[patch, fortran] Fix PR 92113

[Thomas Koenig <tkoenig at netcologne dot de>]

Hello world,

the attached patch fixes an 8/9/10 regression where, to fix PR 84487
by not putting the initializers and vtabs into the read-only section
(for reasons of size, which could grow enormously) led to a regression
on POWER9 and other non-x86 architectures, where the initializer was
sometimes optimized away, depending on optimization levels.

This was a strange beast to hunt down. This only showed up on
the testresults for gcc 8, so I tried to find out what commit
had fixed this on trunk, in order to backport.

However, bisecting this I found that the test case actually
segfaults all the way up to current trunk when run by hand.
By running the testsuite, I didn't see it.  This is strange,
and raises some issues about the testsuite and the possibility
of a latent issue, but I lack the knowledge to hunt this down.

In the meantime, here is this patch, which puts the vtabs and
the initializers where the user actually specified something
into the read-only section again.

Test case: Well, theoretically it is already there, so it makes
little sense to add a new one.

Regression-tested on powerpc64le-unknown-linux-gnu, also
verified by hand that pr51434.f90 now passes with -O2 there.

OK for trunk/9/8?

Regards

	Thomas

2019-11-02  Thomas Koenig  <tkoenig@gcc.gnu.org> 



        PR fortran/92133 

        * trans-decl.c (gfc_get_symbol_decl): If __def_init actually 

        contains a value, put it into  the read-only section.

Index: trans-decl.c
===================================================================
--- trans-decl.c	(Revision 277486)
+++ trans-decl.c	(Arbeitskopie)
@@ -1911,14 +1911,19 @@ gfc_get_symbol_decl (gfc_symbol * sym)
   if (sym->attr.associate_var)
     GFC_DECL_ASSOCIATE_VAR_P (decl) = 1;
 
-  /* We no longer mark __def_init as read-only so it does not take up
-     space in the read-only section and dan go into the BSS instead,
-     see PR 84487.  Marking this as artificial means that OpenMP will
-     treat this as predetermined shared.  */
-  if (sym->attr.vtab
-      || (sym->name[0] == '_' && gfc_str_startswith (sym->name, "__def_init")))
-    DECL_ARTIFICIAL (decl) = 1;
+  /* We only mark __def_init as read-only if it actually has an
+     initializer so it does not needlessly take up space in the
+     read-only section and can go into the BSS instead, see PR 84487.
+     Marking this as artificial means that OpenMP will treat this as
+     predetermined shared.  */
 
+  if (sym->attr.vtab || gfc_str_startswith (sym->name, "__def_init"))
+    {
+      DECL_ARTIFICIAL (decl) = 1;
+      if (sym->attr.vtab || sym->value)
+	TREE_READONLY (decl) = 1;
+    }
+
   return decl;
 }