runtime: make PCDATA_RegMapUnsafe more clear and remove magic number

Change-Id: Ibf3ee755c3fbec03a9396840dc92ce148c49d9f7
GitHub-Last-Rev: 945d8aaa13
GitHub-Pull-Request: golang/go#41262
Reviewed-on: https://go-review.googlesource.com/c/go/+/253377
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/objabi/funcdata.go

@@ -35,7 +35,7 @@ const (
 	// PCDATA_RegMapIndex values.
 	//
 	// Only if !go115ReduceLiveness.
-	PCDATA_RegMapUnsafe = -2 // Unsafe for async preemption
+	PCDATA_RegMapUnsafe = PCDATA_UnsafePointUnsafe // Unsafe for async preemption
 
 	// PCDATA_UnsafePoint values.
 	PCDATA_UnsafePointSafe   = -1 // Safe for async preemption

src/runtime/debugcall.go

@@ -87,7 +87,7 @@ func debugCallCheck(pc uintptr) string {
 				pcdata = 0 // in prologue
 			}
 			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_RegPointerMaps))
-			if pcdata == -2 || stkmap == nil {
+			if pcdata == _PCDATA_RegMapUnsafe || stkmap == nil {
 				// Not at a safe point.
 				ret = debugCallUnsafePoint
 				return

src/runtime/preempt.go

@@ -406,7 +406,7 @@ func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) (bool, uintptr) {
 	var startpc uintptr
 	if !go115ReduceLiveness {
 		smi := pcdatavalue(f, _PCDATA_RegMapIndex, pc, nil)
-		if smi == -2 {
+		if smi == _PCDATA_RegMapUnsafe {
 			// Unsafe-point marked by compiler. This includes
 			// atomic sequences (e.g., write barrier) and nosplit
 			// functions (except at calls).

src/runtime/symtab.go

@@ -284,6 +284,9 @@ const (
 )
 
 const (
+	// Only if !go115ReduceLiveness.
+	_PCDATA_RegMapUnsafe = _PCDATA_UnsafePointUnsafe // Unsafe for async preemption
+
 	// PCDATA_UnsafePoint values.
 	_PCDATA_UnsafePointSafe   = -1 // Safe for async preemption
 	_PCDATA_UnsafePointUnsafe = -2 // Unsafe for async preemption