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From: "sgk at troutmask dot apl.washington.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/67758] [GCC 6 regression] ICE on invalid use of COMMON
Date: Tue, 29 Sep 2015 20:24:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67758

--- Comment #3 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Tue, Sep 29, 2015 at 07:56:16PM +0000, kargl at gcc dot gnu.org wrote:
> 
> troutmask:sgk[204] gfc6 -c r5.f
> f951: internal compiler error: Segmentation fault
> no stack trace because unwind library not available
> Please submit a full bug report,
> with preprocessed source if appropriate.
> See <http://gcc.gnu.org/bugs.html> for instructions.

This fixes the regression.

Index: symbol.c
===================================================================
--- symbol.c    (revision 228264)
+++ symbol.c    (working copy)
@@ -3211,6 +3211,8 @@ gfc_restore_last_undo_checkpoint (void)

              while (csym != p)
                {
+                 if (!csym)
+                   goto syntax;
                  cparent = csym;
                  csym = csym->common_next;
                }
@@ -3242,6 +3244,11 @@ gfc_restore_last_undo_checkpoint (void)

   if (!single_undo_checkpoint_p ())
     pop_undo_change_set (latest_undo_chgset);
+
+  return;
+
+syntax:
+   gfc_error ("syntax or semantics issue at %C");
 }

But, the real issue is that the second COMMON is not
allowed to appear in the execution block of a program.
gfortran should probably error out earlier.