internal/gclayout: make gclayout values constants

The previous versions calculated at init() prevented `interp` from running
in many cases, increasing compile times due to the increased need to revert
the partially interpreted results and also increasing binary runtime because
fewer optimizations had happened during interp.

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
+
+const (
+	// 16-bit int => bits = 4
+	// 32-bit int => bits = 5
+	// 64-bit int => bits = 6
+	sizeBits = 4 + unsafe.Sizeof(uintptr(0))/4
+
+	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))
 )
 
-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))
-}
+func (l Layout) AsPtr() unsafe.Pointer { return unsafe.Pointer(l) }