libstdc++: Fix std::tr2::dynamic_bitset shift operations [PR115399]

The shift operations for dynamic_bitset fail to zero out words where the
non-zero bits were shifted to a completely different word.

For a right shift we don't need to sanitize the unused bits in the high
word, because we know they were already clear and a right shift doesn't
change that.

libstdc++-v3/ChangeLog:

	PR libstdc++/115399
	* include/tr2/dynamic_bitset (operator>>=): Remove redundant
	call to _M_do_sanitize.
	* include/tr2/dynamic_bitset.tcc (_M_do_left_shift): Zero out
	low bits in words that should no longer be populated.
	(_M_do_right_shift): Likewise for high bits.
	* testsuite/tr2/dynamic_bitset/pr115399.cc: New test.

3 files changed, 40 insertions, 8 deletions

diff --git a/libstdc++-v3/include/tr2/dynamic_bitset b/libstdc++-v3/include/tr2/dynamic_bitset
index 0e4e889..274c4f6 100644
--- a/libstdc++-v3/include/tr2/dynamic_bitset
+++ b/libstdc++-v3/include/tr2/dynamic_bitset
@@ -815,10 +815,7 @@ namespace tr2
       operator>>=(size_type __pos)
       {
 	if (__builtin_expect(__pos < this->_M_Nb, 1))
-	  {
-	    this->_M_do_right_shift(__pos);
-	    this->_M_do_sanitize();
-	  }
+	  this->_M_do_right_shift(__pos);
 	else
 	  this->_M_do_reset();
 	return *this;
diff --git a/libstdc++-v3/include/tr2/dynamic_bitset.tcc b/libstdc++-v3/include/tr2/dynamic_bitset.tcc
index 63ba6f2..5aac7d8 100644
--- a/libstdc++-v3/include/tr2/dynamic_bitset.tcc
+++ b/libstdc++-v3/include/tr2/dynamic_bitset.tcc
@@ -60,8 +60,7 @@ namespace tr2
 	      this->_M_w[__wshift] = this->_M_w[0] << __offset;
 	    }
 
-	  //// std::fill(this->_M_w.begin(), this->_M_w.begin() + __wshift,
-	  ////          static_cast<_WordT>(0));
+	  std::fill_n(this->_M_w.begin(), __wshift, _WordT(0));
 	}
     }
 
@@ -88,8 +87,7 @@ namespace tr2
 	      this->_M_w[__limit] = this->_M_w[_M_w.size()-1] >> __offset;
 	    }
 
-	  ////std::fill(this->_M_w.begin() + __limit + 1, this->_M_w.end(),
-	  ////          static_cast<_WordT>(0));
+	  std::fill_n(this->_M_w.end() - __wshift, __wshift, _WordT(0));
 	}
     }
 
diff --git a/libstdc++-v3/testsuite/tr2/dynamic_bitset/pr115399.cc b/libstdc++-v3/testsuite/tr2/dynamic_bitset/pr115399.cc
new file mode 100644
index 0000000..e626e4a
--- /dev/null
+++ b/libstdc++-v3/testsuite/tr2/dynamic_bitset/pr115399.cc
@@ -0,0 +1,37 @@
+// { dg-do run { target c++11 } }
+
+// PR libstdc++/115399
+// std::tr2::dynamic_bitset shift behaves differently from std::bitset
+
+#include <tr2/dynamic_bitset>
+#include <testsuite_hooks.h>
+
+void
+test_left_shift()
+{
+  std::tr2::dynamic_bitset<> b(65);
+  b[0] = 1;
+  auto b2 = b << 64;
+  VERIFY(b2[64] == 1);
+  VERIFY(b2[0] == 0);
+  b <<= 64;
+  VERIFY( b2 == b );
+}
+
+void
+test_right_shift()
+{
+  std::tr2::dynamic_bitset<> b(65);
+  b[64] = 1;
+  auto b2 = b >> 64;
+  VERIFY(b2[64] == 0);
+  VERIFY(b2[0] == 1);
+  b >>= 64;
+  VERIFY( b2 == b );
+}
+
+int main()
+{
+  test_left_shift();
+  test_right_shift();
+}