runtime: use smaller fields for mspan.freeindex and nelems

mspan.freeindex and nelems can fit into uint16 for all possible
values. Use uint16 instead of uintptr.

Change-Id: Ifce20751e81d5022be1f6b5cbb5fbe4fd1728b1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/451359
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

9 files changed, 40 insertions, 41 deletions

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 6376445110..f81e8a9ea1 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -1372,7 +1372,7 @@ func FreeMSpan(s *MSpan) {
 
 func MSpanCountAlloc(ms *MSpan, bits []byte) int {
 	s := (*mspan)(ms)
-	s.nelems = uintptr(len(bits) * 8)
+	s.nelems = uint16(len(bits) * 8)
 	s.gcmarkBits = (*gcBits)(unsafe.Pointer(&bits[0]))
 	result := s.countAlloc()
 	s.gcmarkBits = nil
diff --git a/src/runtime/heapdump.go b/src/runtime/heapdump.go
index 4283aac320..2394a3e9eb 100644
--- a/src/runtime/heapdump.go
+++ b/src/runtime/heapdump.go
@@ -488,8 +488,8 @@ func dumpobjs() {
 			throw("freemark array doesn't have enough entries")
 		}
 
-		for freeIndex := uintptr(0); freeIndex < s.nelems; freeIndex++ {
-			if s.isFree(freeIndex) {
+		for freeIndex := uint16(0); freeIndex < s.nelems; freeIndex++ {
+			if s.isFree(uintptr(freeIndex)) {
 				freemark[freeIndex] = true
 			}
 		}
diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index ec2e547d3f..a071428391 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -886,7 +886,7 @@ var zerobase uintptr
 func nextFreeFast(s *mspan) gclinkptr {
 	theBit := sys.TrailingZeros64(s.allocCache) // Is there a free object in the allocCache?
 	if theBit < 64 {
-		result := s.freeindex + uintptr(theBit)
+		result := s.freeindex + uint16(theBit)
 		if result < s.nelems {
 			freeidx := result + 1
 			if freeidx%64 == 0 && freeidx != s.nelems {
@@ -895,7 +895,7 @@ func nextFreeFast(s *mspan) gclinkptr {
 			s.allocCache >>= uint(theBit + 1)
 			s.freeindex = freeidx
 			s.allocCount++
-			return gclinkptr(result*s.elemsize + s.base())
+			return gclinkptr(uintptr(result)*s.elemsize + s.base())
 		}
 	}
 	return 0
@@ -916,7 +916,7 @@ func (c *mcache) nextFree(spc spanClass) (v gclinkptr, s *mspan, shouldhelpgc bo
 	freeIndex := s.nextFreeIndex()
 	if freeIndex == s.nelems {
 		// The span is full.
-		if uintptr(s.allocCount) != s.nelems {
+		if s.allocCount != s.nelems {
 			println("runtime: s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
 			throw("s.allocCount != s.nelems && freeIndex == s.nelems")
 		}
@@ -931,9 +931,9 @@ func (c *mcache) nextFree(spc spanClass) (v gclinkptr, s *mspan, shouldhelpgc bo
 		throw("freeIndex is not valid")
 	}
 
-	v = gclinkptr(freeIndex*s.elemsize + s.base())
+	v = gclinkptr(uintptr(freeIndex)*s.elemsize + s.base())
 	s.allocCount++
-	if uintptr(s.allocCount) > s.nelems {
+	if s.allocCount > s.nelems {
 		println("s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
 		throw("s.allocCount > s.nelems")
 	}
diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index 4ba25901d4..20323ff82d 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -117,8 +117,8 @@ func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
 // and negates them so that ctz (count trailing zeros) instructions
 // can be used. It then places these 8 bytes into the cached 64 bit
 // s.allocCache.
-func (s *mspan) refillAllocCache(whichByte uintptr) {
-	bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(whichByte)))
+func (s *mspan) refillAllocCache(whichByte uint16) {
+	bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(uintptr(whichByte))))
 	aCache := uint64(0)
 	aCache |= uint64(bytes[0])
 	aCache |= uint64(bytes[1]) << (1 * 8)
@@ -135,7 +135,7 @@ func (s *mspan) refillAllocCache(whichByte uintptr) {
 // or after s.freeindex.
 // There are hardware instructions that can be used to make this
 // faster if profiling warrants it.
-func (s *mspan) nextFreeIndex() uintptr {
+func (s *mspan) nextFreeIndex() uint16 {
 	sfreeindex := s.freeindex
 	snelems := s.nelems
 	if sfreeindex == snelems {
@@ -163,7 +163,7 @@ func (s *mspan) nextFreeIndex() uintptr {
 		// nothing available in cached bits
 		// grab the next 8 bytes and try again.
 	}
-	result := sfreeindex + uintptr(bitIndex)
+	result := sfreeindex + uint16(bitIndex)
 	if result >= snelems {
 		s.freeindex = snelems
 		return snelems
@@ -191,7 +191,7 @@ func (s *mspan) nextFreeIndex() uintptr {
 // been no preemption points since ensuring this (which could allow a
 // GC transition, which would allow the state to change).
 func (s *mspan) isFree(index uintptr) bool {
-	if index < s.freeIndexForScan {
+	if index < uintptr(s.freeIndexForScan) {
 		return false
 	}
 	bytep, mask := s.allocBits.bitp(index)
@@ -751,7 +751,7 @@ func (s *mspan) initHeapBits(forceClear bool) {
 // scanning the allocation bitmap.
 func (s *mspan) countAlloc() int {
 	count := 0
-	bytes := divRoundUp(s.nelems, 8)
+	bytes := divRoundUp(uintptr(s.nelems), 8)
 	// Iterate over each 8-byte chunk and count allocations
 	// with an intrinsic. Note that newMarkBits guarantees that
 	// gcmarkBits will be 8-byte aligned, so we don't have to
diff --git a/src/runtime/mcache.go b/src/runtime/mcache.go
index 757d09787d..d4b6eef13a 100644
--- a/src/runtime/mcache.go
+++ b/src/runtime/mcache.go
@@ -148,7 +148,7 @@ func (c *mcache) refill(spc spanClass) {
 	// Return the current cached span to the central lists.
 	s := c.alloc[spc]
 
