runtime: split out pointer/scalar metadata from heapArena

We're going to want to fork this data in the near future for a
GOEXPERIMENT, so break it out now.

Change-Id: Ia7ded850bb693c443fe439c6b7279dcac638512c
Reviewed-on: https://go-review.googlesource.com/c/go/+/537978
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/runtime/mbitmap.go

@@ -46,6 +46,27 @@ import (
 	"unsafe"
 )
 
+// heapArenaPtrScalar contains the per-heapArena pointer/scalar metadata for the GC.
+type heapArenaPtrScalar struct {
+	// bitmap stores the pointer/scalar bitmap for the words in
+	// this arena. See mbitmap.go for a description.
+	// This array uses 1 bit per word of heap, or 1.6% of the heap size (for 64-bit).
+	bitmap [heapArenaBitmapWords]uintptr
+
+	// If the ith bit of noMorePtrs is true, then there are no more
+	// pointers for the object containing the word described by the
+	// high bit of bitmap[i].
+	// In that case, bitmap[i+1], ... must be zero until the start
+	// of the next object.
+	// We never operate on these entries using bit-parallel techniques,
+	// so it is ok if they are small. Also, they can't be bigger than
+	// uint16 because at that size a single noMorePtrs entry
+	// represents 8K of memory, the minimum size of a span. Any larger
+	// and we'd have to worry about concurrent updates.
+	// This array uses 1 bit per word of bitmap, or .024% of the heap size (for 64-bit).
+	noMorePtrs [heapArenaBitmapWords / 8]uint8
+}
+
 // addb returns the byte pointer p+n.
 //
 //go:nowritebarrier

src/runtime/mheap.go

@@ -239,23 +239,8 @@ var mheap_ mheap
 type heapArena struct {
 	_ sys.NotInHeap
 
-	// bitmap stores the pointer/scalar bitmap for the words in
-	// this arena. See mbitmap.go for a description.
-	// This array uses 1 bit per word of heap, or 1.6% of the heap size (for 64-bit).
-	bitmap [heapArenaBitmapWords]uintptr
-
-	// If the ith bit of noMorePtrs is true, then there are no more
-	// pointers for the object containing the word described by the
-	// high bit of bitmap[i].
-	// In that case, bitmap[i+1], ... must be zero until the start
-	// of the next object.
-	// We never operate on these entries using bit-parallel techniques,
-	// so it is ok if they are small. Also, they can't be bigger than
-	// uint16 because at that size a single noMorePtrs entry
-	// represents 8K of memory, the minimum size of a span. Any larger
-	// and we'd have to worry about concurrent updates.
-	// This array uses 1 bit per word of bitmap, or .024% of the heap size (for 64-bit).
-	noMorePtrs [heapArenaBitmapWords / 8]uint8
+	// heapArenaPtrScalar contains pointer/scalar data about the heap for this heap arena.
+	heapArenaPtrScalar
 
 	// spans maps from virtual address page ID within this arena to *mspan.
 	// For allocated spans, their pages map to the span itself.