runtime: keep objects in free lists marked as allocated.

Restore https://golang.org/cl/41040043 after GC rewrite.
Original description:
On the plus side, we don't need to change the bits on malloc and free.
On the downside, we need to mark objects in the free lists during GC.
But the free lists are small at GC time, so it should be a net win.

benchmark             old ns/op     new ns/op     delta
BenchmarkMalloc8      21.9          20.4          -6.85%
BenchmarkMalloc16     31.1          29.6          -4.82%

LGTM=khr
R=khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/122280043

1 files changed, 88 insertions, 92 deletions

diff --git a/src/pkg/runtime/malloc.go b/src/pkg/runtime/malloc.go
index 71c0a4ecd7..76c06f314b 100644
--- a/src/pkg/runtime/malloc.go
+++ b/src/pkg/runtime/malloc.go
@@ -41,11 +41,9 @@ const (
 	bitsDead           = 0
 	bitsPointer        = 2
 
-	bitMiddle    = 0
-	bitBoundary  = 1
-	bitAllocated = 2
-	bitMarked    = 3
-	bitMask      = bitMiddle | bitBoundary | bitAllocated | bitMarked
+	bitBoundary = 1
+	bitMarked   = 2
+	bitMask     = bitBoundary | bitMarked
 )
 
 // All zero-sized allocations return a pointer to this byte.
@@ -185,110 +183,108 @@ func gomallocgc(size uintptr, typ *_type, flags int) unsafe.Pointer {
 		size = uintptr(s.elemsize)
 	}
 
+	if flags&flagNoScan != 0 {
+		// All objects are pre-marked as noscan.
+		goto marked
+	}
+
 	// From here till marked label marking the object as allocated
 	// and storing type info in the GC bitmap.
-	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
-	if debugMalloc && (((*xbits)>>shift)&bitMask) != bitBoundary {
-		gothrow("bad bits in markallocated")
-	}
+	{
+		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
+		if debugMalloc && ((*xbits>>shift)&(bitMask|bitPtrMask)) != bitBoundary {
+			println("runtime: bits =", (*xbits>>shift)&(bitMask|bitPtrMask))
+			gothrow("bad bits in markallocated")
+		}
 