-	if uintptr(s.allocCount) != s.nelems {
+	if s.allocCount != s.nelems {
 		throw("refill of span with free space remaining")
 	}
 	if s != &emptymspan {
@@ -184,7 +184,7 @@ func (c *mcache) refill(spc spanClass) {
 		throw("out of memory")
 	}
 
-	if uintptr(s.allocCount) == s.nelems {
+	if s.allocCount == s.nelems {
 		throw("span has no free space")
 	}
 
@@ -284,7 +284,7 @@ func (c *mcache) releaseAll() {
 				//
 				// If this span was cached before sweep, then gcController.heapLive was totally
 				// recomputed since caching this span, so we don't do this for stale spans.
-				dHeapLive -= int64(s.nelems-uintptr(s.allocCount)) * int64(s.elemsize)
+				dHeapLive -= int64(s.nelems-s.allocCount) * int64(s.elemsize)
 			}
 
 			// Release the span to the mcentral.
diff --git a/src/runtime/mcentral.go b/src/runtime/mcentral.go
index 78611994f3..b1dcbd7e7d 100644
--- a/src/runtime/mcentral.go
+++ b/src/runtime/mcentral.go
@@ -174,7 +174,7 @@ havespan:
 		traceGCSweepDone()
 	}
 	n := int(s.nelems) - int(s.allocCount)
-	if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
+	if n == 0 || s.freeindex == s.nelems || s.allocCount == s.nelems {
 		throw("span has no free objects")
 	}
 	freeByteBase := s.freeindex &^ (64 - 1)
diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index 68f1aae600..986eb573ca 100644
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -602,8 +602,8 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 		// efficient; allocfreetrace has massive overhead.
 		mbits := s.markBitsForBase()
 		abits := s.allocBitsForIndex(0)
-		for i := uintptr(0); i < s.nelems; i++ {
-			if !mbits.isMarked() && (abits.index < s.freeindex || abits.isMarked()) {
+		for i := uintptr(0); i < uintptr(s.nelems); i++ {
+			if !mbits.isMarked() && (abits.index < uintptr(s.freeindex) || abits.isMarked()) {
 				x := s.base() + i*s.elemsize
 				if debug.allocfreetrace != 0 {
 					tracefree(unsafe.Pointer(x), size)
@@ -634,12 +634,12 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 		//
 		// Check the first bitmap byte, where we have to be
 		// careful with freeindex.
-		obj := s.freeindex
+		obj := uintptr(s.freeindex)
 		if (*s.gcmarkBits.bytep(obj / 8)&^*s.allocBits.bytep(obj / 8))>>(obj%8) != 0 {
 			s.reportZombies()
 		}
 		// Check remaining bytes.
-		for i := obj/8 + 1; i < divRoundUp(s.nelems, 8); i++ {
+		for i := obj/8 + 1; i < divRoundUp(uintptr(s.nelems), 8); i++ {
 			if *s.gcmarkBits.bytep(i)&^*s.allocBits.bytep(i) != 0 {
 				s.reportZombies()
 			}
@@ -666,7 +666,7 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 	// gcmarkBits becomes the allocBits.
 	// get a fresh cleared gcmarkBits in preparation for next GC
 	s.allocBits = s.gcmarkBits
-	s.gcmarkBits = newMarkBits(s.nelems)
+	s.gcmarkBits = newMarkBits(uintptr(s.nelems))
 
