[PATCH] D16170: Registry initialization and linkage interactions

[Rafael Espíndola via llvm-commits]

We currently produce a linkonce_odr for
_ZN4llvm8RegistryINS_10GCStrategyEE4HeadE, so short of some linker
script this should be working (on ELF).

In any case, IMHO giving a home to these symbols is an improvement, so LGTM.

On 13 January 2016 at 21:02, Philip Reames via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> reames created this revision.
> reames added reviewers: chandlerc, rsmith.
> reames added a subscriber: llvm-commits.
>
> Hey C++ template/linkage experts out there, this patch is for you.
>
> The Registry class constructs a linked list of nodes whose storage is inside static variables and nodes are added via static initializers.  The trick is that those static initializers are in both the LLVM code base, and some random plugin that might get loaded in at runtime.  The existing code tries to use C++ templates and their ODR rules to get a single definition of the registry for each type, but, experimentally, this doesn't quite work as designed.   (Well, the entire structure doesn't.  It might not actually be an ODR problem.)
>
> Previously, when I tried moving the GCStrategy class (along with it's registry) from CodeGen to IR, I ran into a problem where asking the GCStrategyRegistry a question would return inconsistent results depending on whether you asked from CodeGen (where the static initializers still were) or Transforms.  My best guess is that this is a result of either a) an order of initialization error, or b) we ended up with two copies of the registry being created.  I remember at the time having convinced myself it was probably (b), but I don't have any of my notes around from that investigation any more.
>
> See http://reviews.llvm.org/rL226311 for the original patch in question.
>
> This patch tries to remove the possibility of (b) above.  It still suffers from (a) since there are static initializers in several files within CodeGen that all modify this structure.  I haven't fixed that, but hopefully, I haven't made it worse either.  Once this is in, I plan to restructure code to put all of the static initializers used in the Registry in a single file to remove that possibility as well.
>
>
>
> http://reviews.llvm.org/D16170
>
> Files:
>   include/llvm/Support/Registry.h
>   lib/CodeGen/GCMetadataPrinter.cpp
>   lib/CodeGen/GCStrategy.cpp
>
> Index: lib/CodeGen/GCStrategy.cpp
> ===================================================================
> --- lib/CodeGen/GCStrategy.cpp
> +++ lib/CodeGen/GCStrategy.cpp
> @@ -20,3 +20,7 @@
>      : UseStatepoints(false), NeededSafePoints(0), CustomReadBarriers(false),
>        CustomWriteBarriers(false), CustomRoots(false), InitRoots(true),
>        UsesMetadata(false) {}
> +
> +// Stamp out the registry of GCStrategy objects so that pulgins can load new
> +// Strategies.
> +DEFINE_REGISTRY(GCStrategy)
> Index: lib/CodeGen/GCMetadataPrinter.cpp
> ===================================================================
> --- lib/CodeGen/GCMetadataPrinter.cpp
> +++ lib/CodeGen/GCMetadataPrinter.cpp
> @@ -17,3 +17,7 @@
>  GCMetadataPrinter::GCMetadataPrinter() {}
>
>  GCMetadataPrinter::~GCMetadataPrinter() {}
> +
> +// Stamp out the registry of GCMetadataPrinter objects so that pulgins can load
> +// new Strategies.
> +DEFINE_REGISTRY(GCMetadataPrinter)
> Index: include/llvm/Support/Registry.h
> ===================================================================
> --- include/llvm/Support/Registry.h
> +++ include/llvm/Support/Registry.h
> @@ -116,14 +116,15 @@
>      };
>    };
>
> -
> -  // Since these are defined in a header file, plugins must be sure to export
> -  // these symbols.
> -  template <typename T>
> -  typename Registry<T>::node *Registry<T>::Head;
> -
> -  template <typename T>
> -  typename Registry<T>::node *Registry<T>::Tail;
> +  // Use this macro to stamp out definitions for required static symbols in an
> +  // appropriate source file.  This is required to avoid getting multiple
> +  // definitions of the Registry's fields in different translation or linkage
> +  // units.
> +#define DEFINE_REGISTRY(T)                      \
> +  template <>                                    \
> +  typename Registry<T>::node *Registry<T>::Head = nullptr; \
> +  template <>                          \
> +  typename Registry<T>::node *Registry<T>::Tail = nullptr;
>  } // end namespace llvm
>
>  #endif // LLVM_SUPPORT_REGISTRY_H
>
>
>
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