code review 9033043: cmd/gc: fix some overflows in the compiler

src/cmd/5g/cgen.c

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 #include <u.h>
 #include <libc.h>
 #include "gg.h"
 
 /*
  * generate:
@@ skipping 662 matching lines @@
                 }
                 break;
 
         case OIND:
                 cgen(nl, res);
                 break;
 
         case ODOT:
                 agen(nl, res);
                 // explicit check for nil if struct is large enough
-                // that we might derive too big a pointer.
+                // that we might derive too big a pointer.  If the left node
+                // was ODOT we have already done the nil check.
+                if(nl->op != ODOT)
                 if(nl->type->width >= unmappedzero) {
                         regalloc(&n1, types[tptr], N);
                         gmove(res, &n1);
                         regalloc(&n2, types[TUINT8], &n1);
                         n1.op = OINDREG;
                         n1.type = types[TUINT8];
                         n1.xoffset = 0;
                         gmove(&n1, &n2);
                         regfree(&n1);
                         regfree(&n2);
@@ skipping 1026 matching lines @@
                 regfree(&nodl);
         return 0;
 
 yes:
         if(freer)
                 regfree(&nodr);
         if(freel)
                 regfree(&nodl);
         return 1;
 }