alias.c (get_alias_set): Return a previously calculated alias set for a VAR_DECL.

        * alias.c (get_alias_set): Return a previously calculated
        alias set for a VAR_DECL.

From-SVN: r44683

gcc/ChangeLog

+2001-08-07  Jason Merrill  <jason_merrill@redhat.com>
+
+	* alias.c (get_alias_set): Return a previously calculated
+	alias set for a VAR_DECL.
+
 2001-08-06  Richard Henderson  <rth@redhat.com>
 
 	* varasm.c (assemble_gc_entry): Remove.

gcc/alias.c

@@ -527,6 +527,13 @@ get_alias_set (t)
 	    return 0;
 	}
 
+      /* If we've already determined the alias set for this decl, just
+	 return it.  This is necessary for C++ anonymous unions, whose
+	 component variables don't look like union members (boo!).  */
+      if (TREE_CODE (t) == VAR_DECL
+	  && DECL_RTL_SET_P (t) && GET_CODE (DECL_RTL (t)) == MEM)
+	return MEM_ALIAS_SET (DECL_RTL (t));
+
       /* Give the language another chance to do something special.  */
       if (orig_t != t
 	  && (set = lang_get_alias_set (t)) != -1)

gcc/testsuite/g++.dg/opt/alias1.C

+// Test that type punning using an anonymous union works with strict aliasing.
+// { dg-do run }
+// { dg-options -O2 -fstrict-aliasing }
+
+extern "C" void abort ();
+
+void f (int i)
+{
+  union
+  {
+    int ui;
+    float uf[2];
+  };
+
+  ui = i;
+  if (uf[0] != 42.0)
+    abort ();
+}
+
+int main ()
+{
+  union U { int i; float f[2]; } u;
+  u.f[0] = 42.0;
+  f (u.i);
+}