6l: eliminate dead code, not just the symbols

editing the firstp list was ineffective,
because follow rebuilds it from the textp list.

the symbols for dead code were being dropped
from the binary but the code was all still there.

text for fmt.Printf("hello, world\n") drops
from 143945 to 128650.

R=r,ken
DELTA=22  (20 added, 0 deleted, 2 changed)
OCL=28255
CL=28290

src/cmd/6l/span.c

@@ -238,7 +238,7 @@ asmsym(void)
 				continue;
 			}
 
-	for(p=textp; p!=P; p=p->pcond) {
+	for(p = textp; p != P; p = p->pcond) {
 		s = p->from.sym;
 
 		/* filenames first */

src/cmd/ld/go.c

@@ -103,11 +103,12 @@ ldpkg(Biobuf *f, int64 len, char *filename)
 		fprint(2, "6l: too much pkg data in %s\n", filename);
 		return;
 	}
-	data = mal(len);
+	data = mal(len+1);
 	if(Bread(f, data, len) != len) {
 		fprint(2, "6l: short pkg read %s\n", filename);
 		return;
 	}
+	data[len] = '\0';
 
 	// first \n$$ marks beginning of exports - skip rest of line
 	p0 = strstr(data, "\n$$");
@@ -554,6 +555,16 @@ sweeplist(Prog **first, Prog **last)
 				if(debug['v'] > 1)
 					Bprint(&bso, "discard %s\n", p->from.sym->name);
 				p->from.sym->type = Sxxx;
+				break;
+			}
+			if(p->as == ATEXT) {
+				// keeping this function; link into textp list
+				if(etextp == P)
+					textp = p;
+				else
+					etextp->pcond = p;
+				etextp = p;
+				etextp->pcond = P;
 			}
 			break;
 		}
@@ -603,6 +614,15 @@ deadcode(void)
 	for(i=0; i<nelem(morename); i++)
 		mark(lookup(morename[i], 0));
 
+	// remove dead code.
+	// sweeplist will rebuild the list of functions at textp
+	textp = P;
+	etextp = P;
+
+	// follow is going to redo the firstp, lastp list
+	// but update it anyway just to keep things consistent.
 	sweeplist(&firstp, &lastp);
+
+	// remove dead data
 	sweeplist(&datap, &edatap);
 }