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* [PATCH] libstdc++: Fix compare_three_way for constexpr and Clang
@ 2021-08-20 20:17 Paul Keir
  2021-09-03 10:31 ` Paul Keir
  2021-10-11 19:48 ` Jonathan Wakely
  0 siblings, 2 replies; 6+ messages in thread
From: Paul Keir @ 2021-08-20 20:17 UTC (permalink / raw)
  To: gcc-patches; +Cc: libstdc++

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Hi,

The current compare_three_way implementation makes provision for constant evaluation contexts (avoiding reinterpret_cast etc.), but the approach fails with Clang; when it compares two const volatile void pointers: "comparison between unequal pointers to void has unspecified result". I include a fix and test.

Could someone commit the attached patch for me?

Thanks,
Paul


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[-- Attachment #2: compare_three_way_constexpr.patch --]
[-- Type: text/x-patch; name="compare_three_way_constexpr.patch", Size: 2947 bytes --]

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 07cc83d98f4..638f00716c8 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2021-08-20  Paul Keir  <paul.keir@uws.ac.uk>
+
+	* libsupc++/compare: Avoid constexpr pointer comparison failure
+	in std::compare_three_way with Clang.
+	* testsuite/18_support/comparisons/pointers/constexpr.cc:
+	New test.
+
 2021-08-19  Jonathan Wakely  <jwakely@redhat.com>
 
 	* doc/xml/manual/status_cxx2020.xml: Move row  earlier in table.
diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 5aee89e3a6e..4081a3f2315 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -553,10 +553,10 @@ namespace std
       {
 	if constexpr (__detail::__3way_builtin_ptr_cmp<_Tp, _Up>)
 	  {
+	    if (__builtin_is_constant_evaluated())
+	      return static_cast<_Tp&&>(__t) <=> static_cast<_Up&&>(__u);
 	    auto __pt = static_cast<const volatile void*>(__t);
 	    auto __pu = static_cast<const volatile void*>(__u);
-	    if (__builtin_is_constant_evaluated())
-	      return __pt <=> __pu;
 	    auto __it = reinterpret_cast<__UINTPTR_TYPE__>(__pt);
 	    auto __iu = reinterpret_cast<__UINTPTR_TYPE__>(__pu);
 	    return __it <=> __iu;
diff --git a/libstdc++-v3/testsuite/18_support/comparisons/pointers/constexpr.cc b/libstdc++-v3/testsuite/18_support/comparisons/pointers/constexpr.cc
new file mode 100644
index 00000000000..8e1dc2ed6d1
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/comparisons/pointers/constexpr.cc
@@ -0,0 +1,43 @@
+// Copyright (C) 2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <functional>
+
+constexpr bool check01()
+{
+  int arr[2];
+  bool b1 = &arr[0] < &arr[1];
+  bool b2 = std::less{}(&arr[0], &arr[1]);
+  bool b3 = std::compare_three_way{}(&arr[0],&arr[1]) < 0;
+  return b1 && b2 && b3;
+}
+
+constexpr bool check02()
+{
+  int *p = new int[2];
+  bool b1 = &p[0] < &p[1];
+  bool b2 = std::less{}(&p[0], &p[1]);
+  bool b3 = std::compare_three_way{}(&p[0],&p[1]) < 0;
+  delete [] p;
+  return b1 && b2 && b3;
+}
+
+static_assert(check01());
+static_assert(check02());

^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-10-13 21:37 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2021-08-20 20:17 [PATCH] libstdc++: Fix compare_three_way for constexpr and Clang Paul Keir
2021-09-03 10:31 ` Paul Keir
2021-10-11 14:33   ` Paul Keir
2021-10-11 19:48 ` Jonathan Wakely
2021-10-11 21:04   ` Jonathan Wakely
2021-10-13 21:37     ` Paul Keir

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