[PATCH] Don't use ManagedStatic.

[Aaron Ballman]

We had to revert use of call_once last year because MinGW did not
support it. IIRC, there were a few other issues as well (I have a
vague recollection of an issue with MSVC as well). May want to check
old mailing list posts, as well as the other toolchains, to see how
feasible this is.

~Aaron

On Mon, Jun 8, 2015 at 3:36 AM, Rui Ueyama <ruiu at google.com> wrote:
> Hi rafael,
>
> ManagedStatic is slow at least on Windows because it calls sys::MemoryFence
> on every access. MSVC profiler reported that LLD is spending 9% on this function.
> This patch is to avoid using that wrapper class and use call_once instead.
>
> http://reviews.llvm.org/D10303
>
> Files:
>   lib/Object/Error.cpp
>
> Index: lib/Object/Error.cpp
> ===================================================================
> --- lib/Object/Error.cpp
> +++ lib/Object/Error.cpp
> @@ -13,7 +13,7 @@
>
>  #include "llvm/Object/Error.h"
>  #include "llvm/Support/ErrorHandling.h"
> -#include "llvm/Support/ManagedStatic.h"
> +#include <mutex>
>
>  using namespace llvm;
>  using namespace object;
> @@ -55,8 +55,10 @@
>                     "defined.");
>  }
>
> -static ManagedStatic<_object_error_category> error_category;
> -
>  const std::error_category &object::object_category() {
> -  return *error_category;
> +  static _object_error_category *Ret;
> +  static std::once_flag Flag;
> +  if (Ret == nullptr)
> +    std::call_once(Flag, []() { Ret = new _object_error_category(); });
> +  return *Ret;
>  }
>
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