runtime: make the GC bitmap a byte array

Half the code in the garbage collector accesses the bitmap
as an array of bytes instead of as an array of uintptrs.
This is tricky to do correctly in a portable fashion,
it breaks on big-endian systems.
Make the bitmap a byte array.
Simplifies markallocated, scanblock and span sweep along the way,
as we don't need to recalculate bitmap position for each word.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/125250043

src/pkg/runtime/asm_386.s

@@ -619,6 +619,14 @@ TEXT runtime·atomicstore64(SB), NOSPLIT, $0-12
 	XADDL	AX, (SP)
 	RET
 
+// void	runtime·atomicor8(byte volatile*, byte);
+TEXT runtime·atomicor8(SB), NOSPLIT, $0-8
+	MOVL	ptr+0(FP), AX
+	MOVB	val+4(FP), BX
+	LOCK
+	ORB	BX, (AX)
+	RET
+
 // void jmpdefer(fn, sp);
 // called from deferreturn.
 // 1. pop the caller

src/pkg/runtime/asm_amd64.s

@@ -702,6 +702,14 @@ TEXT runtime·atomicstore64(SB), NOSPLIT, $0-16
 	XCHGQ	AX, 0(BX)
 	RET
 
+// void	runtime·atomicor8(byte volatile*, byte);
+TEXT runtime·atomicor8(SB), NOSPLIT, $0-16
+	MOVQ	ptr+0(FP), AX
+	MOVB	val+8(FP), BX
+	LOCK
+	ORB	BX, (AX)
+	RET
+
 // void jmpdefer(fn, sp);
 // called from deferreturn.
 // 1. pop the caller

src/pkg/runtime/atomic_arm.c

@@ -167,3 +167,19 @@ runtime·atomicstore64(uint64 volatile *addr, uint64 v)
 	*addr = v;
 	runtime·unlock(LOCK(addr));
 }
+
+#pragma textflag NOSPLIT
+void
+runtime·atomicor8(byte volatile *addr, byte v)
+{
+	uint32 *addr32, old, word, shift;
+
+	// Align down to 4 bytes and use 32-bit CAS.
+	addr32 = (uint32*)((uintptr)addr & ~3);
+	word = ((uint32)v) << (((uintptr)addr & 3) * 8);
+	for(;;) {
+		old = *addr32;
+		if(runtime·cas(addr32, old, old|word))
+			break;
+	}
+}

src/pkg/runtime/heapdump.c

@@ -820,7 +820,8 @@ dumpbvtypes(BitVector *bv, byte *base)
 static BitVector
 makeheapobjbv(byte *p, uintptr size)
 {
-	uintptr off, shift, *bitp, bits, nptr, i;
+	uintptr off, nptr, i;
+	byte shift, *bitp, bits;
 	bool mw;
 
 	// Extend the temp buffer if necessary.
@@ -838,13 +839,13 @@ makeheapobjbv(byte *p, uintptr size)
 	mw = false;
 	for(i = 0; i < nptr; i++) {
 		off = (uintptr*)(p + i*PtrSize) - (uintptr*)runtime·mheap.arena_start;
-		bitp = (uintptr*)runtime·mheap.arena_start - off/wordsPerBitmapWord - 1;
-		shift = (off % wordsPerBitmapWord) * gcBits;
-		bits = (*bitp >> (shift + 2)) & 3;
+		bitp = runtime·mheap.arena_start - off/wordsPerBitmapByte - 1;
+		shift = (off % wordsPerBitmapByte) * gcBits;
+		bits = (*bitp >> (shift + 2)) & BitsMask;
 		if(!mw && bits == BitsDead)
 			break;  // end of heap object
 		mw = !mw && bits == BitsMultiWord;
-		tmpbuf[i*BitsPerPointer/8] &= ~(3<<((i*BitsPerPointer)%8));
+		tmpbuf[i*BitsPerPointer/8] &= ~(BitsMask<<((i*BitsPerPointer)%8));
 		tmpbuf[i*BitsPerPointer/8] |= bits<<((i*BitsPerPointer)%8);
 	}
 	return (BitVector){i*BitsPerPointer, (uint32*)tmpbuf};

src/pkg/runtime/malloc.go

@@ -22,8 +22,8 @@ const (
 	pageSize  = 1 << pageShift
 	pageMask  = pageSize - 1
 
