runtime: test that write barriers are correctly marked unpreemptible

Followon to CL 518055.

Change-Id: I05c4b429f49feb7012070e467fefbf3392260915
Reviewed-on: https://go-review.googlesource.com/c/go/+/518538
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/go/internal/test/test.go

@@ -1026,6 +1026,11 @@ func builderTest(b *work.Builder, ctx context.Context, pkgOpts load.PackageOpts,
 
 	pmain.Dir = testDir
 	pmain.Internal.OmitDebug = !testC && !testNeedBinary()
+	if pmain.ImportPath == "runtime.test" {
+		// The runtime package needs a symbolized binary for its tests.
+		// See runtime/unsafepoint_test.go.
+		pmain.Internal.OmitDebug = false
+	}
 
 	if !cfg.BuildN {
 		// writeTestmain writes _testmain.go,

src/runtime/export_test.go

@@ -7,6 +7,7 @@
 package runtime
 
 import (
+	"internal/abi"
 	"internal/goarch"
 	"internal/goos"
 	"runtime/internal/atomic"
@@ -1940,3 +1941,21 @@ func MyGenericFunc[T any]() {
 		testUintptr = 4
 	})
 }
+
+func UnsafePoint(pc uintptr) bool {
+	fi := findfunc(pc)
+	v := pcdatavalue(fi, abi.PCDATA_UnsafePoint, pc, nil)
+	switch v {
+	case abi.UnsafePointUnsafe:
+		return true
+	case abi.UnsafePointSafe:
+		return false
+	case abi.UnsafePointRestart1, abi.UnsafePointRestart2, abi.UnsafePointRestartAtEntry:
+		// These are all interruptible, they just encode a nonstandard
+		// way of recovering when interrupted.
+		return false
+	default:
+		var buf [20]byte
+		panic("invalid unsafe point code " + string(itoa(buf[:], uint64(v))))
+	}
+}

src/runtime/unsafepoint_test.go

@@ -0,0 +1,122 @@
+// 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 runtime_test
+
+import (
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"reflect"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+// This is the function we'll be testing.
+// It has a simple write barrier in it.
+func setGlobalPointer() {
+	globalPointer = nil
+}
+
+var globalPointer *int
+
+func TestUnsafePoint(t *testing.T) {
+	testenv.MustHaveExec(t)
+	switch runtime.GOARCH {
+	case "amd64", "arm64":
+	default:
+		t.Skipf("test not enabled for %s", runtime.GOARCH)
+	}
+
+	// Get a reference we can use to ask the runtime about
+	// which of its instructions are unsafe preemption points.
+	f := runtime.FuncForPC(reflect.ValueOf(setGlobalPointer).Pointer())
+
+	// Disassemble the test function.
+	// Note that normally "go test runtime" would strip symbols
+	// and prevent this step from working. So there's a hack in
+	// cmd/go/internal/test that exempts runtime tests from
+	// symbol stripping.
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "objdump", "-s", "setGlobalPointer", os.Args[0])
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("can't objdump %v", err)
+	}
+	lines := strings.Split(string(out), "\n")[1:]
+
+	// Walk through assembly instructions, checking preemptible flags.
+	var entry uint64
+	var startedWB bool
+	var doneWB bool
+	instructionCount := 0
+	unsafeCount := 0
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		t.Logf("%s", line)
+		parts := strings.Fields(line)
+		if len(parts) < 4 {
+			continue
+		}
+		if !strings.HasPrefix(parts[0], "unsafepoint_test.go:") {
+			continue
+		}
+		pc, err := strconv.ParseUint(parts[1][2:], 16, 64)
+		if err != nil {
+			t.Fatalf("can't parse pc %s: %v", parts[1], err)
+		}
+		if entry == 0 {
+			entry = pc
+		}
+		// Note that some platforms do ASLR, so the PCs in the disassembly
+		// don't match PCs in the address space. Only offsets from function
+		// entry make sense.
+		unsafe := runtime.UnsafePoint(f.Entry() + uintptr(pc-entry))
+		t.Logf("unsafe: %v\n", unsafe)
+		instructionCount++
+		if unsafe {
+			unsafeCount++
+		}
+
+		// All the instructions inside the write barrier must be unpreemptible.
+		if startedWB && !doneWB && !unsafe {
+			t.Errorf("instruction %s must be marked unsafe, but isn't", parts[1])
+		}
+
+		// Detect whether we're in the write barrier.
+		switch runtime.GOARCH {
+		case "arm64":
+			if parts[3] == "MOVWU" {
+				// The unpreemptible region starts after the
+				// load of runtime.writeBarrier.
+				startedWB = true
+			}
+			if parts[3] == "MOVD" && parts[4] == "ZR," {
+				// The unpreemptible region ends after the
+				// write of nil.
+				doneWB = true
+			}
+		case "amd64":
+			if parts[3] == "CMPL" {
+				startedWB = true
+			}
+			if parts[3] == "MOVQ" && parts[4] == "$0x0," {
+				doneWB = true
+			}
+		}
+	}
+
+	if instructionCount == 0 {
+		t.Errorf("no instructions")
+	}
+	if unsafeCount == instructionCount {
+		t.Errorf("no interruptible instructions")
+	}
+	// Note that there are other instructions marked unpreemptible besides
+	// just the ones required by the write barrier. Those include possibly
+	// the preamble and postamble, as well as bleeding out from the
+	// write barrier proper into adjacent instructions (in both directions).
+	// Hopefully we can clean up the latter at some point.
+}