-	var ti, te uintptr
-	var ptrmask *uint8
-	if flags&flagNoScan != 0 {
-		// bitsDead in the first quadruple means don't scan.
+		var ti, te uintptr
+		var ptrmask *uint8
 		if size == ptrSize {
-			*xbits = (*xbits & ^((bitBoundary | bitPtrMask) << shift)) | ((bitAllocated + (bitsDead << 2)) << shift)
-		} else {
-			xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-			*xbitsb = bitAllocated + (bitsDead << 2)
+			// It's one word and it has pointers, it must be a pointer.
+			*xbits |= (bitsPointer << 2) << shift
+			goto marked
 		}
-		goto marked
-	}
-	if size == ptrSize {
-		// It's one word and it has pointers, it must be a pointer.
-		*xbits = (*xbits & ^((bitBoundary | bitPtrMask) << shift)) | ((bitAllocated | (bitsPointer << 2)) << shift)
-		goto marked
-	}
-	if typ != nil && (uintptr(typ.gc[0])|uintptr(typ.gc[1])) != 0 && uintptr(typ.size) > ptrSize {
-		if typ.kind&kindGCProg != 0 {
-			nptr := (uintptr(typ.size) + ptrSize - 1) / ptrSize
-			masksize := nptr
-			if masksize%2 != 0 {
-				masksize *= 2 // repeated
+		if typ != nil && (uintptr(typ.gc[0])|uintptr(typ.gc[1])) != 0 && uintptr(typ.size) > ptrSize {
+			if typ.kind&kindGCProg != 0 {
+				nptr := (uintptr(typ.size) + ptrSize - 1) / ptrSize
+				masksize := nptr
+				if masksize%2 != 0 {
+					masksize *= 2 // repeated
+				}
+				masksize = masksize * pointersPerByte / 8 // 4 bits per word
+				masksize++                                // unroll flag in the beginning
+				if masksize > maxGCMask && typ.gc[1] != 0 {
+					// If the mask is too large, unroll the program directly
+					// into the GC bitmap. It's 7 times slower than copying
+					// from the pre-unrolled mask, but saves 1/16 of type size
+					// memory for the mask.
+					mp.ptrarg[0] = x
+					mp.ptrarg[1] = unsafe.Pointer(typ)
+					mp.scalararg[0] = uint(size)
+					mp.scalararg[1] = uint(size0)
+					onM(&unrollgcproginplace_m)
+					goto marked
+				}
+				ptrmask = (*uint8)(unsafe.Pointer(uintptr(typ.gc[0])))
+				// Check whether the program is already unrolled.
+				if uintptr(goatomicloadp(unsafe.Pointer(ptrmask)))&0xff == 0 {
+					mp.ptrarg[0] = unsafe.Pointer(typ)
+					onM(&unrollgcprog_m)
+				}
+				ptrmask = (*uint8)(add(unsafe.Pointer(ptrmask), 1)) // skip the unroll flag byte
+			} else {
+				ptrmask = (*uint8)(unsafe.Pointer(&typ.gc[0])) // embed mask
 			}
-			masksize = masksize * pointersPerByte / 8 // 4 bits per word
-			masksize++                                // unroll flag in the beginning
-			if masksize > maxGCMask && typ.gc[1] != 0 {
-				// If the mask is too large, unroll the program directly
-				// into the GC bitmap. It's 7 times slower than copying
-				// from the pre-unrolled mask, but saves 1/16 of type size
-				// memory for the mask.
-				mp.ptrarg[0] = x
-				mp.ptrarg[1] = unsafe.Pointer(typ)
-				mp.scalararg[0] = uint(size)
-				mp.scalararg[1] = uint(size0)
-				onM(&unrollgcproginplace_m)
+			if size == 2*ptrSize {
+				xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
+				*xbitsb = *ptrmask | bitBoundary
 				goto marked
 			}
-			ptrmask = (*uint8)(unsafe.Pointer(uintptr(typ.gc[0])))
-			// Check whether the program is already unrolled.
-			if uintptr(goatomicloadp(unsafe.Pointer(ptrmask)))&0xff == 0 {
-				mp.ptrarg[0] = unsafe.Pointer(typ)
-				onM(&unrollgcprog_m)
+			te = uintptr(typ.size) / ptrSize
+			// If the type occupies odd number of words, its mask is repeated.
+			if te%2 == 0 {
+				te /= 2
 			}
-			ptrmask = (*uint8)(add(unsafe.Pointer(ptrmask), 1)) // skip the unroll flag byte
-		} else {
-			ptrmask = (*uint8)(unsafe.Pointer(&typ.gc[0])) // embed mask
 		}
 		if size == 2*ptrSize {
 			xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-			*xbitsb = *ptrmask | bitAllocated
+			*xbitsb = (bitsPointer << 2) | (bitsPointer << 6) | bitBoundary
 			goto marked
 		}
-		te = uintptr(typ.size) / ptrSize
-		// If the type occupies odd number of words, its mask is repeated.
-		if te%2 == 0 {
-			te /= 2
-		}
-	}
-	if size == 2*ptrSize {
-		xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-		*xbitsb = (bitsPointer << 2) | (bitsPointer << 6) | bitAllocated
-		goto marked
-	}
-	// Copy pointer bitmask into the bitmap.
-	for i := uintptr(0); i < size0; i += 2 * ptrSize {
-		v := uint8((bitsPointer << 2) | (bitsPointer << 6))
-		if ptrmask != nil {
-			v = *(*uint8)(add(unsafe.Pointer(ptrmask), ti))
-			ti++
-			if ti == te {
-				ti = 0
+		// Copy pointer bitmask into the bitmap.
+		for i := uintptr(0); i < size0; i += 2 * ptrSize {
+			v := uint8((bitsPointer << 2) | (bitsPointer << 6))
+			if ptrmask != nil {
+				v = *(*uint8)(add(unsafe.Pointer(ptrmask), ti))
+				ti++
+				if ti == te {
+					ti = 0
+				}
 			}
+			if i == 0 {
+				v |= bitBoundary
+			}
+			if i+ptrSize == size0 {
+				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
 		}
-		if i == 0 {
-			v |= bitAllocated
-		}
-		if i+ptrSize == size0 {
-			v &= ^uint8(bitPtrMask << 4)
+		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
 		}
-
-		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
-	}
-	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
 	}
 marked:
 	mp.mallocing = 0