-	wordsPerBitmapWord = ptrSize * 8 / 4
 	gcBits             = 4
+	wordsPerBitmapByte = 8 / gcBits
 	bitsPerPointer     = 2
 	bitsMask           = 1<<bitsPerPointer - 1
 	pointersPerByte    = 8 / bitsPerPointer
@@ -211,8 +211,8 @@ func gomallocgc(size uintptr, typ *_type, flags int) unsafe.Pointer {
 	{
 		arena_start := uintptr(unsafe.Pointer(mheap_.arena_start))
 		off := (uintptr(x) - arena_start) / ptrSize
-		xbits := (*uintptr)(unsafe.Pointer(arena_start - off/wordsPerBitmapWord*ptrSize - ptrSize))
-		shift := (off % wordsPerBitmapWord) * gcBits
+		xbits := (*uint8)(unsafe.Pointer(arena_start - off/wordsPerBitmapByte - 1))
+		shift := (off % wordsPerBitmapByte) * gcBits
 		if debugMalloc && ((*xbits>>shift)&(bitMask|bitPtrMask)) != bitBoundary {
 			println("runtime: bits =", (*xbits>>shift)&(bitMask|bitPtrMask))
 			gothrow("bad bits in markallocated")
@@ -260,8 +260,7 @@ func gomallocgc(size uintptr, typ *_type, flags int) unsafe.Pointer {
 			ptrmask = (*uint8)(unsafe.Pointer(&typ.gc[0])) // embed mask
 		}
 		if size == 2*ptrSize {
-			xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-			*xbitsb = *ptrmask | bitBoundary
+			*xbits = *ptrmask | bitBoundary
 			goto marked
 		}
 		te = uintptr(typ.size) / ptrSize
@@ -283,19 +282,12 @@ func gomallocgc(size uintptr, typ *_type, flags int) unsafe.Pointer {
 				v &^= uint8(bitPtrMask << 4)
 			}
 
-			off := (uintptr(x) + i - arena_start) / ptrSize
-			xbits := (*uintptr)(unsafe.Pointer(arena_start - off/wordsPerBitmapWord*ptrSize - ptrSize))
-			shift := (off % wordsPerBitmapWord) * gcBits
-			xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-			*xbitsb = v
+			*xbits = v
+			xbits = (*byte)(add(unsafe.Pointer(xbits), ^uintptr(0)))
 		}
 		if size0%(2*ptrSize) == 0 && size0 < size {
 			// Mark the word after last object's word as bitsDead.
-			off := (uintptr(x) + size0 - arena_start) / ptrSize
-			xbits := (*uintptr)(unsafe.Pointer(arena_start - off/wordsPerBitmapWord*ptrSize - ptrSize))
-			shift := (off % wordsPerBitmapWord) * gcBits
-			xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-			*xbitsb = bitsDead << 2
+			*xbits = bitsDead << 2
 		}
 	}
 marked:

src/pkg/runtime/mgc0.c

@@ -212,8 +212,8 @@ static struct {
 static void
 scanblock(byte *b, uintptr n, byte *ptrmask)
 {
-	byte *obj, *p, *arena_start, *arena_used, **wp, *scanbuf[8], bits8;
-	uintptr i, nobj, size, idx, *bitp, bits, xbits, shift, x, off, cached, scanbufpos;
+	byte *obj, *p, *arena_start, *arena_used, **wp, *scanbuf[8], *ptrbitp, *bitp, bits, xbits, shift, cached;
+	uintptr i, nobj, size, idx, x, off, scanbufpos;
 	intptr ncached;
 	Workbuf *wbuf;
 	String *str;
@@ -237,6 +237,10 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 	for(i = 0; i < nelem(scanbuf); i++)
 		scanbuf[i] = nil;
 
