cpu: get hwcap/auxv from the Go 1.21+ runtime

Depends on https://go.dev/cl/458256

This change only does Linux for now.

Updates golang/go#57336

Change-Id: I0659697c1bdc6e2577c6251b964a0df32047ee12
Reviewed-on: https://go-review.googlesource.com/c/sys/+/465295
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

Author: bradfitz

cpu/hwcap_linux.go

@@ -24,6 +24,21 @@ var hwCap uint
 var hwCap2 uint
 
 func readHWCAP() error {
+	// For Go 1.21+, get auxv from the Go runtime.
+	if a := getAuxv(); len(a) > 0 {
+		for len(a) >= 2 {
+			tag, val := a[0], uint(a[1])
+			a = a[2:]
+			switch tag {
+			case _AT_HWCAP:
+				hwCap = val
+			case _AT_HWCAP2:
+				hwCap2 = val
+			}
+		}
+		return nil
+	}
+
 	buf, err := ioutil.ReadFile(procAuxv)
 	if err != nil {
 		// e.g. on android /proc/self/auxv is not accessible, so silently

cpu/runtime_auxv.go

@@ -0,0 +1,16 @@
+// 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.
+
+package cpu
+
+// getAuxvFn is non-nil on Go 1.21+ (via runtime_auxv_go121.go init)
+// on platforms that use auxv.
+var getAuxvFn func() []uintptr
+
+func getAuxv() []uintptr {
+	if getAuxvFn == nil {
+		return nil
+	}
+	return getAuxvFn()
+}

cpu/runtime_auxv_go121.go

@@ -0,0 +1,19 @@
+// 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 go1.21
+// +build go1.21
+
+package cpu
+
+import (
+	_ "unsafe" // for linkname
+)
+
+//go:linkname runtime_getAuxv runtime.getAuxv
+func runtime_getAuxv() []uintptr
+
+func init() {
+	getAuxvFn = runtime_getAuxv
+}

cpu/runtime_auxv_go121_test.go

@@ -0,0 +1,21 @@
+// 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 go1.21
+// +build go1.21
+
+package cpu
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestAuxvFromRuntime(t *testing.T) {
+	got := getAuxv()
+	t.Logf("got: %v", got) // notably: we didn't panic
+	if runtime.GOOS == "linux" && len(got) == 0 {
+		t.Errorf("expected auxv on linux; got zero entries")
+	}
+}