[gcc r15-10616] libstdc++: Do not optimize std::copy to memcpy for bool output [PR122907]

[Jonathan Wakely]

https://gcc.gnu.org/g:e7664c01527f59b65438fe033b84f89191754300

commit r15-10616-ge7664c01527f59b65438fe033b84f89191754300
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Dec 2 15:26:31 2025 +0000

    libstdc++: Do not optimize std::copy to memcpy for bool output [PR122907]
    
    Copying narrow characters to a range of bool using std::copy cannot be
    optimized to use std::memcpy. Assignment of an arbitrary integer to a
    bool needs to convert all non-zero values to true, so is not a simple
    memcpy-like or bit_cast-like operation. We currently get this wrong and
    optimize it to memcpy, producing invalid bool values.
    
    By making __memcpyable_integer<bool> false we disable memcpy
    optimizations for heterogeneous std::copy and std::move calls where
    either the source or destination type is bool. Copies where both types
    are bool can still optimize to memcpy, because we don't check
    __memcpyable_integer in that case.
    
    This disables the memcpy optimization for bool as the source type,
    which isn't actually necessary (the representation of bool in GCC is
    0x00 or 0x01 and so copying bool to char is just a bit_cast). We don't
    currently have a straightforward way to allow memcpy for bool to char
    but disallow the inverse. This seems acceptable as using std::copy with
    bool inputs and narrow character outputs is probably not common enough
    for this to be an important optimization to do in the library code.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/122907
            * include/bits/cpp_type_traits.h (__memcpyable_integer<bool>):
            Define as false.
            * testsuite/25_algorithms/copy/122907.cc: New test.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    (cherry picked from commit 7a5ad555965d103e73e606417f4289a4238e82d4)

Diff:
---
 libstdc++-v3/include/bits/cpp_type_traits.h        |  7 ++++
 .../testsuite/25_algorithms/copy/122907.cc         | 43 ++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
index b1a6206ce1eb..0e1c4418782d 100644
--- a/libstdc++-v3/include/bits/cpp_type_traits.h
+++ b/libstdc++-v3/include/bits/cpp_type_traits.h
@@ -503,6 +503,13 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
     struct __memcpyable_integer<volatile _Tp>
     { enum { __width = 0 }; };
 
+  // Assigning an integer to bool needs to convert all non-zero values to true
+  // so it is not a memcpyable integer.
+  // __memcpyable<bool*, bool*> is still true though.
+  template<>
+    struct __memcpyable_integer<bool>
+    { enum { __width = 0 }; };
+
   // Specializations for __intNN types with padding bits.
 #if defined __GLIBCXX_TYPE_INT_N_0 && __GLIBCXX_BITSIZE_INT_N_0 % __CHAR_BIT__
   __extension__
diff --git a/libstdc++-v3/testsuite/25_algorithms/copy/122907.cc b/libstdc++-v3/testsuite/25_algorithms/copy/122907.cc
new file mode 100644
index 000000000000..02276cea7b9b
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/copy/122907.cc
@@ -0,0 +1,43 @@
+// { dg-do run }
+
+// Bug libstdc++/122907
+// std::copy incorrectly uses memcpy when copying from signed or unsigned char
+// buffer to bool buffer
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+
+template<typename T>
+__attribute__((noinline,noipa))
+void
+test_pr122907(T (&buf)[4])
+{
+  unsigned char uc[4];
+  bool bool_buf[4];
+  std::copy(buf, buf+1, bool_buf);
+  std::copy(bool_buf, bool_buf+1, uc);
+  VERIFY(uc[0] == bool(buf[0]));
+  std::copy(buf, buf+4, bool_buf);
+  std::copy(bool_buf, bool_buf+4, uc);
+  VERIFY(uc[0] == bool(buf[0]));
+  VERIFY(uc[1] == bool(buf[1]));
+  VERIFY(uc[2] == bool(buf[2]));
+  VERIFY(uc[3] == bool(buf[3]));
+}
+
+template<typename T>
+void
+test_pr122907()
+{
+  T buf[4] = { (T)3,  (T)2, (T)1, (T)0 };
+  test_pr122907(buf);
+}
+
+int main()
+{
+  test_pr122907<char>();
+  test_pr122907<signed char>();
+  test_pr122907<unsigned char>();
+  test_pr122907<bool>();
+  test_pr122907<int>();
+}