[gcc r14-1452] libstdc++: Express std::vector's size() <= capacity() invariant in code

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:fb409a15d9babc78fe1d9957afcbaf1102cce58f

commit r14-1452-gfb409a15d9babc78fe1d9957afcbaf1102cce58f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu May 25 09:57:46 2023 +0100

    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:
---
 libstdc++-v3/include/bits/stl_vector.h             | 30 +++++++++++++++++++---
 .../23_containers/vector/capacity/invariant.cc     | 16 ++++++++++++
 .../testsuite/23_containers/vector/types/1.cc      |  2 +-
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index acb29396d26..e593be443bc 100644
--- 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
index 00000000000..d68db694add
--- /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 079e5af9556..9be07d9fd5c 100644
--- 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>