+	ptrbitp = nil;
+	cached = 0;
+	ncached = 0;
+
 	// ptrmask can have 3 possible values:
 	// 1. nil - obtain pointer mask from GC bitmap.
 	// 2. ScanConservatively - don't use any mask, scan conservatively.
@@ -295,8 +299,15 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 			}
 			ptrmask = ScanConservatively;
 		}
-		cached = 0;
-		ncached = 0;
+		// Find bits of the beginning of the object.
+		if(ptrmask == nil) {
+			off = (uintptr*)b - (uintptr*)arena_start;
+			ptrbitp = arena_start - off/wordsPerBitmapByte - 1;
+			shift = (off % wordsPerBitmapByte) * gcBits;
+			cached = *ptrbitp >> shift;
+			cached &= ~bitBoundary;
+			ncached = (8 - shift)/gcBits;
+		}
 		for(i = 0; i < n; i += PtrSize) {
 			obj = nil;
 			// Find bits for this word.
@@ -308,16 +319,13 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 				// Consult GC bitmap.
 				if(ncached <= 0) {
 					// Refill cache.
-					off = (uintptr*)(b+i) - (uintptr*)arena_start;
-					bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-					shift = (off % wordsPerBitmapWord) * gcBits;
-					cached = *bitp >> shift;
-					ncached = (PtrSize*8 - shift)/gcBits;
+					cached = *--ptrbitp;
+					ncached = 2;
 				}
 				bits = cached;
 				cached >>= gcBits;
 				ncached--;
-				if(i != 0 && (bits&bitBoundary) != 0)
+				if((bits&bitBoundary) != 0)
 					break; // reached beginning of the next object
 				bits = (bits>>2)&BitsMask;
 				if(bits == BitsDead)
@@ -336,11 +344,9 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 			// Find the next pair of bits.
 			if(ptrmask == nil) {
 				if(ncached <= 0) {
-					off = (uintptr*)(b+i+PtrSize) - (uintptr*)arena_start;
-					bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-					shift = (off % wordsPerBitmapWord) * gcBits;
-					cached = *bitp >> shift;
-					ncached = (PtrSize*8 - shift)/gcBits;
+					// Refill cache.
+					cached = *--ptrbitp;
+					ncached = 2;
 				}
 				bits = (cached>>2)&BitsMask;
 			} else
@@ -383,8 +389,14 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 
 			if(bits == BitsSlice) {
 				i += 2*PtrSize;
-				cached >>= 2*gcBits;
-				ncached -= 2;
+				if(ncached == 2)
+					ncached = 0;
+				else if(ptrmask == nil) {
+					// Refill cache and consume one quadruple.
+					cached = *--ptrbitp;
+					cached >>= gcBits;
+					ncached = 1;
+				}
 			} else {
 				i += PtrSize;
 				cached >>= gcBits;
@@ -398,20 +410,12 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 				continue;
 			// Mark the object.
 			off = (uintptr*)obj - (uintptr*)arena_start;
-			bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-			shift = (off % wordsPerBitmapWord) * gcBits;
+			bitp = arena_start - off/wordsPerBitmapByte - 1;
+			shift = (off % wordsPerBitmapByte) * gcBits;
 			xbits = *bitp;
 			bits = (xbits >> shift) & bitMask;
 			if((bits&bitBoundary) == 0) {
-				// Not a beginning of a block, check if we have block boundary in xbits.
-				while(shift > 0) {
-					obj -= PtrSize;
-					shift -= gcBits;
-					bits = (xbits >> shift) & bitMask;
-					if((bits&bitBoundary) != 0)
-						goto havebits;
-				}
-				// Otherwise consult span table to find the block beginning.
+				// Not a beginning of a block, consult span table to find the block beginning.
 				k = (uintptr)obj>>PageShift;
 				x = k;
 				x -= (uintptr)arena_start>>PageShift;
@@ -433,7 +437,6 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 				goto markobj;
 			}
 
-		havebits:
 			// Now we have bits, bitp, and shift correct for
 			// obj pointing at the base of the object.
 			// Only care about not marked objects.
@@ -449,22 +452,12 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
 			// For 8-byte objects we use non-atomic store, if the other
 			// quadruple is already marked. Otherwise we resort to CAS
 			// loop for marking.
-			bits8 = xbits>>(shift&~7);
-			if((bits8&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) ||
+			if((xbits&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) ||
 				work.nproc == 1)
-				((uint8*)bitp)[shift/8] = bits8 | (bitMarked<<(shift&7));
-			else {
-				for(;;) {
-					if(runtime·casp((void**)bitp, (void*)xbits, (void*)(xbits|(bitMarked<<shift))))
-						break;
-					xbits = *bitp;
-					bits = (xbits>>shift) & bitMask;
-					if((bits&bitMarked) != 0)
-						break;
-				}
-				if((bits&bitMarked) != 0)
-					continue;
-			}
+				*bitp = xbits | (bitMarked<<shift);
+			else
+				runtime·atomicor8(bitp, bitMarked<<shift);
+
 			if(((xbits>>(shift+2))&BitsMask) == BitsDead)
 				continue;  // noscan object
 
