[gcc r15-9111] libstdc++: Fix -Wstringop-overread warning in std::vector [PR114758]

[Jonathan Wakely via Libstdc++-cvs]

https://gcc.gnu.org/g:1f6c19f307c8de9830130a0ba071c24e3835beb3

commit r15-9111-g1f6c19f307c8de9830130a0ba071c24e3835beb3
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Mar 28 15:41:41 2025 +0000

    libstdc++: Fix -Wstringop-overread warning in std::vector<bool> [PR114758]
    
    As in r13-4393-gcca06f0d6d76b0 and a few other commits, we can avoid
    bogus warnings in std::vector<bool> by hoisting some loads to before the
    allocation that calls operator new. This means that the compiler has
    enough info to remove the dead branches that trigger bogus warnings.
    
    On trunk this is only needed with -fno-assume-sane-operators-new-delete
    but it will help on the branches where that option doesn't exist.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/114758
            * include/bits/vector.tcc (vector<bool, A>::_M_fill_insert):
            Hoist loads of begin() and end() before allocation.
            * testsuite/23_containers/vector/bool/capacity/114758.cc: New
            test.
    
    Reviewed-by: Tomasz Kamiński <tkami...@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/vector.tcc                         |  5 +++--
 .../testsuite/23_containers/vector/bool/capacity/114758.cc   | 12 ++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/vector.tcc 
b/libstdc++-v3/include/bits/vector.tcc
index f197278d52e0..29aa63e47428 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -1134,11 +1134,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
        {
          const size_type __len = 
            _M_check_len(__n, "vector<bool>::_M_fill_insert");
+         iterator __begin = begin(), __end = end();
          _Bit_pointer __q = this->_M_allocate(__len);
          iterator __start(std::__addressof(*__q), 0);
-         iterator __i = _M_copy_aligned(begin(), __position, __start);
+         iterator __i = _M_copy_aligned(__begin, __position, __start);
          std::fill(__i, __i + difference_type(__n), __x);
-         iterator __finish = std::copy(__position, end(),
+         iterator __finish = std::copy(__position, __end,
                                        __i + difference_type(__n));
          this->_M_deallocate();
          this->_M_impl._M_end_of_storage = __q + _S_nword(__len);
diff --git 
a/libstdc++-v3/testsuite/23_containers/vector/bool/capacity/114758.cc 
b/libstdc++-v3/testsuite/23_containers/vector/bool/capacity/114758.cc
new file mode 100644
index 000000000000..c56d50afddb1
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/bool/capacity/114758.cc
@@ -0,0 +1,12 @@
+// { dg-options "-O3 -Werror=stringop-overread 
-fno-assume-sane-operators-new-delete" }
+// { dg-do compile }
+
+// Bug libstdc++/114758 The layout of a std::vector<bool> reports a warning
+
+#include <vector>
+
+void pr114758(std::vector<bool>& v)
+{
+  v.resize(3);
+  v = std::vector<bool>(3, false);
+}