runtime: add gc layout info for some basic types

src/internal/gclayout/gclayout.go

@@ -0,0 +1,33 @@
+package gclayout
+
+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
+)
+
+func init() {
+	var sizeBits uintptr
+
+	switch unsafe.Sizeof(uintptr(0)) {
+	case 8:
+		sizeBits = 6
+	case 4:
+		sizeBits = 5
+	case 2:
+		sizeBits = 4
+	}
+
+	var sizeShift = sizeBits + 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))
+}