runtime: eliminate write barriers from alloc/mark bitmaps

This introduces a new type, *gcBits, to use for alloc/mark bitmap
allocations instead of *uint8. This type is marked go:notinheap, so
uses of it correctly eliminate write barriers. Since we now have a
type, this also extracts some common operations to methods both for
convenience and to avoid (*uint8) casts at most use sites.

For #19325.

Change-Id: Id51f734fb2e96b8b7715caa348c8dcd4aef0696a
Reviewed-on: https://go-review.googlesource.com/38580
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

2 files changed, 32 insertions, 22 deletions

diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index eb36450508..8075f66115 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -182,10 +182,8 @@ type markBits struct {
 
 //go:nosplit
 func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
-	whichByte := allocBitIndex / 8
-	whichBit := allocBitIndex % 8
-	bytePtr := addb(s.allocBits, whichByte)
-	return markBits{bytePtr, uint8(1 << whichBit), allocBitIndex}
+	bytep, mask := s.allocBits.bitp(allocBitIndex)
+	return markBits{bytep, mask, allocBitIndex}
 }
 
 // refillaCache takes 8 bytes s.allocBits starting at whichByte
@@ -193,7 +191,7 @@ func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
 // 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(addb(s.allocBits, whichByte)))
+	bytes := (*[8]uint8)(unsafe.Pointer(s.allocBits.bytep(whichByte)))
 	aCache := uint64(0)
 	aCache |= uint64(bytes[0])
 	aCache |= uint64(bytes[1]) << (1 * 8)
@@ -265,10 +263,8 @@ func (s *mspan) isFree(index uintptr) bool {
 	if index < s.freeindex {
 		return false
 	}
-	whichByte := index / 8
-	whichBit := index % 8
-	byteVal := *addb(s.allocBits, whichByte)
-	return byteVal&uint8(1<<whichBit) == 0
+	bytep, mask := s.allocBits.bitp(index)
+	return *bytep&mask == 0
 }
 
 func (s *mspan) objIndex(p uintptr) uintptr {
@@ -290,14 +286,12 @@ func markBitsForAddr(p uintptr) markBits {
 }
 
 func (s *mspan) markBitsForIndex(objIndex uintptr) markBits {
-	whichByte := objIndex / 8
-	bitMask := uint8(1 << (objIndex % 8)) // low 3 bits hold the bit index
-	bytePtr := addb(s.gcmarkBits, whichByte)
-	return markBits{bytePtr, bitMask, objIndex}
+	bytep, mask := s.gcmarkBits.bitp(objIndex)
+	return markBits{bytep, mask, objIndex}
 }
 
 func (s *mspan) markBitsForBase() markBits {
-	return markBits{s.gcmarkBits, uint8(1), 0}
+	return markBits{(*uint8)(s.gcmarkBits), uint8(1), 0}
 }
 
 // isMarked reports whether mark bit m is set.
@@ -827,11 +821,11 @@ func (s *mspan) countAlloc() int {
 	count := 0
 	maxIndex := s.nelems / 8
 	for i := uintptr(0); i < maxIndex; i++ {
-		mrkBits := *addb(s.gcmarkBits, i)
+		mrkBits := *s.gcmarkBits.bytep(i)
 		count += int(oneBitCount[mrkBits])
 	}
 	if bitsInLastByte := s.nelems % 8; bitsInLastByte != 0 {
-		mrkBits := *addb(s.gcmarkBits, maxIndex)
+		mrkBits := *s.gcmarkBits.bytep(maxIndex)
 		mask := uint8((1 << bitsInLastByte) - 1)
 		bits := mrkBits & mask
 		count += int(oneBitCount[bits])
diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index ec2cdc1e9c..bf0ae785a9 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -229,8 +229,8 @@ type mspan struct {
 	// The sweep will free the old allocBits and set allocBits to the
 	// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
 	// out memory.
-	allocBits  *uint8
-	gcmarkBits *uint8
+	allocBits  *gcBits
+	gcmarkBits *gcBits
 
 	// sweep generation:
 	// if sweepgen == h->sweepgen - 2, the span needs sweeping
@@ -1417,6 +1417,22 @@ func freespecial(s *special, p unsafe.Pointer, size uintptr) {
 	}
 }
 
+// gcBits is an alloc/mark bitmap. This is always used as *gcBits.
+//
+//go:notinheap
+type gcBits uint8
+
+// bytep returns a pointer to the n'th byte of b.
+func (b *gcBits) bytep(n uintptr) *uint8 {
+	return addb((*uint8)(b), n)
+}
+
+// bitp returns a pointer to the byte containing bit n and a mask for
+// selecting that bit from *bytep.
+func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) {
+	return b.bytep(n / 8), 1 << (n % 8)
+}
+
 const gcBitsChunkBytes = uintptr(64 << 10)
 const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{})
 
@@ -1430,7 +1446,7 @@ type gcBitsArena struct {
 	// gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand.
 	free uintptr // free is the index into bits of the next free byte; read/write atomically
 	next *gcBitsArena
-	bits [gcBitsChunkBytes - gcBitsHeaderBytes]uint8
+	bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits
 }
 
 var gcBitsArenas struct {
@@ -1443,7 +1459,7 @@ var gcBitsArenas struct {
 
 // tryAlloc allocates from b or returns nil if b does not have enough room.
 // This is safe to call concurrently.
-func (b *gcBitsArena) tryAlloc(bytes uintptr) *uint8 {
+func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits {
 	if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) {
 		return nil
 	}
@@ -1459,7 +1475,7 @@ func (b *gcBitsArena) tryAlloc(bytes uintptr) *uint8 {
 
 // newMarkBits returns a pointer to 8 byte aligned bytes
 // to be used for a span's mark bits.
-func newMarkBits(nelems uintptr) *uint8 {
+func newMarkBits(nelems uintptr) *gcBits {
 	blocksNeeded := uintptr((nelems + 63) / 64)
 	bytesNeeded := blocksNeeded * 8
 
@@ -1515,7 +1531,7 @@ func newMarkBits(nelems uintptr) *uint8 {
 // allocation bits. For spans not being initialized the
 // the mark bits are repurposed as allocation bits when
 // the span is swept.
-func newAllocBits(nelems uintptr) *uint8 {
+func newAllocBits(nelems uintptr) *gcBits {
 	return newMarkBits(nelems)
 }