Ensure that ManagedStatic is constant initialized in MSVC 2017 & 2019

Fixes PR41367.

This effectively relands r357655 with a workaround for MSVC 2017.

I tried various approaches with unions, but I ended up going with this
ifdef approach because it lets us write the proper C++11 code that we
want to write, with a separate workaround that we can delete when we
drop MSVC 2017 support.

This also adds LLVM_REQUIRE_CONSTANT_INITIALIZATION, which wraps
[[clang::require_constant_initialization]]. This actually detected a
minor issue when using clang-cl where clang wasn't able to use the
constexpr constructor in MSVC's STL, so I switched back to using the
default ctor of std::atomic<void*>.

llvm-svn: 357714

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Support/CommandLine.cpp b/llvm/lib/Support/CommandLine.cpp
index c1193f9..d2cfef9f 100644
--- a/llvm/lib/Support/CommandLine.cpp
+++ b/llvm/lib/Support/CommandLine.cpp
@@ -373,11 +373,16 @@ void OptionCategory::registerCategory() {
   GlobalParser->registerCategory(this);
 }
 
-// A special subcommand representing no subcommand
-ManagedStatic<SubCommand> llvm::cl::TopLevelSubCommand;
+// A special subcommand representing no subcommand. It is particularly important
+// that this ManagedStatic uses constant initailization and not dynamic
+// initialization because it is referenced from cl::opt constructors, which run
+// dynamically in an arbitrary order.
+LLVM_REQUIRE_CONSTANT_INITIALIZATION ManagedStatic<SubCommand>
+    llvm::cl::TopLevelSubCommand;
 
 // A special subcommand that can be used to put an option into all subcommands.
-ManagedStatic<SubCommand> llvm::cl::AllSubCommands;
+LLVM_REQUIRE_CONSTANT_INITIALIZATION ManagedStatic<SubCommand>
+    llvm::cl::AllSubCommands;
 
 void SubCommand::registerSubCommand() {
   GlobalParser->registerSubCommand(this);