[Bug tree-optimization/40635] bogus name and location in 'may be used uninitialized' warning

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40635

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2019-02-24 00:00:00         |2021-3-25
      Known to fail|9.0                         |10.2.0, 11.0, 9.3.0

--- Comment #13 from Martin Sebor <msebor at gcc dot gnu.org> ---
I think the reason why the location for the last PHI argument isn't set is
because the argument is itself a PHI whose arguments have different locations:

  <bb 8> [local count: 1073741824]:
  # _16 = PHI <[pr40635.c:21:20] -1(13), [pr40635.c:19:15] s42_9(7),
[pr40635.c:28:16] -1(15), s42_21(9)>
  [pr40635.c:37:5] foo ();
  [pr40635.c:38:8] _28 = _16 < 0;
  [pr40635.c:38:8] _5 = (int) _28;
  [pr40635.c:38:8] _4 = -_5;
  return _4;

  <bb 12> [local count: 39298952]:

  <bb 9> [local count: 383953502]:
  # s42_21 = PHI <[pr40635.c:13:9] s42_18(D)(12), [pr40635.c:19:15] s42_9(14)>
  goto <bb 8>; [100.00%]

}

Even if -Wuninitialized is changed to extract the location from the
uninitialized argument to s42_21(9), it doesn't use it because it prefers to
use the location of the statement where the variable us used.  -Wuninitialized
usually prints a note pointing to the uninitialized variable but it has the
code below that guards is:

      if (xloc.file != floc.file
          || linemap_location_before_p (line_table, location, cfun_loc)
          || linemap_location_before_p (line_table, cfun->function_end_locus,
                                        location))
        inform (DECL_SOURCE_LOCATION (var), "%qD was declared here", var);

Because the location of the variable is in a different function than the
current one the note isn't printed.  The patch below removes this IMO pointless
test and improves the output a bit:

pr40635.c:38:8: warning: ‘s42’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   38 |     if (sockt_rd < 0)
      |        ^
pr40635.c:13:9: note: ‘s42’ was declared here
   13 |     int s42, x;
      |         ^~~

diff --git a/gcc/tree-ssa-uninit.c b/gcc/tree-ssa-uninit.c
index 0800f596ab1..a578a596fee 100644
--- a/gcc/tree-ssa-uninit.c
+++ b/gcc/tree-ssa-uninit.c
@@ -126,8 +126,6 @@ warn_uninit (enum opt_code wc, tree t, tree expr, tree var,
             const char *gmsgid, void *data, location_t phiarg_loc)
 {
   gimple *context = (gimple *) data;
-  location_t location, cfun_loc;
-  expanded_location xloc, floc;

   /* Ignore COMPLEX_EXPR as initializing only a part of a complex
      turns in a COMPLEX_EXPR with the not initialized part being
@@ -170,6 +168,7 @@ warn_uninit (enum opt_code wc, tree t, tree expr, tree var,
       || TREE_NO_WARNING (expr))
     return;

+  location_t location;
   if (context != NULL && gimple_has_location (context))
     location = gimple_location (context);
   else if (phiarg_loc != UNKNOWN_LOCATION)
@@ -178,9 +177,7 @@ warn_uninit (enum opt_code wc, tree t, tree expr, tree var,
     location = DECL_SOURCE_LOCATION (var);
   location = linemap_resolve_location (line_table, location,
                                       LRK_SPELLING_LOCATION, NULL);
-  cfun_loc = DECL_SOURCE_LOCATION (cfun->decl);
-  xloc = expand_location (location);
-  floc = expand_location (cfun_loc);
+
   auto_diagnostic_group d;
   if (warning_at (location, wc, gmsgid, expr))
     {
@@ -188,11 +185,7 @@ warn_uninit (enum opt_code wc, tree t, tree expr, tree
var,

       if (location == DECL_SOURCE_LOCATION (var))
        return;
-      if (xloc.file != floc.file
-         || linemap_location_before_p (line_table, location, cfun_loc)
-         || linemap_location_before_p (line_table, cfun->function_end_locus,
-                                       location))
-       inform (DECL_SOURCE_LOCATION (var), "%qD was declared here", var);
+      inform (DECL_SOURCE_LOCATION (var), "%qD was declared here", var);
     }
 }