Re: [PATCH] Help compiler detect invalid code

["François Dumont" <frs.dumont@gmail.com>]

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Some more tests have revealed  a small problem in is_sorted test. It was 
revealed by the Debug mode but not in a clean ways so for the moment I 
prefer to fix it and I'll add a _neg test when Debug is able to report 
it correctly.

I've also added a _neg test for equal which doesn't need Debug mode.

OK to commit ?

François



On 9/20/19 7:08 AM, François Dumont wrote:
> I already realized that previous patch will be too controversial to be 
> accepted.
>
> In this new version I just implement a real memmove in __memmove so 
> that in copy_backward there is no need for a shortcut to a more 
> defensive code.
>
> I'll see if in Debug mode I can do something.
>
> François
>
>
> On 9/19/19 10:27 PM, François Dumont wrote:
>> Hi
>>
>>     I start working on making recently added constexpr tests to work 
>> in Debug mode.
>>
>>     It appears that the compiler is able to detect code mistakes 
>> pretty well as long we don't try to hide the code intention with a 
>> defensive approach. This is why I'd like to propose to replace '__n > 
>> 0' conditions with '__n != 0'.
>>
>>     The result is demonstrated by the constexpr_neg.cc tests. What do 
>> you think ?
>>
>>     * include/bits/stl_algobase.h (__memmove): Return _Tp*.
>>     (__memmove): Loop as long as __n is not 0.
>>     (__copy_move<>::__copy_m): Likewise.
>>     (__copy_move_backward<>::__copy_move_b): Likewise.
>>     * testsuite/25_algorithms/copy/constexpr.cc: Add check on copied 
>> values.
>>     * testsuite/25_algorithms/copy_backward/constexpr.cc: Likewise.
>>     * testsuite/25_algorithms/copy/constexpr_neg.cc: New.
>>     * testsuite/25_algorithms/copy_backward/constexpr.cc: New.
>>
>>     I'll submit the patch to fix Debug mode depending on the decision 
>> for this one.
>>
>> François
>>
>


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diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 4eba053ac75..94a79b85d15 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -83,27 +83,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    */
   template<bool _IsMove, typename _Tp>
     _GLIBCXX14_CONSTEXPR
-    inline void*
-    __memmove(_Tp* __dst, const _Tp* __src, size_t __num)
+    inline void
+    __memmove(_Tp* __dst, const _Tp* __src, ptrdiff_t __num)
     {
 #ifdef __cpp_lib_is_constant_evaluated
       if (std::is_constant_evaluated())
 	{
-	  for(; __num > 0; --__num)
+	  __dst += __num;
+	  __src += __num;
+	  for (; __num != 0; --__num)
 	    {
 	      if constexpr (_IsMove)
-		*__dst = std::move(*__src);
+		*--__dst = std::move(*--__src);
 	      else
-		*__dst = *__src;
-	      ++__src;
-	      ++__dst;
+		*--__dst = *--__src;
 	    }
-	  return __dst;
 	}
       else
 #endif
-	return __builtin_memmove(__dst, __src, sizeof(_Tp) * __num);
-      return __dst;
+	__builtin_memmove(__dst, __src, sizeof(_Tp) * __num);
     }
 
   /*
@@ -730,12 +728,6 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 			     && __is_pointer<_BI2>::__value
 			     && __are_same<_ValueType1, _ValueType2>::__value);
 
-#ifdef __cpp_lib_is_constant_evaluated
-      if (std::is_constant_evaluated())
-	return std::__copy_move_backward<true, false,
-				 _Category>::__copy_move_b(__first, __last,
-							   __result);
-#endif
       return std::__copy_move_backward<_IsMove, __simple,
 				       _Category>::__copy_move_b(__first,
 								 __last,
diff --git a/libstdc++-v3/testsuite/25_algorithms/copy/constexpr.cc b/libstdc++-v3/testsuite/25_algorithms/copy/constexpr.cc
index 67910b8773e..a0460603496 100644
--- a/libstdc++-v3/testsuite/25_algorithms/copy/constexpr.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/copy/constexpr.cc
@@ -24,12 +24,12 @@
 constexpr bool
 test()
 {
-  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  constexpr std::array<int, 12> ca0{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}};
   std::array<int, 12> ma0{{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}};
 
   const auto out6 = std::copy(ca0.begin(), ca0.begin() + 8, ma0.begin() + 2);
 
