cmd/cc, cmd/gc, runtime: Uniquely encode iface and eface pointers in the pointer map.

Prior to this change, pointer maps encoded the disposition of
a word using a single bit.  A zero signaled a non-pointer
value and a one signaled a pointer value.  Interface values,
which are a effectively a union type, were conservatively
labeled as a pointer.

This change widens the logical element size of the pointer map
to two bits per word.  As before, zero signals a non-pointer
value and one signals a pointer value.  Additionally, a two
signals an iface pointer and a three signals an eface pointer.

Following other changes to the runtime, values two and three
will allow a type information to drive interpretation of the
subsequent word so only those interface values containing a
pointer value will be scanned.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12689046

src/cmd/cc/pgen.c

@@ -31,6 +31,8 @@
 #include "gc.h"
 #include "../../pkg/runtime/funcdata.h"
 
+enum { BitsPerPointer = 2 };
+
 static void dumpgcargs(Type *fn, Sym *sym);
 
 int
@@ -674,7 +676,7 @@ walktype1(Type *t, int32 offset, Bvec *bv)
 		// pointer types
 		if((offset + t->offset) % ewidth[TIND] != 0)
 			yyerror("unaligned pointer");
-		bvset(bv, (offset + t->offset) / ewidth[TIND]);
+		bvset(bv, ((offset + t->offset) / ewidth[TIND])*BitsPerPointer);
 		break;
 
 	case TSTRUCT:
@@ -701,6 +703,7 @@ dumpgcargs(Type *fn, Sym *sym)
 	Bvec *bv;
 	Type *t;
 	int32 i;
+	int32 argbytes;
 	int32 symoffset, argoffset;
 
 	if(hasdotdotdot()) {
@@ -709,7 +712,8 @@ dumpgcargs(Type *fn, Sym *sym)
 		gextern(sym, nodconst(0), 0, 4); // nptrs=0
 		symoffset = 4;
 	} else {
-		bv = bvalloc((argsize() + ewidth[TIND] - 1) / ewidth[TIND]);
+		argbytes = (argsize() + ewidth[TIND] - 1);
+		bv = bvalloc((argbytes  / ewidth[TIND]) * BitsPerPointer);
 		argoffset = align(0, fn->link, Aarg0, nil);
 		if(argoffset > 0) {
 			// The C calling convention returns structs by

src/cmd/gc/pgen.c

@@ -8,6 +8,8 @@
 #include	"opt.h"
 #include	"../../pkg/runtime/funcdata.h"
 
+enum { BitsPerPointer = 2 };
+
 static void allocauto(Prog* p);
 static void dumpgcargs(Node*, Sym*);
 static void dumpgclocals(Node*, Sym*);
@@ -227,7 +229,7 @@ walktype1(Type *t, vlong *xoffset, Bvec *bv)
 	case TMAP:
 		if(*xoffset % widthptr != 0)
 			fatal("walktype1: invalid alignment, %T", t);
-		bvset(bv, *xoffset / widthptr);
+		bvset(bv, (*xoffset / widthptr) * BitsPerPointer);
 		*xoffset += t->width;
 		break;
 
@@ -235,7 +237,7 @@ walktype1(Type *t, vlong *xoffset, Bvec *bv)
 		// struct { byte *str; intgo len; }
 		if(*xoffset % widthptr != 0)
 			fatal("walktype1: invalid alignment, %T", t);
-		bvset(bv, *xoffset / widthptr);
+		bvset(bv, (*xoffset / widthptr) * BitsPerPointer);
 		*xoffset += t->width;
 		break;
 
@@ -245,8 +247,10 @@ walktype1(Type *t, vlong *xoffset, Bvec *bv)
 		// struct { Type* type; union { void* ptr, uintptr val } data; }
 		if(*xoffset % widthptr != 0)
 			fatal("walktype1: invalid alignment, %T", t);
-		bvset(bv, *xoffset / widthptr);
-		bvset(bv, (*xoffset + widthptr) / widthptr);
+		bvset(bv, ((*xoffset / widthptr) * BitsPerPointer) + 1);
+		if(isnilinter(t))
+			bvset(bv, ((*xoffset / widthptr) * BitsPerPointer));
+		bvset(bv, ((*xoffset + widthptr) / widthptr) * BitsPerPointer);
 		*xoffset += t->width;
 		break;
 
@@ -259,7 +263,7 @@ walktype1(Type *t, vlong *xoffset, Bvec *bv)
 			// struct { byte* array; uintgo len; uintgo cap; }
 			if(*xoffset % widthptr != 0)
 				fatal("walktype1: invalid TARRAY alignment, %T", t);
-			bvset(bv, *xoffset / widthptr);
+			bvset(bv, (*xoffset / widthptr) * BitsPerPointer);
 			*xoffset += t->width;
 		} else if(!haspointers(t->type))
 				*xoffset += t->width;
@@ -309,7 +313,7 @@ dumpgcargs(Node *fn, Sym *sym)
 	thistype = getthisx(fn->type);
 	inargtype = getinargx(fn->type);
 	outargtype = getoutargx(fn->type);
-	bv = bvalloc(fn->type->argwid / widthptr);
+	bv = bvalloc((fn->type->argwid / widthptr) * BitsPerPointer);
 	if(thistype != nil)
 		walktype(thistype, bv);
 	if(inargtype != nil)
@@ -336,7 +340,7 @@ dumpgclocals(Node* fn, Sym *sym)
 	int32 i;
 	int off;
 
-	bv = bvalloc(stkptrsize / widthptr);
+	bv = bvalloc((stkptrsize / widthptr) * BitsPerPointer);
 	for(ll = fn->dcl; ll != nil; ll = ll->next) {
 		node = ll->n;
 		if(node->class == PAUTO && node->op == ONAME) {

src/pkg/runtime/mgc0.c

@@ -33,6 +33,9 @@ enum {
 	PRECISE = 1,
 	LOOP = 2,
 	PC_BITS = PRECISE | LOOP,
+
+	// Pointer map
+	BitsPerPointer = 2,
 };
 
 // Bits in per-word bitmap.
@@ -1409,10 +1412,11 @@ scanbitvector(byte *scanp, BitVector *bv)
 			i = remptrs;
 		else
 			i = 32;
+		i /= BitsPerPointer;
 		for(; i > 0; i--) {
-			if(w & 1)
+			if(w & 3)
 				addroot((Obj){scanp, PtrSize, 0});
-			w >>= 1;
+			w >>= BitsPerPointer;
 			scanp += PtrSize;
 		}
 	}
@@ -1444,7 +1448,7 @@ addframeroots(Stkframe *frame, void*)
 		} else if(locals->n > 0) {
 			// Locals bitmap information, scan just the
 			// pointers in locals.
-			size = locals->n*PtrSize;
+			size = (locals->n*PtrSize) / BitsPerPointer;
 			scanbitvector(frame->varp - size, locals);
 		}
 	}