[PATCH] call mark_dfs_back_edges() before testing EDGE_DFS_BACK [PR104761]

[Martin Sebor via Gcc-patches]

The -Wdangling-pointer code tests the EDGE_DFS_BACK but the pass never
calls the mark_dfs_back_edges() function that initializes the bit (I
didn't know about it).  As a result the bit is not set when expected,
which can cause false positives under the right conditions.

The attached patch adds a call to the warning pass to initialize
the bit.  Tested on x86_64-linux.

Martin
Call mark_dfs_back_edges before testing EDGE_DFS_BACK [PR104761].

Resolves:
PR middle-end/104761 - bogus -Wdangling-pointer with cleanup and infinite loop

gcc/ChangeLog:

	PR middle-end/104761
	* gimple-ssa-warn-access.cc (pass_waccess::execute): Call
	mark_dfs_back_edges.

gcc/testsuite/ChangeLog:

	PR middle-end/104761
	* g++.dg/warn/Wdangling-pointer-4.C: New test.
	* gcc.dg/Wdangling-pointer-4.c: New test.

diff --git a/gcc/gimple-ssa-warn-access.cc b/gcc/gimple-ssa-warn-access.cc
index b7cdad517b3..b519712d76e 100644
--- a/gcc/gimple-ssa-warn-access.cc
+++ b/gcc/gimple-ssa-warn-access.cc
@@ -47,7 +47,7 @@
 #include "tree-object-size.h"
 #include "tree-ssa-strlen.h"
 #include "calls.h"
-#include "cfgloop.h"
+#include "cfganal.h"
 #include "intl.h"
 #include "gimple-range.h"
 #include "stringpool.h"
@@ -4710,6 +4710,9 @@ pass_waccess::execute (function *fun)
   calculate_dominance_info (CDI_DOMINATORS);
   calculate_dominance_info (CDI_POST_DOMINATORS);
 
+  /* Set or clear EDGE_DFS_BACK bits on back edges.  */
+  mark_dfs_back_edges (fun);
+
   /* Create a new ranger instance and associate it with FUN.  */
   m_ptr_qry.rvals = enable_ranger (fun);
   m_func = fun;
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-pointer-4.C b/gcc/testsuite/g++.dg/warn/Wdangling-pointer-4.C
new file mode 100644
index 00000000000..3608cc79e9a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-pointer-4.C
@@ -0,0 +1,23 @@
+/* PR middle-end/104761 - False positive -Wdangling-pointer warning
+   in NetworkManager
+   { dg-do compile }
+   { dg-options "-O -Wall -fno-exceptions" } */
+
+struct S { int i; };
+
+struct X { ~X (); };
+
+void g (int);
+
+void test (int i)
+{
+  S s = { 0 };
+
+  X x;
+
+  if (i)
+    {
+      g (s.i);                // { dg-bogus "-Wdangling-pointer" }
+      for ( ; ; );
+    }
+}
diff --git a/gcc/testsuite/gcc.dg/Wdangling-pointer-4.c b/gcc/testsuite/gcc.dg/Wdangling-pointer-4.c
new file mode 100644
index 00000000000..b2716c7b634
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wdangling-pointer-4.c
@@ -0,0 +1,23 @@
+/* PR middle-end/104761 - False positive -Wdangling-pointer warning
+   in NetworkManager
+   { dg-do compile }
+   { dg-options "-O -Wall" } */
+
+typedef struct { int i; } S;
+
+void f (S **);
+
+int g (int);
+
+void nowarn (int x)
+{
+  S s = { 0 };
+
+  __attribute__((__cleanup__ (f))) S *p = 0;
+
+  if (x)
+    {
+      g (s.i);                // { dg-bogus "-Wdangling-pointer" }
+      for ( ; ; );
+    }
+}