@@ -901,9 +894,9 @@ bool
 runtime·MSpan_Sweep(MSpan *s, bool preserve)
 {
 	int32 cl, n, npages, nfree;
-	uintptr size, off, *bitp, shift, xbits, bits;
+	uintptr size, off, step;
 	uint32 sweepgen;
-	byte *p;
+	byte *p, *bitp, shift, xbits, bits;
 	MCache *c;
 	byte *arena_start;
 	MLink head, *end, *link;
@@ -939,8 +932,8 @@ runtime·MSpan_Sweep(MSpan *s, bool preserve)
 	// Mark any free objects in this span so we don't collect them.
 	for(link = s->freelist; link != nil; link = link->next) {
 		off = (uintptr*)link - (uintptr*)arena_start;
-		bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-		shift = (off % wordsPerBitmapWord) * gcBits;
+		bitp = arena_start - off/wordsPerBitmapByte - 1;
+		shift = (off % wordsPerBitmapByte) * gcBits;
 		*bitp |= bitMarked<<shift;
 	}
 
@@ -951,8 +944,8 @@ runtime·MSpan_Sweep(MSpan *s, bool preserve)
 		// A finalizer can be set for an inner byte of an object, find object beginning.
 		p = (byte*)(s->start << PageShift) + special->offset/size*size;
 		off = (uintptr*)p - (uintptr*)arena_start;
-		bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-		shift = (off % wordsPerBitmapWord) * gcBits;
+		bitp = arena_start - off/wordsPerBitmapByte - 1;
+		shift = (off % wordsPerBitmapByte) * gcBits;
 		bits = (*bitp>>shift) & bitMask;
 		if((bits&bitMarked) == 0) {
 			// Find the exact byte for which the special was setup
@@ -977,10 +970,27 @@ runtime·MSpan_Sweep(MSpan *s, bool preserve)
 	// This thread owns the span now, so it can manipulate
 	// the block bitmap without atomic operations.
 	p = (byte*)(s->start << PageShift);
+	// Find bits for the beginning of the span.
+	off = (uintptr*)p - (uintptr*)arena_start;
+	bitp = arena_start - off/wordsPerBitmapByte - 1;
+	shift = 0;
+	step = size/(PtrSize*wordsPerBitmapByte);
+	// Rewind to the previous quadruple as we move to the next
+	// in the beginning of the loop.
+	bitp += step;
+	if(step == 0) {
+		// 8-byte objects.
+		bitp++;
+		shift = gcBits;
+	}
 	for(; n > 0; n--, p += size) {
-		off = (uintptr*)p - (uintptr*)arena_start;
-		bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-		shift = (off % wordsPerBitmapWord) * gcBits;
+		bitp -= step;
+		if(step == 0) {
+			if(shift != 0)
+				bitp--;
+			shift = gcBits - shift;
+		}
+
 		xbits = *bitp;
 		bits = (xbits>>shift) & bitMask;
 
@@ -1759,8 +1769,8 @@ runtime·wakefing(void)
 static byte*
 unrollgcprog1(byte *mask, byte *prog, uintptr *ppos, bool inplace, bool sparse)
 {
-	uintptr *b, off, shift, pos, siz, i;
-	byte *arena_start, *prog1, v;
+	uintptr pos, siz, i, off;
+	byte *arena_start, *prog1, v, *bitp, shift;
 
 	arena_start = runtime·mheap.arena_start;
 	pos = *ppos;
@@ -1777,11 +1787,11 @@ unrollgcprog1(byte *mask, byte *prog, uintptr *ppos, bool inplace, bool sparse)
 				if(inplace) {
 					// Store directly into GC bitmap.
 					off = (uintptr*)(mask+pos) - (uintptr*)arena_start;
-					b = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-					shift = (off % wordsPerBitmapWord) * gcBits;
-					if((shift%8)==0)
-						((byte*)b)[shift/8] = 0;
-					((byte*)b)[shift/8] |= v<<((shift%8)+2);
+					bitp = arena_start - off/wordsPerBitmapByte - 1;
+					shift = (off % wordsPerBitmapByte) * gcBits;
+					if(shift==0)
+						*bitp = 0;
+					*bitp |= v<<(shift+2);
 					pos += PtrSize;
 				} else if(sparse) {
 					// 4-bits per word
@@ -1847,8 +1857,8 @@ unrollglobgcprog(byte *prog, uintptr size)
 void
 runtime·unrollgcproginplace_m(void)
 {
-	uintptr size, size0, *b, off, shift, pos;
-	byte *arena_start, *prog;
+	uintptr size, size0, pos, off;
+	byte *arena_start, *prog, *bitp, shift;
 	Type *typ;
 	void *v;
 
