build: enable experimental Cxx20 support (#4600)

* build: Add optional c++20 compilation option

* build: update ax_cxx_compile_stdcxx.m4 to be compatible with c++20

* fix: fix c++20 build error for undefined identifier

* ci: enable c++20 build during ci

* cxx17 -> cxx20

Signed-off-by: pasta <pasta@dashboost.org>

.gitlab-ci.yml

@@ -241,12 +241,12 @@ linux64-build:
   variables:
     BUILD_TARGET: linux64
 
-linux64_cxx17-build:
+linux64_cxx20-build:
   extends: .build-template
   needs:
     - x86_64-unknown-linux-gnu-debug
   variables:
-    BUILD_TARGET: linux64_cxx17
+    BUILD_TARGET: linux64_cxx20
 
 linux64_nowallet-build:
   extends:

build-aux/m4/ax_cxx_compile_stdcxx.m4

@@ -10,8 +10,9 @@
 #
 #   Check for baseline language coverage in the compiler for the specified
 #   version of the C++ standard.  If necessary, add switches to CXX and
-#   CXXCPP to enable support.  VERSION may be '11' (for the C++11 standard)
-#   or '14' (for the C++14 standard).
+#   CXXCPP to enable support.  VERSION may be '11' (for the C++11 standard),
+#   '14' (for the C++14 standard), '17' (for the C++17 standard) or
+#   '20' (for the C++20 standard)
 #
 #   The second argument, if specified, indicates whether you insist on an
 #   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
@@ -46,10 +47,14 @@
 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl  (serial version number 13).
 
+dnl  Modifications:
+dnl  Add support for C++20, with no new tests
+
 AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
   m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
         [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
+        [$1], [20], [ax_cxx_compile_alternatives="20 2a"],
         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
   m4_if([$2], [], [],
         [$2], [ext], [],
@@ -154,6 +159,22 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
   _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
 )
 
+
+dnl Test body for checking C++20 support: R modification
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_20],
+#ifndef __cplusplus
+#error "This is not a C++ compiler"
+dnl value from 2020-01-14 draft, clang 11 has 202002L
+#elif __cplusplus < 201703L
+#error "This is not a C++20 compiler"
+#else
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+#endif
+)
+
+
 dnl  Tests for new features in C++11
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
@@ -251,7 +272,7 @@ namespace cxx11
     }
 
     int
-    test(const int c, volatile int v)
+    test(const int c, volatile int v) // 'volatile is deprecated in C++20'
     {
       static_assert(is_same<int, decltype(0)>::value == true, "");
       static_assert(is_same<int, decltype(c)>::value == false, "");

ci/matrix.sh

@@ -59,10 +59,10 @@ elif [ "$BUILD_TARGET" = "linux64" ]; then
   export BITCOIN_CONFIG="--enable-zmq --enable-reduce-exports --enable-crash-hooks --with-sanitizers=undefined"
   export CPPFLAGS="-DDEBUG_LOCKORDER -DENABLE_DASH_DEBUG -DARENA_DEBUG"
   export PYZMQ=true
-elif [ "$BUILD_TARGET" = "linux64_cxx17" ]; then
+elif [ "$BUILD_TARGET" = "linux64_cxx20" ]; then
   export HOST=x86_64-unknown-linux-gnu
   export DEP_OPTS="NO_UPNP=1 DEBUG=1"
-  export BITCOIN_CONFIG="--enable-zmq --enable-reduce-exports --enable-crash-hooks --enable-c++17 --enable-suppress-external-warnings --enable-werror --with-sanitizers=undefined"
+  export BITCOIN_CONFIG="--enable-zmq --enable-reduce-exports --enable-crash-hooks --enable-c++20 --enable-suppress-external-warnings --enable-werror --with-sanitizers=undefined"
   export CPPFLAGS="-DDEBUG_LOCKORDER -DENABLE_DASH_DEBUG -DARENA_DEBUG"
   export PYZMQ=true
   export RUN_INTEGRATIONTESTS=false

configure.ac

@@ -62,8 +62,18 @@ case $host in
   ;;
 esac
 
-dnl Require C++17 compiler (no GNU extensions)
+AC_ARG_ENABLE([c++20],
+  [AS_HELP_STRING([--enable-c++20],
+  [enable compilation in c++20 mode (disabled by default)])],
+  [use_cxx20=$enableval],
+  [use_cxx20=no])
+
+dnl Require C++17 or C++20 compiler (no GNU extensions)
+if test "x$use_cxx20" = xyes; then
+  AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
+else
   AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
+fi
 
 dnl Check if -latomic is required for <std::atomic>
 CHECK_ATOMIC

src/leveldb/util/env_posix.cc

@@ -850,7 +850,7 @@ class SingletonEnv {
  public:
   SingletonEnv() {
 #if !defined(NDEBUG)
-    env_initialized_.store(true, std::memory_order::memory_order_relaxed);
+    env_initialized_.store(true, std::memory_order_relaxed);
 #endif  // !defined(NDEBUG)
     static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                   "env_storage_ will not fit the Env");
@@ -867,7 +867,7 @@ class SingletonEnv {
 
   static void AssertEnvNotInitialized() {
 #if !defined(NDEBUG)
-    assert(!env_initialized_.load(std::memory_order::memory_order_relaxed));
+    assert(!env_initialized_.load(std::memory_order_relaxed));
 #endif  // !defined(NDEBUG)
   }