 	// refresh pinnerBits if they exists
 	if s.pinnerBits != nil {
@@ -760,7 +760,7 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 				return true
 			}
 			// Return span back to the right mcentral list.
-			if uintptr(nalloc) == s.nelems {
+			if nalloc == s.nelems {
 				mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
 			} else {
 				mheap_.central[spc].mcentral.partialSwept(sweepgen).push(s)
@@ -829,10 +829,10 @@ func (s *mspan) reportZombies() {
 	print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n")
 	mbits := s.markBitsForBase()
 	abits := s.allocBitsForIndex(0)
-	for i := uintptr(0); i < s.nelems; i++ {
+	for i := uintptr(0); i < uintptr(s.nelems); i++ {
 		addr := s.base() + i*s.elemsize
 		print(hex(addr))
-		alloc := i < s.freeindex || abits.isMarked()
+		alloc := i < uintptr(s.freeindex) || abits.isMarked()
 		if alloc {
 			print(" alloc")
 		} else {
diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index 0ba45009eb..42318ca04c 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -435,10 +435,17 @@ type mspan struct {
 	// undefined and should never be referenced.
 	//
 	// Object n starts at address n*elemsize + (start << pageShift).
-	freeindex uintptr
+	freeindex uint16
 	// TODO: Look up nelems from sizeclass and remove this field if it
 	// helps performance.
-	nelems uintptr // number of object in the span.
+	nelems uint16 // number of object in the span.
+	// freeIndexForScan is like freeindex, except that freeindex is
+	// used by the allocator whereas freeIndexForScan is used by the
+	// GC scanner. They are two fields so that the GC sees the object
+	// is allocated only when the object and the heap bits are
+	// initialized (see also the assignment of freeIndexForScan in
+	// mallocgc, and issue 54596).
+	freeIndexForScan uint16
 
 	// Cache of the allocBits at freeindex. allocCache is shifted
 	// such that the lowest bit corresponds to the bit freeindex.
@@ -495,14 +502,6 @@ type mspan struct {
 	speciallock           mutex         // guards specials list and changes to pinnerBits
 	specials              *special      // linked list of special records sorted by offset.
 	userArenaChunkFree    addrRange     // interval for managing chunk allocation
-
-	// freeIndexForScan is like freeindex, except that freeindex is
-	// used by the allocator whereas freeIndexForScan is used by the
-	// GC scanner. They are two fields so that the GC sees the object
-	// is allocated only when the object and the heap bits are
-	// initialized (see also the assignment of freeIndexForScan in
-	// mallocgc, and issue 54596).
-	freeIndexForScan uintptr
 }
 
 func (s *mspan) base() uintptr {
@@ -1403,7 +1402,7 @@ func (h *mheap) initSpan(s *mspan, typ spanAllocType, spanclass spanClass, base,
 			s.divMul = 0
 		} else {
 			s.elemsize = uintptr(class_to_size[sizeclass])
-			s.nelems = nbytes / s.elemsize
+			s.nelems = uint16(nbytes / s.elemsize)
 			s.divMul = class_to_divmagic[sizeclass]
 		}
 
@@ -1411,8 +1410,8 @@ func (h *mheap) initSpan(s *mspan, typ spanAllocType, spanclass spanClass, base,
 		s.freeindex = 0
 		s.freeIndexForScan = 0
 		s.allocCache = ^uint64(0) // all 1s indicating all free.
-		s.gcmarkBits = newMarkBits(s.nelems)
-		s.allocBits = newAllocBits(s.nelems)
+		s.gcmarkBits = newMarkBits(uintptr(s.nelems))
+		s.allocBits = newAllocBits(uintptr(s.nelems))
 
 		// It's safe to access h.sweepgen without the heap lock because it's
 		// only ever updated with the world stopped and we run on the
diff --git a/src/runtime/pinner.go b/src/runtime/pinner.go
index c0db91eba8..0cc48e2164 100644
--- a/src/runtime/pinner.go
+++ b/src/runtime/pinner.go
@@ -267,14 +267,14 @@ func (p *pinnerBits) ofObject(n uintptr) pinState {
 }
 
 func (s *mspan) pinnerBitSize() uintptr {
-	return divRoundUp(s.nelems*2, 8)
+	return divRoundUp(uintptr(s.nelems)*2, 8)
 }
 
 // newPinnerBits returns a pointer to 8 byte aligned bytes to be used for this
 // span's pinner bits. newPinneBits is used to mark objects that are pinned.
 // They are copied when the span is swept.
 func (s *mspan) newPinnerBits() *pinnerBits {
-	return (*pinnerBits)(newMarkBits(s.nelems * 2))
+	return (*pinnerBits)(newMarkBits(uintptr(s.nelems) * 2))
 }
 
 // nosplit, because it's called by isPinned, which is nosplit