constinit, here we come

Following up on b13421011d9377676e1adc282634991d5064a866 "better data
structures for some static const vars":

* Make the o3tl::sorted_vector ctor taking an initializer_list constexpr.
  <https://wg21.link/P0202R3> "Add Constexpr Modifiers to Functions in
  <algorithm> and <utility> Headers", which will be in C++2a and is already
  implemented by recent libc++ and libstdc++ according to
  <https://en.cppreference.com/w/cpp/compiler_support
  #C.2B.2B2a_library_features>, makes std::sort constexpr but explicitly keeps
  std::stable_sort non-constexpr.  ("Algorithms stable_partition, inplace_merge
  and stable_sort allocate memory, construct variables using placement new, use
  unique_ptr and do other things not acceptable in constexpr expressions.
  Making those algorithms constexpr seems to be a hard task that would require
  a lot of intrinsics.  Those algorithms are not marked with constexpr in this
  wording.")  But keep o3tl::sorted_vector::Resort (which was introduced in
  c59355e936446fe55960209e543b072acb6b2170 "fdo#58793: re-implement
  SwpHintsArray::Resort()") using std::stable_sort instead of std::sort, in
  case that is relevant for its pre-exisiting uses.

* <https://wg21.link/P1004R2> "Making std::vector constexpr", which was voted
  into C++2a according to <https://wg21.link/n4829> "Editors' Report --
  Programming Languages -- C++", will make the relevant parts of std::vector
  constexpr.  But none of libc++, libstdc++, and MSVC implement that as of now.

* Introduce HAVE_CPP_CONSTINIT_SORTED_VECTOR to hide the constinit behind for
  now for the one case from b13421011d9377676e1adc282634991d5064a866 "better
  data structures for some static const vars" that can clearly be constinit
  once constexpr std::vector is supported by compilers.  The other three cases
  (s_aContainerDocumentCommands in
  chart2/source/controller/main/CommandDispatchContainer.cxx,
  aMetricCompatibleMap in vcl/source/font/PhysicalFontCollection.cxx, and
  aBlacklist in writerfilter/source/dmapper/PropertyMap.cxx) would each need a
  constexpr OUString first.  (Technically, the constinit would not even be
  needed, but it nicely documents our intent that this will actually be
  initialized at compile-time once compilers support that.)

Change-Id: Ibeb138f120528be3a7a09b3912143bf795fbab29
Reviewed-on: https://gerrit.libreoffice.org/79556
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 88c512c2635f..eb08a5883eda 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6618,6 +6618,28 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 CXXFLAGS=$save_CXXFLAGS
 AC_LANG_POP([C++])
 
+AC_MSG_CHECKING([whether $CXX_BASE supports C++2a constinit sorted vectors])
+AC_LANG_PUSH([C++])
+save_CXXFLAGS=$CXXFLAGS
+CXXFLAGS="$CXXFLAGS $CXXFLAGS_CXX11"
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+        #include <algorithm>
+        #include <initializer_list>
+        #include <vector>
+        template<typename T> class S {
+        private:
+            std::vector<T> v_;
+        public:
+            constexpr S(std::initializer_list<T> i): v_(i) { std::sort(v_.begin(), v_.end()); }
+        };
+        constinit S<int> s{3, 2, 1};
+    ])], [
+        AC_DEFINE([HAVE_CPP_CONSTINIT_SORTED_VECTOR],[1])
+        AC_MSG_RESULT([yes])
+    ], [AC_MSG_RESULT([no])])
+CXXFLAGS=$save_CXXFLAGS
+AC_LANG_POP([C++])
+
 AC_MSG_CHECKING([whether $CXX_BASE has GCC bug 87150])
 AC_LANG_PUSH([C++])
 save_CXXFLAGS=$CXXFLAGS