runtime: simplify heapBitsSetType doubleCheck

The heapBitsSetType function has a slow doubleCheck debugging mode
that checks the bitmap written out by the rest of the function using
far more obvious logic. But even this has some surprisingly complex
logic in it. Simplify it a bit. This also happens to fix the logic on
32-bit.

Fixes golang#40335.

Change-Id: I5cee482ad8adbd01cf5b98e35a270fe941ba4940
Reviewed-on: https://go-review.googlesource.com/c/go/+/244538
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mbitmap.go

@@ -1403,17 +1403,20 @@ Phase4:
 	// Double check the whole bitmap.
 	if doubleCheck {
 		// x+size may not point to the heap, so back up one
-		// word and then call next().
-		end := heapBitsForAddr(x + size - sys.PtrSize).next()
-		endAI := arenaIdx(end.arena)
-		if !outOfPlace && (end.bitp == nil || (end.shift == 0 && end.bitp == &mheap_.arenas[endAI.l1()][endAI.l2()].bitmap[0])) {
-			// The unrolling code above walks hbitp just
-			// past the bitmap without moving to the next
-			// arena. Synthesize this for end.bitp.
-			end.arena--
-			endAI = arenaIdx(end.arena)
-			end.bitp = addb(&mheap_.arenas[endAI.l1()][endAI.l2()].bitmap[0], heapArenaBitmapBytes)
-			end.last = nil
+		// word and then advance it the way we do above.
+		end := heapBitsForAddr(x + size - sys.PtrSize)
+		if outOfPlace {
+			// In out-of-place copying, we just advance
+			// using next.
+			end = end.next()
+		} else {
+			// Don't use next because that may advance to
+			// the next arena and the in-place logic
+			// doesn't do that.
+			end.shift += heapBitsShift
+			if end.shift == 4*heapBitsShift {
+				end.bitp, end.shift = add1(end.bitp), 0
+			}
 		}
 		if typ.kind&kindGCProg == 0 && (hbitp != end.bitp || (w == nw+2) != (end.shift == 2)) {
 			println("ended at wrong bitmap byte for", typ.string(), "x", dataSize/typ.size)
@@ -1437,8 +1440,9 @@ Phase4:
 			var have, want uint8
 			have = (*h.bitp >> h.shift) & (bitPointer | bitScan)
 			if i >= totalptr {
-				want = 0 // deadmarker
 				if typ.kind&kindGCProg != 0 && i < (totalptr+3)/4*4 {
+					// heapBitsSetTypeGCProg always fills
+					// in full nibbles of bitScan.
 					want = bitScan
 				}
 			} else {