Re: [PATCH] Improvements to valid range checks in debug mode

[Jonathan Wakely <jwakely@redhat.com>]

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On Mon, 16 Aug 2021 at 20:26, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Fri, 13 Aug 2021 at 18:10, François Dumont via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > I just come back to this email and it sounds like the right moment to
> > commit it.
>
> Ah yes, thanks for the reminder!
>
> I'll push it tomorrow.
>
> >
> > On 27/02/20 2:06 pm, Jonathan Wakely wrote:
> > > These should wait for stage 1 but I'm posting them now for comment.
> > >
> > > With the change to __gnu_debug::__valid_range we now get a debug
> > > assertion for:
> > >
> > >   std::string s;
> > >   std::min_element(std::string::iterator{}, s.end());
> > >
> > > where previously it would just crash with undefined behaviour.

Actually, that change doesn't work. Some of our container iterators
use a value-initialized iterator as the past-the-end value, so the
check in the new __valid_range_aux function incorrectly rejects some
valid ranges. Maybe I can make it work for bidirectional iterators,
which must be attached to a container to be valid.

For now I've only pushed the second half of the patch, optimizing the
std::min/max/minmax overloads taking an initializer_list (as
attached).

Tested powerpc64le-linux, pushed to trunk.

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commit de44eee5d52f5980b6b2b2120940f70cc2fa007e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Aug 18 12:24:12 2021

    libstdc++: Minor optimization for min/max/minmax
    
    The debug mode checks for a valid range are redundant when we have an
    initializer_list argument, because we know it's a valid range already.
    By making std::min(initialier_list<T>) call the internal __min_element
    function directly we avoid a function call and skip those checks. The
    same can be done for the overload taking a comparison function, and also
    for the std::max and std::minmax overloads for initializer_list
    arguments.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_algo.h (min(initializer_list<T>))
            (min(initializer_list<T>, Compare)): Call __min_element directly to
            avoid redundant debug checks for valid ranges.
            (max(initializer_list<T>), max(initializer_list<T>, Compare)):
            Likewise, for __max_element.
            (minmax(initializer_list<T>), minmax(initializer_list<T>, Compare)):
            Likewise, for __minmax_element.

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index 54ad383711f..ac4f2d0f721 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -3445,38 +3445,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				   __gnu_cxx::__ops::__iter_comp_iter(__comp));
     }
 
-  // N2722 + DR 915.
-  template<typename _Tp>
-    _GLIBCXX14_CONSTEXPR
-    inline _Tp
-    min(initializer_list<_Tp> __l)
-    { return *std::min_element(__l.begin(), __l.end()); }
-
-  template<typename _Tp, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
-    inline _Tp
-    min(initializer_list<_Tp> __l, _Compare __comp)
-    { return *std::min_element(__l.begin(), __l.end(), __comp); }
-
-  template<typename _Tp>
-    _GLIBCXX14_CONSTEXPR
-    inline _Tp
-    max(initializer_list<_Tp> __l)
-    { return *std::max_element(__l.begin(), __l.end()); }
-
-  template<typename _Tp, typename _Compare>
-    _GLIBCXX14_CONSTEXPR
-    inline _Tp
-    max(initializer_list<_Tp> __l, _Compare __comp)
-    { return *std::max_element(__l.begin(), __l.end(), __comp); }
-
   template<typename _Tp>
     _GLIBCXX14_CONSTEXPR
     inline pair<_Tp, _Tp>
     minmax(initializer_list<_Tp> __l)
     {
+      __glibcxx_requires_irreflexive(__l.begin(), __l.end());
       pair<const _Tp*, const _Tp*> __p =
-	std::minmax_element(__l.begin(), __l.end());
+	std::__minmax_element(__l.begin(), __l.end(),
+			      __gnu_cxx::__ops::__iter_less_iter());
       return std::make_pair(*__p.first, *__p.second);
     }
 
@@ -3485,8 +3462,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline pair<_Tp, _Tp>
     minmax(initializer_list<_Tp> __l, _Compare __comp)
     {
+      __glibcxx_requires_irreflexive_pred(__l.begin(), __l.end(), __comp);
       pair<const _Tp*, const _Tp*> __p =
-	std::minmax_element(__l.begin(), __l.end(), __comp);
+	std::__minmax_element(__l.begin(), __l.end(),
+			      __gnu_cxx::__ops::__iter_comp_iter(__comp));
       return std::make_pair(*__p.first, *__p.second);
     }
 
@@ -3793,7 +3772,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       for (_RandomAccessIterator __i = __first + 1; __i != __last; ++__i)
 	std::iter_swap(__i, __first + __d(__g, __p_type(0, __i - __first)));
     }
-#endif
+#endif // USE C99_STDINT
 
 #endif // C++11
 
@@ -5746,6 +5725,49 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 				__gnu_cxx::__ops::__iter_comp_iter(__comp));
     }
 
+#if __cplusplus >= 201103L
+  // N2722 + DR 915.
+  template<typename _Tp>
+    _GLIBCXX14_CONSTEXPR
+    inline _Tp
+    min(initializer_list<_Tp> __l)
+    {
+      __glibcxx_requires_irreflexive(__l.begin(), __l.end());
+      return *_GLIBCXX_STD_A::__min_element(__l.begin(), __l.end(),
+	  __gnu_cxx::__ops::__iter_less_iter());
+    }
+
+  template<typename _Tp, typename _Compare>
+    _GLIBCXX14_CONSTEXPR
+    inline _Tp
+    min(initializer_list<_Tp> __l, _Compare __comp)
+    {
+      __glibcxx_requires_irreflexive_pred(__l.begin(), __l.end(), __comp);
+      return *_GLIBCXX_STD_A::__min_element(__l.begin(), __l.end(),
+	  __gnu_cxx::__ops::__iter_comp_iter(__comp));
+    }
+
+  template<typename _Tp>
+    _GLIBCXX14_CONSTEXPR
+    inline _Tp
+    max(initializer_list<_Tp> __l)
+    {
+      __glibcxx_requires_irreflexive(__l.begin(), __l.end());
+      return *_GLIBCXX_STD_A::__max_element(__l.begin(), __l.end(),
+	  __gnu_cxx::__ops::__iter_less_iter());
+    }
+
+  template<typename _Tp, typename _Compare>
+    _GLIBCXX14_CONSTEXPR
+    inline _Tp
+    max(initializer_list<_Tp> __l, _Compare __comp)
+    {
+      __glibcxx_requires_irreflexive_pred(__l.begin(), __l.end(), __comp);
+      return *_GLIBCXX_STD_A::__max_element(__l.begin(), __l.end(),
+	  __gnu_cxx::__ops::__iter_comp_iter(__comp));
+    }
+#endif // C++11
+
 #if __cplusplus >= 201402L
   /// Reservoir sampling algorithm.
   template<typename _InputIterator, typename _RandomAccessIterator,