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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108517] std::sort of empty range yield "warning: 'this' pointer is null"
Date: Tue, 24 Jan 2023 12:41:59 +0000	[thread overview]
Message-ID: <bug-108517-4-OhfhBIBYTv@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-108517-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108517

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Libstdc++ isn't causing it, but we can give the compiler enough information to
make it shut up:

--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -1814,6 +1814,10 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
     {
       if (__first == __last) return;

+      if (__first == _RandomAccessIterator()
+            || __last == _RandomAccessIterator())
+        __builtin_unreachable();
+
       for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)
        {
          if (__comp(__i, __first))


As you can see there, we already return early and never reach line 1819 where
the dereference happens. We can tell the compiler that it's impossible to get
to that that dereference with a value-initialized iterator, because if one is
null the other must be, and so we'd already have returned.

  parent reply	other threads:[~2023-01-24 12:41 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-24 12:01 [Bug libstdc++/108517] New: " hanicka at hanicka dot net
2023-01-24 12:33 ` [Bug libstdc++/108517] " redi at gcc dot gnu.org
2023-01-24 12:35 ` redi at gcc dot gnu.org
2023-01-24 12:37 ` [Bug c++/108517] " redi at gcc dot gnu.org
2023-01-24 12:41 ` redi at gcc dot gnu.org [this message]
2023-01-24 13:01 ` [Bug c++/108517] [11/12/13 Regression] " redi at gcc dot gnu.org
2023-01-24 13:07 ` rguenth at gcc dot gnu.org
2023-02-01 12:32 ` jakub at gcc dot gnu.org
2023-02-01 12:40 ` jakub at gcc dot gnu.org
2023-02-15 15:28 ` jamborm at gcc dot gnu.org
2023-02-21 12:59 ` rguenth at gcc dot gnu.org
2023-05-29 10:07 ` [Bug c++/108517] [11/12/13/14 " jakub at gcc dot gnu.org

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