@@ -1866,15 +1876,15 @@ runtime·unrollgcproginplace_m(void)
 	// Mark first word as bitAllocated.
 	arena_start = runtime·mheap.arena_start;
 	off = (uintptr*)v - (uintptr*)arena_start;
-	b = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-	shift = (off % wordsPerBitmapWord) * gcBits;
-	*b |= bitBoundary<<shift;
+	bitp = arena_start - off/wordsPerBitmapByte - 1;
+	shift = (off % wordsPerBitmapByte) * gcBits;
+	*bitp |= bitBoundary<<shift;
 	// Mark word after last as BitsDead.
 	if(size0 < size) {
 		off = (uintptr*)((byte*)v + size0) - (uintptr*)arena_start;
-		b = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
-		shift = (off % wordsPerBitmapWord) * gcBits;
-		*b &= ~(bitPtrMask<<shift) | ((uintptr)BitsDead<<(shift+2));
+		bitp = arena_start - off/wordsPerBitmapByte - 1;
+		shift = (off % wordsPerBitmapByte) * gcBits;
+		*bitp &= ~(bitPtrMask<<shift) | ((uintptr)BitsDead<<(shift+2));
 	}
 }
 
@@ -1916,60 +1926,67 @@ runtime·unrollgcprog_m(void)
 void
 runtime·markspan(void *v, uintptr size, uintptr n, bool leftover)
 {
-	uintptr *b, *b0, off, shift, x;
-	byte *p;
+	uintptr i, off, step;
+	byte *b;
 
 	if((byte*)v+size*n > (byte*)runtime·mheap.arena_used || (byte*)v < runtime·mheap.arena_start)
 		runtime·throw("markspan: bad pointer");
 
-	p = v;
-	if(leftover)	// mark a boundary just past end of last block too
-		n++;
-
-	b0 = nil;
-	x = 0;
-	for(; n-- > 0; p += size) {
-		// Okay to use non-atomic ops here, because we control
-		// the entire span, and each bitmap word has bits for only
-		// one span, so no other goroutines are changing these
-		// bitmap words.
-		off = (uintptr*)p - (uintptr*)runtime·mheap.arena_start;  // word offset
-		b = (uintptr*)runtime·mheap.arena_start - off/wordsPerBitmapWord - 1;
-		shift = (off % wordsPerBitmapWord) * gcBits;
-		if(b0 != b) {
-			if(b0 != nil)
-				*b0 = x;
-			b0 = b;
-			x = 0;
-		}
-		x |= (bitBoundary<<shift) | ((uintptr)BitsDead<<(shift+2));
+	// Find bits of the beginning of the span.
+	off = (uintptr*)v - (uintptr*)runtime·mheap.arena_start;  // word offset
+	b = runtime·mheap.arena_start - off/wordsPerBitmapByte - 1;
+	if((off%wordsPerBitmapByte) != 0)
+		runtime·throw("markspan: unaligned length");
+
+	// Okay to use non-atomic ops here, because we control
+	// the entire span, and each bitmap byte has bits for only
+	// one span, so no other goroutines are changing these bitmap words.
+
+	if(size == PtrSize) {
+		// Possible only on 64-bits (minimal size class is 8 bytes).
+		// Poor man's memset(0x11).
+		if(0x11 != ((bitBoundary+BitsDead)<<gcBits) + (bitBoundary+BitsDead))
+			runtime·throw("markspan: bad bits");
+		if((n%(wordsPerBitmapByte*PtrSize)) != 0)
+			runtime·throw("markspan: unaligned length");
+		b = b - n/wordsPerBitmapByte + 1;	// find first byte
+		if(((uintptr)b%PtrSize) != 0)
+			runtime·throw("markspan: unaligned pointer");
+		for(i = 0; i != n; i += wordsPerBitmapByte*PtrSize, b += PtrSize)
+			*(uintptr*)b = (uintptr)0x1111111111111111ULL;  // bitBoundary+BitsDead
+		return;
 	}
-	*b0 = x;
+
+	if(leftover)
+		n++;	// mark a boundary just past end of last block too
+	step = size/(PtrSize*wordsPerBitmapByte);
+	for(i = 0; i != n; i++, b -= step)
+		*b = bitBoundary|(BitsDead<<2);
 }
 
