libstdc++: Express std::vector's size() <= capacity() invariant in code

This adds optimizer hints so that GCC knows that size() <= capacity() is
always true. This allows the compiler to optimize away re-allocating
paths when assigning new values to the vector without resizing it, e.g.,
vec.assign(vec.size(), new_val).

libstdc++-v3/ChangeLog:

	* include/bits/stl_vector.h (_Vector_base::_M_invariant()): New
	function.
	(vector::size(), vector::capacity()): Call _M_invariant().
	* testsuite/23_containers/vector/capacity/invariant.cc: New test.
	* testsuite/23_containers/vector/types/1.cc: Add suppression for
	false positive warning (PR110060).

diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index acb29396d2640974e6e959f000895855318a4591..e593be443bc2a0a941b1aad8ce1bb7ed9f120c15 100644 (file)
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -388,6 +388,24 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       }
 
     protected:
+
+      __attribute__((__always_inline__))
+      _GLIBCXX20_CONSTEXPR void
+      _M_invariant() const
+      {
+#if __OPTIMIZE__
+       if (this->_M_impl._M_finish < this->_M_impl._M_start)
+         __builtin_unreachable();
+       if (this->_M_impl._M_finish > this->_M_impl._M_end_of_storage)
+         __builtin_unreachable();
+
+       size_t __sz = this->_M_impl._M_finish - this->_M_impl._M_start;
+       size_t __cap = this->_M_impl._M_end_of_storage - this->_M_impl._M_start;
+       if (__sz > __cap)
+         __builtin_unreachable();
+#endif
+      }
+
       _GLIBCXX20_CONSTEXPR
       void
       _M_create_storage(size_t __n)
@@ -987,7 +1005,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return size_type(this->_M_impl._M_finish - this->_M_impl._M_start); }
+      {
+       _Base::_M_invariant();
+       return size_type(this->_M_impl._M_finish - this->_M_impl._M_start);
+      }
 
       /**  Returns the size() of the largest possible %vector.  */
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
@@ -1073,8 +1094,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
       size_type
       capacity() const _GLIBCXX_NOEXCEPT
-      { return size_type(this->_M_impl._M_end_of_storage
-                        - this->_M_impl._M_start); }
+      {
+       _Base::_M_invariant();
+       return size_type(this->_M_impl._M_end_of_storage
+                          - this->_M_impl._M_start);
+      }
 
       /**
        *  Returns true if the %vector is empty.  (Thus begin() would

diff --git a/libstdc++-v3/testsuite/23_containers/vector/capacity/invariant.cc b/libstdc++-v3/testsuite/23_containers/vector/capacity/invariant.cc
new file mode 100644 (file)
index 0000000..d68db69
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/capacity/invariant.cc
@@ -0,0 +1,16 @@
+// { dg-do compile }
+// { dg-options "-O3 -g0" }
+// { dg-final { scan-assembler-not "_Znw" } }
+// GCC should be able to optimize away the paths involving reallocation.
+
+#include <vector>
+
+void fill(std::vector<int>& vec)
+{
+  vec.assign(vec.size(), 0);
+}
+
+void fill_val(std::vector<int>& vec, int i)
+{
+  vec.assign(vec.size(), i);
+}

diff --git a/libstdc++-v3/testsuite/23_containers/vector/types/1.cc b/libstdc++-v3/testsuite/23_containers/vector/types/1.cc
index 079e5af95567792ee5e6e4290d640ad76bf5d6dc..9be07d9fd5c60958ff093ae15c31e350b590312b 100644 (file)
--- a/libstdc++-v3/testsuite/23_containers/vector/types/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/vector/types/1.cc
@@ -18,7 +18,7 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-do compile }
-// { dg-options "-Wno-unused-result" }
+// { dg-options "-Wno-unused-result -Wno-stringop-overread" }
 
 #include <vector>
 #include <testsuite_greedy_ops.h>