internal/gclayout: make gclayout values constants

src/internal/gclayout/gclayout.go

@@ -5,29 +5,20 @@ import "unsafe"
 // Internal constants for gc layout
 // See runtime/gc_precise.go
 
-var (
-	NoPtrs  unsafe.Pointer
-	Pointer unsafe.Pointer
-	String  unsafe.Pointer
-	Slice   unsafe.Pointer
-)
+type Layout uintptr
 
-func init() {
-	var sizeBits uintptr
+const (
+	// 16-bit int => bits = 4
+	// 32-bit int => bits = 5
+	// 64-bit int => bits = 6
+	sizeBits = 4 + unsafe.Sizeof(uintptr(0))/4
 
-	switch unsafe.Sizeof(uintptr(0)) {
-	case 8:
-		sizeBits = 6
-	case 4:
-		sizeBits = 5
-	case 2:
-		sizeBits = 4
-	}
+	sizeShift = sizeBits + 1
 
-	var sizeShift = sizeBits + 1
+	NoPtrs  = Layout(uintptr(0b0<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
+	Pointer = Layout(uintptr(0b1<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
+	String  = Layout(uintptr(0b01<<sizeShift) | uintptr(0b10<<1) | uintptr(1))
+	Slice   = Layout(uintptr(0b001<<sizeShift) | uintptr(0b11<<1) | uintptr(1))
+)
 
-	NoPtrs = unsafe.Pointer(uintptr(0b0<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
-	Pointer = unsafe.Pointer(uintptr(0b1<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
-	String = unsafe.Pointer(uintptr(0b01<<sizeShift) | uintptr(0b10<<1) | uintptr(1))
-	Slice = unsafe.Pointer(uintptr(0b001<<sizeShift) | uintptr(0b11<<1) | uintptr(1))
-}
+func (l Layout) AsPtr() unsafe.Pointer { return unsafe.Pointer(l) }

src/internal/reflectlite/type.go

@@ -708,16 +708,16 @@ func (r *RawType) gcLayout() unsafe.Pointer {
 	kind := r.Kind()
 
 	if kind < String {
-		return gclayout.NoPtrs
+		return gclayout.NoPtrs.AsPtr()
 	}
 
 	switch kind {
 	case Pointer, UnsafePointer, Chan, Map:
-		return gclayout.Pointer
+		return gclayout.Pointer.AsPtr()
 	case String:
-		return gclayout.String
+		return gclayout.String.AsPtr()
 	case Slice:
-		return gclayout.Slice
+		return gclayout.Slice.AsPtr()
 	}
 
 	// Unknown (for now); let the conservative pointer scanning handle it

src/runtime/gc_boehm.go

@@ -89,7 +89,7 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 	gcLock.Lock()
 	needsResumeWorld = false
 	var ptr unsafe.Pointer
-	if layout == gclayout.NoPtrs {
+	if layout == gclayout.NoPtrs.AsPtr() {
 		// This object is entirely pointer free, for example make([]int, ...).
 		// Make sure the GC knows this so it doesn't scan the object
 		// unnecessarily to improve performance.

src/runtime/slice.go

@@ -51,7 +51,7 @@ func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr)
 	var layout unsafe.Pointer
 	// less type info here; can only go off element size
 	if elemSize < unsafe.Sizeof(uintptr(0)) {
-		layout = gclayout.NoPtrs
+		layout = gclayout.NoPtrs.AsPtr()
 	}
 
 	buf := alloc(newCap*elemSize, layout)

src/runtime/string.go

@@ -60,7 +60,7 @@ func stringConcat(x, y _string) _string {
 		return x
 	} else {
 		length := x.length + y.length
-		buf := alloc(length, gclayout.NoPtrs)
+		buf := alloc(length, gclayout.NoPtrs.AsPtr())
 		memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 		memcpy(unsafe.Add(buf, x.length), unsafe.Pointer(y.ptr), y.length)
 		return _string{ptr: (*byte)(buf), length: length}
@@ -73,7 +73,7 @@ func stringFromBytes(x struct {
 	len uintptr
 	cap uintptr
 }) _string {
-	buf := alloc(x.len, gclayout.NoPtrs)
+	buf := alloc(x.len, gclayout.NoPtrs.AsPtr())
 	memcpy(buf, unsafe.Pointer(x.ptr), x.len)
 	return _string{ptr: (*byte)(buf), length: x.len}
 }
@@ -84,7 +84,7 @@ func stringToBytes(x _string) (slice struct {
 	len uintptr
 	cap uintptr
 }) {
-	buf := alloc(x.length, gclayout.NoPtrs)
+	buf := alloc(x.length, gclayout.NoPtrs.AsPtr())
 	memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 	slice.ptr = (*byte)(buf)
 	slice.len = x.length
@@ -101,7 +101,7 @@ func stringFromRunes(runeSlice []rune) (s _string) {
 	}
 
 	// Allocate memory for the string.
-	s.ptr = (*byte)(alloc(s.length, gclayout.NoPtrs))
+	s.ptr = (*byte)(alloc(s.length, gclayout.NoPtrs.AsPtr()))
 
 	// Encode runes to UTF-8 and store the resulting bytes in the string.
 	index := uintptr(0)