[PATCH] Fix removing of df problem in df_finish_pass

["Thomas Preud'homme" <thomas dot preudhomme at arm dot com>]

Hi,

In df_finish_pass, optional problems are removed manually making non null
entries in df->problems_in_order non contiguous. This may lead to null pointer
dereference when accessing all problems from df->problems_in_order[0] to
df->problems_in_order[df->num_problems_defined - 1] and miss some other
problems. Such a scenario was actually encountered when working on a patch.
This patch use the existing function df_remove_problem to do the deletion,
which require iterating on problems via the df->problems_by_index[] array
since each call mess up with df->num_problems_defined and order of
problems in df->problems_in_order[].

ChangeLog entry is as follows:

2015-02-12  Thomas Preud'homme  <thomas.preudhomme@arm.com>

        * df-core.c (df_finish_pass): Iterate over df->problems_by_index[] and
        use df_remove_problem rather than manually removing problems, living
        holes in df->problems_in_order[].

diff --git a/gcc/df-core.c b/gcc/df-core.c
index 82f1364..67040a1 100644
--- a/gcc/df-core.c
+++ b/gcc/df-core.c
@@ -642,7 +642,6 @@ void
 df_finish_pass (bool verify ATTRIBUTE_UNUSED)
 {
   int i;
-  int removed = 0;
 
 #ifdef ENABLE_DF_CHECKING
   int saved_flags;
@@ -658,21 +657,15 @@ df_finish_pass (bool verify ATTRIBUTE_UNUSED)
   saved_flags = df->changeable_flags;
 #endif
 
-  for (i = 0; i < df->num_problems_defined; i++)
+  /* We iterate over problems by index as each problem removed will
+     lead to problems_in_order to be reordered.  */
+  for (i = 0; i < DF_LAST_PROBLEM_PLUS1; i++)
     {
-      struct dataflow *dflow = df->problems_in_order[i];
-      struct df_problem *problem = dflow->problem;
+      struct dataflow *dflow = df->problems_by_index[i];
 
-      if (dflow->optional_p)
-	{
-	  gcc_assert (problem->remove_problem_fun);
-	  (problem->remove_problem_fun) ();
-	  df->problems_in_order[i] = NULL;
-	  df->problems_by_index[problem->id] = NULL;
-	  removed++;
-	}
+      if (dflow && dflow->optional_p)
+	df_remove_problem (dflow);
     }
-  df->num_problems_defined -= removed;
 
   /* Clear all of the flags.  */
   df->changeable_flags = 0;


Testsuite was run with a bootstrapped x86_64 native compiler and an
arm-none-eabi GCC cross-compiler targetting Cortex-M3 without any
regression.

Although the problem is real, it doesn't seem that GCC hits it now
(I stumbled upon it while working on a patch). Therefore I'm not sure
if this should go in stage4 or not. Please advise me on this.

Ok for trunk/stage1?

Best regards,

Thomas