runtime: redo stack map entries to avoid false retention

Change two-bit stack map entries to encode:
0 = dead
1 = scalar
2 = pointer
3 = multiword

If multiword, the two-bit entry for the following word encodes:
0 = string
1 = slice
2 = iface
3 = eface

That way, during stack scanning we can check if a string
is zero length or a slice has zero capacity.  We can avoid
following the contained pointer in those cases.  It is safe
to do so because it can never be dereferenced, and it is
desirable to do so because it may cause false retention
of the following block in memory.

Slice feature turned off until issue 7564 is fixed.

Update #7549

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/76380043

1 files changed, 41 insertions, 17 deletions

diff --git a/src/pkg/runtime/stack.c b/src/pkg/runtime/stack.c
index c73991470e..6e5d9f1f58 100644
--- a/src/pkg/runtime/stack.c
+++ b/src/pkg/runtime/stack.c
@@ -354,7 +354,11 @@ adjustpointers(byte **scanp, BitVector *bv, AdjustInfo *adjinfo, Func *f)
 		if(StackDebug >= 4)
 			runtime·printf("        %p:%s:%p\n", &scanp[i], mapnames[bv->data[i / (32 / BitsPerPointer)] >> (i * BitsPerPointer & 31) & 3], scanp[i]);
 		switch(bv->data[i / (32 / BitsPerPointer)] >> (i * BitsPerPointer & 31) & 3) {
-		case BitsNoPointer:
+		case BitsDead:
+			if(runtime·debug.gcdead)
+				scanp[i] = (byte*)0x6868686868686868LL;
+			break;
+		case BitsScalar:
 			break;
 		case BitsPointer:
 			p = scanp[i];
@@ -370,33 +374,53 @@ adjustpointers(byte **scanp, BitVector *bv, AdjustInfo *adjinfo, Func *f)
 				scanp[i] = p + delta;
 			}
 			break;
-		case BitsEface:
-			t = (Type*)scanp[i];
-			if(t != nil && (t->size > PtrSize || (t->kind & KindNoPointers) == 0)) {
-				p = scanp[i+1];
+		case BitsMultiWord:
+			switch(bv->data[(i+1) / (32 / BitsPerPointer)] >> ((i+1) * BitsPerPointer & 31) & 3) {
+			case BitsString:
+				// string referents are never on the stack, never need to be adjusted
+				i++; // skip len
+				break;
+			case BitsSlice:
+				p = scanp[i];
 				if(minp <= p && p < maxp) {
 					if(StackDebug >= 3)
-						runtime·printf("adjust eface %p\n", p);
-					if(t->size > PtrSize) // currently we always allocate such objects on the heap
-						runtime·throw("large interface value found on stack");
-					scanp[i+1] = p + delta;
+						runtime·printf("adjust slice %p\n", p);
+					scanp[i] = p + delta;
 				}
-			}
-			break;
-		case BitsIface:
-			tab = (Itab*)scanp[i];
-			if(tab != nil) {
-				t = tab->type;
-				if(t->size > PtrSize || (t->kind & KindNoPointers) == 0) {
+				i += 2; // skip len, cap
+				break;
+			case BitsEface:
+				t = (Type*)scanp[i];
+				if(t != nil && (t->size > PtrSize || (t->kind & KindNoPointers) == 0)) {
 					p = scanp[i+1];
 					if(minp <= p && p < maxp) {
 						if(StackDebug >= 3)
-							runtime·printf("adjust iface %p\n", p);
+							runtime·printf("adjust eface %p\n", p);
 						if(t->size > PtrSize) // currently we always allocate such objects on the heap
 							runtime·throw("large interface value found on stack");
 						scanp[i+1] = p + delta;
 					}
 				}
+				i++;
+				break;
+			case BitsIface:
+				tab = (Itab*)scanp[i];
+				if(tab != nil) {
+					t = tab->type;
+					//runtime·printf("          type=%p\n", t);
+					if(t->size > PtrSize || (t->kind & KindNoPointers) == 0) {
+						p = scanp[i+1];
+						if(minp <= p && p < maxp) {
+							if(StackDebug >= 3)
+								runtime·printf("adjust iface %p\n", p);
+							if(t->size > PtrSize) // currently we always allocate such objects on the heap
+								runtime·throw("large interface value found on stack");
+							scanp[i+1] = p + delta;
+						}
+					}
+				}
+				i++;
+				break;
 			}
 			break;
 		}