internal/cpu, runtime: make linux/loong64 HWCAP data available

This can be used to toggle runtime usages of ISA extensions as such
usages appear.

Only the CRC32 bit is exposed for now, as the others are not going to be
utilized in the standard library for a while.

Change-Id: I774032ca84dc8bcf1c9f17558917315af07c7314
Reviewed-on: https://go-review.googlesource.com/c/go/+/482416
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: xiaodong liu <teaofmoli@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>

src/internal/cpu/cpu.go

@@ -78,6 +78,14 @@ var ARM64 struct {
 	_          CacheLinePad
 }
 
+// The booleans in Loong64 contain the correspondingly named cpu feature bit.
+// The struct is padded to avoid false sharing.
+var Loong64 struct {
+	_        CacheLinePad
+	HasCRC32 bool
+	_        CacheLinePad
+}
+
 var MIPS64X struct {
 	_      CacheLinePad
 	HasMSA bool // MIPS SIMD architecture

src/internal/cpu/cpu_loong64.go

@@ -10,4 +10,10 @@ package cpu
 // We choose 64 because Loongson 3A5000 the L1 Dcache is 4-way 256-line 64-byte-per-line.
 const CacheLinePadSize = 64
 
-func doinit() {}
+func doinit() {
+	options = []option{
+		{Name: "crc32", Feature: &Loong64.HasCRC32},
+	}
+
+	osInit()
+}

src/internal/cpu/cpu_loong64_hwcap.go

@@ -0,0 +1,30 @@
+// Copyright 2023 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.
+
+//go:build loong64 && linux
+
+package cpu
+
+// This is initialized by archauxv and should not be changed after it is
+// initialized.
+var HWCap uint
+
+// HWCAP bits. These are exposed by the Linux kernel.
+const (
+	hwcap_LOONGARCH_CRC32 = 1 << 6
+)
+
+func hwcapInit() {
+	// It is not taken from CPUCFG data regardless of availability of
+	// CPUCFG, because the CPUCFG data only reflects capabilities of the
+	// hardware, but not kernel support.
+	//
+	// As of 2023, we do not know for sure if the CPUCFG data can be
+	// patched in software, nor does any known LoongArch kernel do that.
+	Loong64.HasCRC32 = isSet(HWCap, hwcap_LOONGARCH_CRC32)
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}

src/internal/cpu/cpu_loong64_linux.go

@@ -0,0 +1,11 @@
+// Copyright 2023 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.
+
+//go:build loong64 && linux
+
+package cpu
+
+func osInit() {
+	hwcapInit()
+}

src/runtime/os_linux_loong64.go

@@ -6,6 +6,13 @@
 
 package runtime
 
-func archauxv(tag, val uintptr) {}
+import "internal/cpu"
+
+func archauxv(tag, val uintptr) {
+	switch tag {
+	case _AT_HWCAP:
+		cpu.HWCap = uint(val)
+	}
+}
 
 func osArchInit() {}