Source File
	cpu.go

Belonging Package
	internal/cpu

// 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
	HasOSXSAVE   bool
	HasPCLMULQDQ bool
	HasPOPCNT    bool
	HasRDTSCP    bool
	HasSHA       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
	HasSHA512  bool
	HasCRC32   bool
	HasATOMICS bool
	HasCPUID   bool
	IsNeoverse 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)
	IsPOWER10 bool // ISA v3.1  (POWER10)
	_         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) {
	doinit()
	processOptions(env)
}

// 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
}

// 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) {
field:
	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." {
			continue
		}
		i = indexByte(field, '=')
		if i < 0 {
			print("GODEBUG: no value specified for \"", field, "\"\n")
			continue
		}
		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
		default:
			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
			}
			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 {
			continue
		}

		if o.Enable && !*o.Feature {
			print("GODEBUG: can not enable \"", o.Name, "\", missing CPU support\n")
			continue
		}

		*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
}

The pages are generated with Golds v0.6.7. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @Go100and1 (reachable from the left QR code) to get the latest news of Golds.