From patchwork Tue Jun  8 07:02:15 2010
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Subject: Fix g++.dg/torture/pr32304.C
Date: Mon, 07 Jun 2010 21:02:15 -0000
From: Jan Hubicka <hubicka@ucw.cz>
X-Patchwork-Id: 54937
Message-Id: <20100608070215.GA12438@kam.mff.cuni.cz>
To: gcc-patches@gcc.gnu.org

Hi,
the testcase:

struct S {
        S() {}
};
S f() {
        static S s;
        return s;
}

fails with my last ipa-pure-const changes because variable S that is
TYPE_NEEDS_CONSTRCTING become read only.  Old pure-const implementation never
did this change because TYPE_NEEDS_CONSTRUCTIONG vars was left out of analysis.

I am not too familiar with TYPE_NEEDS_CONSTRUCTING mechanism, but I think in
this case S can become read only.  The reason is that S is empty and there is
nothing to construct.  We compile the code into:
S f() ()
{ 
  struct S D.2138;
  bool D.2122;
  int D.2130;
  bool retval.1;
  char D.2126;
  char * _ZGVZ1fvE1s.0;

<bb 2>:
  _ZGVZ1fvE1s.0_1 = (char *) &_ZGVZ1fvE1s;
  D.2126_2 = *_ZGVZ1fvE1s.0_1;
  if (D.2126_2 == 0)
    goto <bb 3>;
  else
    goto <bb 5>;

<bb 3>:
  D.2130_3 = __cxa_guard_acquire (&_ZGVZ1fvE1s);
  if (D.2130_3 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

<bb 4>:
  __cxa_guard_release (&_ZGVZ1fvE1s);

<bb 5>:
  return D.2138;

}

where __cxa_guard_acquire/__cxa_guard_release pair is locking the empty
initialization code, at least in my understanding.  Looking at places
TYPE_NEEDS_CONSTRUCTING is handled in backend, I think we should have
everything gimplified to level that we do not need to care about the flag.

Note also that D.2138 is undefined. It is comming so from frontend while in
older versions we at least used to initialize it from static s.  I wonder if
this is because the var is empty.

Does the attached patch seem to make sense?

Bootstrapped/regtested x86_64-linux.

	* emit-rtl.c (set_mem_attributes_minus_bitpos): Remove TYPE_NEEDS_CONSTRUCTING
	sanity check.


Index: emit-rtl.c
===================================================================
--- emit-rtl.c	(revision 160379)
+++ emit-rtl.c	(working copy)
@@ -1668,12 +1668,7 @@ set_mem_attributes_minus_bitpos (rtx ref
       if (base && DECL_P (base)
 	  && TREE_READONLY (base)
 	  && (TREE_STATIC (base) || DECL_EXTERNAL (base)))
-	{
-	  tree base_type = TREE_TYPE (base);
-	  gcc_assert (!(base_type && TYPE_NEEDS_CONSTRUCTING (base_type))
-		      || DECL_ARTIFICIAL (base));
-	  MEM_READONLY_P (ref) = 1;
-	}
+	MEM_READONLY_P (ref) = 1;
 
       /* If this expression uses it's parent's alias set, mark it such
 	 that we won't change it.  */