1 files changed, 41 insertions, 0 deletions
diff --git a/src/pkg/runtime/mgc0.h b/src/pkg/runtime/mgc0.h
index f8abe6c9c..16000d1ae 100644
--- a/src/pkg/runtime/mgc0.h
+++ b/src/pkg/runtime/mgc0.h
@@ -44,3 +44,44 @@ enum {
 	//  - at most GC_STACK_CAPACITY allocations because of GC_ARRAY_START
 	GC_STACK_CAPACITY = 8,	
 };
+
+enum {
+	ScanStackByFrames = 1,
+	IgnorePreciseGC = 0,
+
+	// Four bits per word (see #defines below).
+	wordsPerBitmapWord = sizeof(void*)*8/4,
+	bitShift = sizeof(void*)*8/4,
+};
+
+// Bits in per-word bitmap.
+// #defines because enum might not be able to hold the values.
+//
+// Each word in the bitmap describes wordsPerBitmapWord words
+// of heap memory.  There are 4 bitmap bits dedicated to each heap word,
+// so on a 64-bit system there is one bitmap word per 16 heap words.
+// The bits in the word are packed together by type first, then by
+// heap location, so each 64-bit bitmap word consists of, from top to bottom,
+// the 16 bitMarked bits for the corresponding heap words,
+// then the 16 bitScan/bitBlockBoundary bits, then the 16 bitAllocated bits.
+// This layout makes it easier to iterate over the bits of a given type.
+//
+// The bitmap starts at mheap.arena_start and extends *backward* from
+// there.  On a 64-bit system the off'th word in the arena is tracked by
+// the off/16+1'th word before mheap.arena_start.  (On a 32-bit system,
+// the only difference is that the divisor is 8.)
+//
+// To pull out the bits corresponding to a given pointer p, we use:
+//
+//	off = p - (uintptr*)mheap.arena_start;  // word offset
+//	b = (uintptr*)mheap.arena_start - off/wordsPerBitmapWord - 1;
+//	shift = off % wordsPerBitmapWord
+//	bits = *b >> shift;
+//	/* then test bits & bitAllocated, bits & bitMarked, etc. */
+//
+#define bitAllocated		((uintptr)1<<(bitShift*0))	/* block start; eligible for garbage collection */
+#define bitScan			((uintptr)1<<(bitShift*1))	/* when bitAllocated is set */
+#define bitMarked		((uintptr)1<<(bitShift*2))	/* when bitAllocated is set */
+#define bitBlockBoundary	((uintptr)1<<(bitShift*1))	/* when bitAllocated is NOT set - mark for FlagNoGC objects */
+
+#define bitMask (bitAllocated | bitScan | bitMarked)