[committed] libstdc++: Make byte-sized std::fill_n a constant expression (PR 94933)

[Jonathan Wakely via Gcc-patches]

The overload for byte types uses memset and isn't constexpr. This adds
the specifier and uses std::is_constant_evaluated() to provide a
compile-time alternative.

        PR libstdc++/94933
        * include/bits/stl_algobase.h (__fill_a1): Make overload for byte types
        usable in constant expressions.
        * testsuite/25_algorithms/fill_n/constexpr.cc: Test with bytes and
        non-scalars.

Tested powerpc64le-linux. Committed to master.

This should be backported to gcc-10 but I'll wait for 10.2



commit 22b6b5d6cfb76deb68ca5bc0bcae8b4245df946d
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Sun May 3 12:46:15 2020 +0100

    libstdc++: Make byte-sized std::fill_n a constant expression (PR 94933)
    
    The overload for byte types uses memset and isn't constexpr. This adds
    the specifier and uses std::is_constant_evaluated() to provide a
    compile-time alternative.
    
            PR libstdc++/94933
            * include/bits/stl_algobase.h (__fill_a1): Make overload for byte 
types
            usable in constant expressions.
            * testsuite/25_algorithms/fill_n/constexpr.cc: Test with bytes and
            non-scalars.

diff --git a/libstdc++-v3/include/bits/stl_algobase.h 
b/libstdc++-v3/include/bits/stl_algobase.h
index a7e92d4b473..bed61fd9d00 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -875,11 +875,20 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 
   // Specialization: for char types we can use memset.
   template<typename _Tp>
+    _GLIBCXX20_CONSTEXPR
     inline typename
     __gnu_cxx::__enable_if<__is_byte<_Tp>::__value, void>::__type
     __fill_a1(_Tp* __first, _Tp* __last, const _Tp& __c)
     {
       const _Tp __tmp = __c;
+#if __cpp_lib_is_constant_evaluated
+      if (std::is_constant_evaluated())
+       {
+         for (; __first != __last; ++__first)
+           *__first = __tmp;
+         return;
+       }
+#endif
       if (const size_t __len = __last - __first)
        __builtin_memset(__first, static_cast<unsigned char>(__tmp), __len);
     }
diff --git a/libstdc++-v3/testsuite/25_algorithms/fill_n/constexpr.cc 
b/libstdc++-v3/testsuite/25_algorithms/fill_n/constexpr.cc
index c18b6c99eda..17c9aa5a8d7 100644
--- a/libstdc++-v3/testsuite/25_algorithms/fill_n/constexpr.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/fill_n/constexpr.cc
@@ -28,7 +28,37 @@ test()
 
   const auto outd = std::fill_n(ma0.begin(), 6, 77);
 
-  return outd == ma0.begin() + 6;
+  return outd == ma0.begin() + 6 && ma0[5] == 77 && ma0[6] == 0;
 }
 
 static_assert(test());
+
+constexpr bool
+test_byte()
+{
+  // PR libstdc++/94933
+  std::array<char, 12> ma0{};
+
+  const auto outd = std::fill_n(ma0.begin(), 6, 77);
+
+  return outd == ma0.begin() + 6 && ma0[5] == 77 && ma0[6] == 0;
+}
+
+static_assert( test_byte() );
+
+struct S
+{
+  int i = 0;
+};
+
+constexpr bool
+test_nonscalar()
+{
+  std::array<S, 12> ma0{};
+
+  const auto outd = std::fill_n(ma0.begin(), 6, S{77});
+
+  return outd == ma0.begin() + 6 && ma0[5].i == 77 && ma0[6].i == 0;
+}
+
+static_assert( test_nonscalar() );