code review 4641044: gc: implement goto restriction

src/cmd/8g/gsubr.c

 // Derived from Inferno utils/8c/txt.c
 // http://code.google.com/p/inferno-os/source/browse/utils/8c/txt.c
 //
 //      Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
 //      Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
 //      Portions Copyright © 1997-1999 Vita Nuova Limited
 //      Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
 //      Portions Copyright © 2004,2006 Bruce Ellis
 //      Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
 //      Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
@@ skipping 118 matching lines @@
         else
                 plast->link = pl;
         plast = pl;
 
         pc = mal(sizeof(*pc));
         clearp(pc);
         pl->firstpc = pc;
 
         return pl;
 }
+
+void
+clearstk(void)
+{
+        Plist *pl;
+        Prog *p1, *p2;
+        Node sp, di, cx, con, ax;
+
+        if(plast->firstpc->to.offset <= 0)
+                return;
+
+        // reestablish context for inserting code
+        // at beginning of function.
+        pl = plast;
+        p1 = pl->firstpc;
+        p2 = p1->link;
+        pc = mal(sizeof(*pc));
+        clearp(pc);
+        p1->link = pc;
+········
+        // zero stack frame
+        nodreg(&sp, types[tptr], D_SP);
+        nodreg(&di, types[tptr], D_DI);
+        nodreg(&cx, types[TUINT32], D_CX);
+        nodconst(&con, types[TUINT32], p1->to.offset / widthptr);
+        gins(ACLD, N, N);
+        gins(AMOVL, &sp, &di);
+        gins(AMOVL, &con, &cx);
+        nodconst(&con, types[TUINT32], 0);
+        nodreg(&ax, types[TUINT32], D_AX);
+        gins(AMOVL, &con, &ax);
+        gins(AREP, N, N);
+        gins(ASTOSL, N, N);
+
+        // continue with original code.
+        gins(ANOP, N, N)->link = p2;
+        pc = P;
+}·······
 
 void
 gused(Node *n)
 {
         gins(ANOP, n, N);       // used
 }
 
 Prog*
 gjmp(Prog *to)
 {
@@ skipping 1765 matching lines @@
 void
 sudoclean(void)
 {
 }
 
 int
 sudoaddable(int as, Node *n, Addr *a)
 {
         return 0;
 }