From ff3fa1b32dba4448bed45282b1038a4f8e9c23dc Mon Sep 17 00:00:00 2001
From: Dmitriy Vyukov <dvyukov@google.com>
Date: Tue, 19 Aug 2014 17:38:00 +0400
Subject: 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/malloc.go | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

(limited to 'src/pkg/runtime/malloc.go')

diff --git a/src/pkg/runtime/malloc.go b/src/pkg/runtime/malloc.go
index 152b3b6b68..8ee460755f 100644
--- a/src/pkg/runtime/malloc.go
+++ b/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:
-- 
cgit v1.3-5-g9baa