libstdc++: Micro-optimizaion for std::bitset stream extraction

Don't bother trying to copy any characters for bitset<0>.

libstdc++-v3/ChangeLog:

	* include/std/bitset (operator>>): Do not copy for N==0.
	* testsuite/20_util/bitset/io/input.cc: Add comment.

diff --git a/libstdc++-v3/include/std/bitset b/libstdc++-v3/include/std/bitset
index 83c6416b7706292a72c304c6d2ec7fa72d8a50df..6dbc58c6429e12cc9f4bde60851887a75666e519 100644 (file)
--- a/libstdc++-v3/include/std/bitset
+++ b/libstdc++-v3/include/std/bitset
@@ -1615,7 +1615,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
       if (__tmp.empty() && _Nb)
        __state |= __ios_base::failbit;
-      else
+      else if _GLIBCXX17_CONSTEXPR (_Nb)
        __x._M_copy_from_string(__tmp, static_cast<size_t>(0), _Nb,
                                __zero, __one);
       if (__state)

diff --git a/libstdc++-v3/testsuite/20_util/bitset/io/input.cc b/libstdc++-v3/testsuite/20_util/bitset/io/input.cc
index 939861b171eaa2f8a023ecce0a83246fa3d488dd..0f22cefbb5b7aef9276f37b374229f8d638e72a5 100644 (file)
--- a/libstdc++-v3/testsuite/20_util/bitset/io/input.cc
+++ b/libstdc++-v3/testsuite/20_util/bitset/io/input.cc
@@ -39,7 +39,7 @@ void test01()
   ss.clear();
   ss.str("*");
   ss >> b0;
-  VERIFY( ss.rdstate() == ios_base::goodbit );
+  VERIFY( ss.rdstate() == ios_base::goodbit ); // LWG 3199
 }
 
 int main()