-  return out6 == ma0.begin() + 10;
+  return out6 == ma0.begin() + 10 && *(ma0.begin() + 2) == 1 && *out6 == 0;
 }
 
 static_assert(test());
diff --git a/libstdc++-v3/testsuite/25_algorithms/copy/constexpr_neg.cc b/libstdc++-v3/testsuite/25_algorithms/copy/constexpr_neg.cc
new file mode 100644
index 00000000000..49052467409
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/copy/constexpr_neg.cc
@@ -0,0 +1,38 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a xfail *-*-* } }
+
+#include <algorithm>
+#include <array>
+
+constexpr bool
+test()
+{
+  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  std::array<int, 12> ma0{{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}};
+
+  const auto out6 = std::copy(ca0.begin() + 8, ca0.begin(), ma0.begin() + 2);
+
+  return out6 == ma0.begin() + 10;
+}
+
+static_assert(test()); // { dg-error "outside the bounds" }
+
+// { dg-prune-output "non-constant condition" }
+// { dg-prune-output "in 'constexpr'" }
diff --git a/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr.cc b/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr.cc
index ed7487905a8..c0e1a747832 100644
--- a/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr.cc
@@ -22,15 +22,27 @@
 #include <array>
 
 constexpr bool
-test()
+test1()
 {
-  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  constexpr std::array<int, 12> ca0{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}};
 
   std::array<int, 12> ma0{{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}};
   const auto out7 = std::copy_backward(ca0.begin(), ca0.begin() + 8,
 				       ma0.begin() + 10);
 
-  return true;
+  return out7 == ma0.begin() + 2 && *out7 == 1 && *(ma0.begin() + 10) == 0;
 }
 
-static_assert(test());
+static_assert(test1());
+
+constexpr bool
+test2()
+{
+  std::array<int, 12> ma0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  const auto out7 = std::copy_backward(ma0.begin(), ma0.begin() + 8,
+				       ma0.begin() + 10);
+
+  return out7 == ma0.begin() + 2 && *out7 == 0 && *(ma0.begin() + 9) == 7;
+}
+
+static_assert(test2());
diff --git a/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr_neg.cc b/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr_neg.cc
new file mode 100644
index 00000000000..3dc595d239f
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/copy_backward/constexpr_neg.cc
@@ -0,0 +1,39 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a xfail *-*-* } }
+
+#include <algorithm>
+#include <array>
+
+constexpr bool
+test()
+{
+  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  std::array<int, 12> ma0{{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}};
+
+  const auto out7 = std::copy_backward(ca0.begin() + 8, ca0.begin(),
+				       ma0.begin() + 10);
+
+  return out7 == ma0.begin() + 2;
+}
+
+static_assert(test()); // { dg-error "outside the bounds" }
+
+// { dg-prune-output "non-constant condition" }
+// { dg-prune-output "in 'constexpr'" }
diff --git a/libstdc++-v3/testsuite/25_algorithms/equal/constexpr_neg.cc b/libstdc++-v3/testsuite/25_algorithms/equal/constexpr_neg.cc
new file mode 100644
index 00000000000..065ca1e89ff
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/equal/constexpr_neg.cc
@@ -0,0 +1,49 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a xfail *-*-* } }
+
+#include <algorithm>
+#include <array>
+
+constexpr bool
+test01()
+{
+  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  constexpr std::array<int, 12> ca1{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+
+  const auto outa = std::equal(ca0.end(), ca0.begin(), ca1.begin());
+  return outa;
+}
+
+static_assert(test01()); // { dg-error "outside the bounds" }
+
+constexpr bool
+test02()
+{
+  constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
+  constexpr std::array<int, 11> ca1{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}};
+
+  const auto outa = std::equal(ca0.begin(), ca0.end(), ca1.begin());
+  return outa;
+}
+
+static_assert(test02()); // { dg-error "outside the bounds" }
+
+// { dg-prune-output "non-constant condition" }
+// { dg-prune-output "in 'constexpr'" }
diff --git a/libstdc++-v3/testsuite/25_algorithms/is_sorted/constexpr.cc b/libstdc++-v3/testsuite/25_algorithms/is_sorted/constexpr.cc
index 0be2f5fed62..f549b3d9307 100644
--- a/libstdc++-v3/testsuite/25_algorithms/is_sorted/constexpr.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/is_sorted/constexpr.cc
@@ -26,7 +26,7 @@ constexpr std::array<int, 12> ca0{{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}};
 constexpr auto outv = std::is_sorted(ca0.begin(), ca0.end());
 
 constexpr auto outw = std::is_sorted(ca0.begin(), ca0.end(),
-				     std::equal_to<int>());
+				     std::less<int>());
 
 constexpr bool
 test()