[PATCH] PR libstdc++/90920 restore previous checks for empty ranges

[Jonathan Wakely]

The change in r263433 broke the contract of the __rotate functions, by no
longer accepting empty ranges. That means that callers which inlined the
old version of std::rotate (without checks) that end up linking to a new
definition of std::__rotate (also without checks) could perform a divide
by zero and crash.

This restores the old contract of the __rotate overloads.

	PR libstdc++/90920 partially revert r263433
	* include/bits/stl_algo.h (__rotate): Restore checks for empty ranges.
	(rotate): Remove checks.
	* testsuite/25_algorithms/rotate/90920.cc: New test.


Tested x86_64-linux, committed to trunk.

This needs to be backported to gcc-9-branch too.

-------------- next part --------------
commit 100ba82c20a03d6ff316eb34fb46fe29e5c920fc
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed Jun 19 22:57:02 2019 +0000

    PR libstdc++/90920 restore previous checks for empty ranges
    
    The change in r263433 broke the contract of the __rotate functions, by no
    longer accepting empty ranges. That means that callers which inlined the
    old version of std::rotate (without checks) that end up linking to a new
    definition of std::__rotate (also without checks) could perform a divide
    by zero and crash.
    
    This restores the old contract of the __rotate overloads.
    
            PR libstdc++/90920 partially revert r263433
            * include/bits/stl_algo.h (__rotate): Restore checks for empty ranges.
            (rotate): Remove checks.
            * testsuite/25_algorithms/rotate/90920.cc: New test.
    
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@272489 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index ca957e0b9a7..ac21c55ca1b 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -1251,6 +1251,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	     _ForwardIterator __last,
 	     forward_iterator_tag)
     {
+      if (__first == __middle)
+	return __last;
+      else if (__last == __middle)
+	return __first;
+
       _ForwardIterator __first2 = __middle;
       do
 	{
@@ -1291,6 +1296,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_function_requires(_Mutable_BidirectionalIteratorConcept<
 				  _BidirectionalIterator>)
 
+      if (__first == __middle)
+	return __last;
+      else if (__last == __middle)
+	return __first;
+
       std::__reverse(__first,  __middle, bidirectional_iterator_tag());
       std::__reverse(__middle, __last,   bidirectional_iterator_tag());
 
@@ -1324,6 +1334,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_function_requires(_Mutable_RandomAccessIteratorConcept<
 				  _RandomAccessIterator>)
 
+      if (__first == __middle)
+	return __last;
+      else if (__last == __middle)
+	return __first;
+
       typedef typename iterator_traits<_RandomAccessIterator>::difference_type
 	_Distance;
       typedef typename iterator_traits<_RandomAccessIterator>::value_type
@@ -1425,11 +1440,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __glibcxx_requires_valid_range(__first, __middle);
       __glibcxx_requires_valid_range(__middle, __last);
 
-      if (__first == __middle)
-	return __last;
-      else if (__last  == __middle)
-	return __first;
-
       return std::__rotate(__first, __middle, __last,
 			   std::__iterator_category(__first));
     }
diff --git a/libstdc++-v3/testsuite/25_algorithms/rotate/90920.cc b/libstdc++-v3/testsuite/25_algorithms/rotate/90920.cc
new file mode 100644
index 00000000000..cae3ae748d8
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/rotate/90920.cc
@@ -0,0 +1,48 @@
+// Copyright (C) 2019 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-do run }
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+
+namespace gnu_test
+{
+  // This is the definition from GCC 8.x, with no checks for first==middle
+  // or middle==last.
+  template<typename _ForwardIterator>
+    inline _ForwardIterator
+    rotate(_ForwardIterator __first, _ForwardIterator __middle,
+	   _ForwardIterator __last)
+    {
+      return std::__rotate(__first, __middle, __last,
+			   std::__iterator_category(__first));
+    }
+}
+
+void
+test01()
+{
+  int i = 0;
+  gnu_test::rotate(&i, &i, &i+1);
+}
+
+int
+main()
+{
+  test01();
+}