 // unmark the span of memory at v of length n bytes.
 void
 runtime·unmarkspan(void *v, uintptr n)
 {
-	uintptr *p, *b, off;
+	uintptr off;
+	byte *b;
 
 	if((byte*)v+n > (byte*)runtime·mheap.arena_used || (byte*)v < runtime·mheap.arena_start)
 		runtime·throw("markspan: bad pointer");
 
-	p = v;
-	off = p - (uintptr*)runtime·mheap.arena_start;  // word offset
-	if((off % wordsPerBitmapWord) != 0)
+	off = (uintptr*)v - (uintptr*)runtime·mheap.arena_start;  // word offset
+	if((off % (PtrSize*wordsPerBitmapByte)) != 0)
 		runtime·throw("markspan: unaligned pointer");
-	b = (uintptr*)runtime·mheap.arena_start - off/wordsPerBitmapWord - 1;
+	b = runtime·mheap.arena_start - off/wordsPerBitmapByte - 1;
 	n /= PtrSize;
-	if(n%wordsPerBitmapWord != 0)
+	if(n%(PtrSize*wordsPerBitmapByte) != 0)
 		runtime·throw("unmarkspan: unaligned length");
 	// Okay to use non-atomic ops here, because we control
 	// the entire span, and each bitmap word has bits for only
 	// one span, so no other goroutines are changing these
 	// bitmap words.
-	n /= wordsPerBitmapWord;
-	runtime·memclr((byte*)(b - n + 1), n*PtrSize);
+	n /= wordsPerBitmapByte;
+	runtime·memclr(b - n + 1, n);
 }
 
 void
@@ -1983,7 +2000,7 @@ runtime·MHeap_MapBits(MHeap *h)
 	};
 	uintptr n;
 
-	n = (h->arena_used - h->arena_start) / wordsPerBitmapWord;
+	n = (h->arena_used - h->arena_start) / (PtrSize*wordsPerBitmapByte);
 	n = ROUND(n, bitmapChunk);
 	n = ROUND(n, PhysPageSize);
 	if(h->bitmap_mapped >= n)
@@ -2011,8 +2028,8 @@ void
 runtime·getgcmask(byte *p, Type *t, byte **mask, uintptr *len)
 {
 	Stkframe frame;
-	uintptr i, n, off, bits, shift, *b;
-	byte *base;
+	uintptr i, n, off;
+	byte *base, bits, shift, *b;
 
 	*mask = nil;
 	*len = 0;
@@ -2047,8 +2064,8 @@ runtime·getgcmask(byte *p, Type *t, byte **mask, uintptr *len)
 		*mask = runtime·mallocgc(*len, nil, 0);
 		for(i = 0; i < n; i += PtrSize) {
 			off = (uintptr*)(base+i) - (uintptr*)runtime·mheap.arena_start;
-			b = (uintptr*)runtime·mheap.arena_start - off/wordsPerBitmapWord - 1;
-			shift = (off % wordsPerBitmapWord) * gcBits;
+			b = runtime·mheap.arena_start - off/wordsPerBitmapByte - 1;
+			shift = (off % wordsPerBitmapByte) * gcBits;
 			bits = (*b >> (shift+2))&BitsMask;
 			(*mask)[i/PtrSize] = bits;
 		}

src/pkg/runtime/mgc0.h

@@ -8,8 +8,8 @@ enum {
 	ScanStackByFrames = 1,
 
 	// Four bits per word (see #defines below).
-	wordsPerBitmapWord = sizeof(void*)*8/4,
 	gcBits = 4,
+	wordsPerBitmapByte = 8/gcBits,
 
 	// GC type info programs.
 	// The programs allow to store type info required for GC in a compact form.

src/pkg/runtime/runtime.h

@@ -913,6 +913,7 @@ void	runtime·atomicstore64(uint64 volatile*, uint64);
 uint64	runtime·atomicload64(uint64 volatile*);
 void*	runtime·atomicloadp(void* volatile*);
 void	runtime·atomicstorep(void* volatile*, void*);
+void	runtime·atomicor8(byte volatile*, byte);
 
 void	runtime·setg(G*);
 void	runtime·newextram(void);