[gcc r15-9112] libstdc++: Fix -Warray-bounds warning in std::vector<bool> [PR110498]

[Jonathan Wakely]

https://gcc.gnu.org/g:aa3aaf2bfb8fcc17076993df4297597b68bc5f60

commit r15-9112-gaa3aaf2bfb8fcc17076993df4297597b68bc5f60
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 28 15:41:41 2025 +0000

    libstdc++: Fix -Warray-bounds warning in std::vector<bool> [PR110498]
    
    In this case, we need to tell the compiler that the current size is not
    larger than the new size so that all the existing elements can be copied
    to the new storage. This avoids bogus warnings about overflowing the new
    storage when the compiler can't tell that that cannot happen.
    
    We might as well also hoist the loads of begin() and end() before the
    allocation too. All callers will have loaded at least begin() before
    calling _M_reallocate.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/110498
            * include/bits/vector.tcc (vector<bool, A>::_M_reallocate):
            Hoist loads of begin() and end() before allocation and use them
            to state an unreachable condition.
            * testsuite/23_containers/vector/bool/capacity/110498.cc: New
            test.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/vector.tcc                   |  5 ++++-
 .../23_containers/vector/bool/capacity/110498.cc       | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 29aa63e47428..7a92f34ec644 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -1106,9 +1106,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     vector<bool, _Alloc>::
     _M_reallocate(size_type __n)
     {
+      const iterator __begin = begin(), __end = end();
+      if (size_type(__end - __begin) > __n)
+	__builtin_unreachable();
       _Bit_pointer __q = this->_M_allocate(__n);
       iterator __start(std::__addressof(*__q), 0);
-      iterator __finish(_M_copy_aligned(begin(), end(), __start));
+      iterator __finish(_M_copy_aligned(__begin, __end, __start));
       this->_M_deallocate();
       this->_M_impl._M_start = __start;
       this->_M_impl._M_finish = __finish;
diff --git a/libstdc++-v3/testsuite/23_containers/vector/bool/capacity/110498.cc b/libstdc++-v3/testsuite/23_containers/vector/bool/capacity/110498.cc
new file mode 100644
index 000000000000..f848edcb25e1
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/bool/capacity/110498.cc
@@ -0,0 +1,18 @@
+// { dg-options "-O3 -Werror=array-bounds -fno-assume-sane-operators-new-delete" }
+// { dg-do compile }
+
+// Bug libstdc++/110498
+// Spurious warnings stringop-overflow and array-bounds copying data as bytes
+// into vector::reserve
+
+#include <vector>
+
+void f(std::vector<bool>& v)
+{
+  // Warning emitted when set to any number in the range [1,64].
+  const std::size_t reserve_size = 30;
+
+  v.reserve(reserve_size);
+  v.push_back(0);
+}
+