From 6cc31cc59386d2f38e1af5ee188fa37caab7cd12 Mon Sep 17 00:00:00 2001
From: Kefu Chai <tchaikov@gmail.com>
Date: Sat, 3 Sep 2022 00:21:57 +0800
Subject: [PATCH] denc: support non-c++20 compliant C++ standard library

when compiling with the standard library comes with GCC-10, we have
FTBFS like:

```
src/include/denc.h:517:49: error: 'bit_cast' is not a member of 'std';
did you mean 'bad_cast'?
  517 |   unsigned lowznib = v ?
      (std::countr_zero(std::bit_cast<uint64_t>(v)) / 4) : 0u;
      |                                                 ^~~~~~~~
      |                                                 bad_cast
```

to address this issue, an implementation of std::bit_cast<> is defined
if it is not available. in the long run, we should use a better C++
compiler for compiling the tree.

Fixes: https://tracker.ceph.com/issues/57355
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
---
 src/include/cpp_lib_backport.h | 30 ++++++++++++++++++++++++++++++
 src/include/denc.h             |  1 +
 2 files changed, 31 insertions(+)
 create mode 100644 src/include/cpp_lib_backport.h

diff --git a/src/include/cpp_lib_backport.h b/src/include/cpp_lib_backport.h
new file mode 100644
index 00000000000..ea956c44605
--- /dev/null
+++ b/src/include/cpp_lib_backport.h
@@ -0,0 +1,30 @@
+#pragma once
+
+#include <cstring>
+#include <type_traits>
+
+namespace std {
+
+#ifndef __cpp_lib_bit_cast
+#define __cpp_lib_bit_cast 201806L
+
+/// Create a value of type `To` from the bits of `from`.
+template<typename To, typename From>
+requires (sizeof(To) == sizeof(From)) &&
+         std::is_trivially_copyable_v<From> &&
+         std::is_trivially_copyable_v<To>
+[[nodiscard]] constexpr To
+bit_cast(const From& from) noexcept {
+#if __has_builtin(__builtin_bit_cast)
+  return __builtin_bit_cast(To, from);
+#else
+  static_assert(std::is_trivially_constructible_v<To>);
+  To to;
+  std::memcpy(&to, &from, sizeof(To));
+  return to;
+#endif
+}
+
+#endif // __cpp_lib_bit_cast
+
+} // namespace std
diff --git a/src/include/denc.h b/src/include/denc.h
index 0f080abc678..d075dd51831 100644
--- a/src/include/denc.h
+++ b/src/include/denc.h
@@ -41,6 +41,7 @@
 #include <boost/intrusive/set.hpp>
 #include <boost/optional.hpp>
 
+#include "include/cpp_lib_backport.h"
 #include "include/compat.h"
 #include "include/int_types.h"
 #include "include/scope_guard.h"
-- 
2.39.5