runtime: use divRoundUp

There are a handful of places where the runtime wants to round up the
result of a division. We just introduced a helper to do this. This CL
replaces all of the hand-coded round-ups (that I could find) with this
helper.

Change-Id: I465d152157ff0f3cad40c0aa57491e4f2de510ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/224385
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/malloc.go

@@ -1035,9 +1035,9 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 		} else {
 			var sizeclass uint8
 			if size <= smallSizeMax-8 {
-				sizeclass = size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]
+				sizeclass = size_to_class8[divRoundUp(size, smallSizeDiv)]
 			} else {
-				sizeclass = size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]
+				sizeclass = size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]
 			}
 			size = uintptr(class_to_size[sizeclass])
 			spc := makeSpanClass(sizeclass, noscan)

src/runtime/mgcmark.go

@@ -54,7 +54,7 @@ func gcMarkRootPrepare() {
 
 	// Compute how many data and BSS root blocks there are.
 	nBlocks := func(bytes uintptr) int {
-		return int((bytes + rootBlockBytes - 1) / rootBlockBytes)
+		return int(divRoundUp(bytes, rootBlockBytes))
 	}
 
 	work.nDataRoots = 0

src/runtime/mgcsweepbuf.go

@@ -144,7 +144,7 @@ func (b *gcSweepBuf) pop() *mspan {
 // intervening pops. Spans that are pushed after the call may also
 // appear in these blocks.
 func (b *gcSweepBuf) numBlocks() int {
-	return int((atomic.Load(&b.index) + gcSweepBlockEntries - 1) / gcSweepBlockEntries)
+	return int(divRoundUp(uintptr(atomic.Load(&b.index)), gcSweepBlockEntries))
 }
 
 // block returns the spans in the i'th block of buffer b. block is

src/runtime/msize.go

@@ -13,9 +13,9 @@ package runtime
 func roundupsize(size uintptr) uintptr {
 	if size < _MaxSmallSize {
 		if size <= smallSizeMax-8 {
-			return uintptr(class_to_size[size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]])
+			return uintptr(class_to_size[size_to_class8[divRoundUp(size, smallSizeDiv)]])
 		} else {
-			return uintptr(class_to_size[size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]])
+			return uintptr(class_to_size[size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]])
 		}
 	}
 	if size+_PageSize < size {