasVX bool // vector facility. Note: the runtime sets this when it processes auxv records.
HasVXE bool // vector-enhancements facility 1
HasKDSA bool // elliptic curve functions
HasECDSA bool // NIST curves
HasEDDSA bool // Edwards curves
_ CacheLinePad
// Initialize examines the processor and sets the relevant variables above.
// This is called by the runtime package early in program initialization,
// before normal init functions are run. env is set by runtime if the OS supports
// cpu feature options in GODEBUG.
func Initialize(env string) {
// options contains the cpu debug options that can be used in GODEBUG.
// Options are arch dependent and are added by the arch specific doinit functions.
// Features that are mandatory for the specific GOARCH should not be added to options
// (e.g. SSE2 on amd64).
var options []option
// Option names should be lower case. e.g. avx instead of AVX.
type option struct {
Name string
Feature *bool
Specified bool // whether feature value was specified in GODEBUG
Enable bool // whether feature should be enabled
Required bool // whether feature is mandatory and can not be disabled
// processOptions enables or disables CPU feature values based on the parsed env string.
// The env string is expected to be of the form cpu.feature1=value1,cpu.feature2=value2...
// where feature names is one of the architecture specific list stored in the
// cpu packages options variable and values are either 'on' or 'off'.
// If env contains cpu.all=off then all cpu features referenced through the options
// variable are disabled. Other feature names and values result in warning messages.
func processOptions(env string) {
for env != "" {
field := ""
i := indexByte(env, ',')
if i < 0 {
field, env = env, ""
} else {
field, env = env[:i], env[i+1:]
if len(field) < 4 || field[:4] != "cpu." {
i = indexByte(field, '=')
if i < 0 {
print("GODEBUG: no value specified for \"", field, "\"\n")
key, value := field[4:i], field[i+1:] // e.g. "SSE2", "on"
var enable bool
switch value {
case "on":
enable = true
case "off":
enable = false
print("GODEBUG: value \"", value, "\" not supported for cpu option \"", key, "\"\n")
continue field
if key == "all" {
for i := range options {
options[i].Specified = true
options[i].Enable = enable || options[i].Required
continue field
for i := range options {
if options[i].Name == key {
options[i].Specified = true
options[i].Enable = enable
continue field
print("GODEBUG: unknown cpu feature \"", key, "\"\n")
for _, o := range options {
if !o.Specified {
if o.Enable && !*o.Feature {
print("GODEBUG: can not enable \"", o.Name, "\", missing CPU support\n")
if !o.Enable && o.Required {
print("GODEBUG: can not disable \"", o.Name, "\", required CPU feature\n")
*o.Feature = o.Enable
// indexByte returns the index of the first instance of c in s,
// or -1 if c is not present in s.
func indexByte(s string, c byte) int {
for i := 0; i < len(s); i++ {
if s[i] == c {
return i
return -1