Re: [PATCH] Do not erase warning data in gimple_set_location

[Eric Botcazou via Gcc-patches]

> Hmm, I think instead of special-casing UNKNOWN_LOCATION
> what gimple_set_location should probably do is either not copy
> warnings at all or union them.  Btw, gimple_set_location also
> removes a previously set BLOCK (but gimple_set_block preserves
> the location locus and diagnostic override).
> 
> So I'd be tempted to axe the copy_warning () completely here. 

The first thing I tried, but it regressed the original testcase IIRC.

Even my minimal patch manages to break bootstrap on ARM:

buildslave/workspace/tcwg_gnu_1/abe/snapshots/gcc.git~master/libcpp/lex.cc:
1523:9: error: pointer used after ‘void operator delete(void*, std::size_t)’ 
[-Werror=use-after-free]
# 00:31:04 make[3]: *** [Makefile:227: lex.o] Error 1
# 00:31:04 make[2]: *** [Makefile:9527: all-stage3-libcpp] Error 2
# 00:31:35 make[1]: *** [Makefile:25887: stage3-bubble] Error 2
# 00:31:35 make: *** [Makefile:1072: all] Error 2

      /* Don't warn for cases like when a cdtor returns 'this' on ARM.  */
      else if (warning_suppressed_p (var, OPT_Wuse_after_free))
        return;

because warning-control.cc:copy_warning also clobbers the warning data of the 
destination.  We have in cp/decl.cc:maybe_return_this the lines:

      /* Return the address of the object.  */
      tree val = DECL_ARGUMENTS (current_function_decl);
      suppress_warning (val, OPT_Wuse_after_free);

-Wuse-after-free is suppressed for the location of VAL and the TREE_NO_WARNING 
bit set on it.  But other expressions may have the same location as VAL and 
the TREE_NO_WARNING bit _not_ set, so when you call copy_warning (expr, expr) 
(we do that a lot after failed folding) for them, copy_warning erases the 
warning data of the location.

I have installed the obvious fixlet after testing on x86-64/Linux, but the 
decoupling between TREE_NO_WARNING bit and location looks a bit problematic.


        * warning-control.cc (copy_warning) [generic version]: Do not erase
        the warning data of the destination location when the no-warning
        bit is not set on the source.
        (copy_warning) [tree version]: Return early if TO is equal to FROM.
        (copy_warning) [gimple version]: Likewise.
testsuite/
        * g++.dg/warn/Wuse-after-free5.C: New test.

-- 
Eric Botcazou
diff --git a/gcc/warning-control.cc b/gcc/warning-control.cc
index 0cbb4f079fa..7e9e701cfbe 100644
--- a/gcc/warning-control.cc
+++ b/gcc/warning-control.cc
@@ -191,7 +191,7 @@ void copy_warning (ToType to, FromType from)
 {
   const location_t to_loc = get_location (to);
 
-  bool supp = get_no_warning_bit (from);
+  const bool supp = get_no_warning_bit (from);
 
   nowarn_spec_t *from_spec = get_nowarn_spec (from);
   if (RESERVED_LOCATION_P (to_loc))
@@ -209,7 +209,7 @@ void copy_warning (ToType to, FromType from)
 	  nowarn_spec_t tem = *from_spec;
 	  nowarn_map->put (to_loc, tem);
 	}
-      else
+      else if (supp)
 	{
 	  if (nowarn_map)
 	    nowarn_map->remove (to_loc);
@@ -226,6 +226,8 @@ void copy_warning (ToType to, FromType from)
 void
 copy_warning (tree to, const_tree from)
 {
+  if (to == from)
+    return;
   copy_warning<tree, const_tree>(to, from);
 }
 
@@ -250,5 +252,7 @@ copy_warning (gimple *to, const_tree from)
 void
 copy_warning (gimple *to, const gimple *from)
 {
+  if (to == from)
+    return;
   copy_warning<gimple *, const gimple *>(to, from);
 }

// Check the suppression of -Wuse-after-free for destructors on ARM
// { dg-do compile }
// { dg-options "-Wuse-after-free" }

struct range_label {
  virtual ~range_label();
};

struct unpaired_bidi_rich_location {
  struct custom_range_label : range_label {};
  unpaired_bidi_rich_location(int);
  custom_range_label m_custom_label;
};

void maybe_warn_bidi_on_close() { unpaired_bidi_rich_location(0); }