dexonsmith added a comment.
In D105959#2883954 <https://reviews.llvm.org/D105959#2883954>, @mehdi_amini
wrote:
> That's interesting!
>
> I'm not sure how to get there though: a complete solution should be able to
> "degrade" to global constructor when the platform-specific logic isn't
> available (unless we're confident that no such environment can exist?).
Right, it needs to downgrade (and I think there should be a CMake flag to turn
it off in case there are reasons some LLVM user doesn't want it).
I think this would work:
#ifdef USE_DARWIN_CLOPT
// Add F to the list of initializers for LLVMSupport to iterate through.
#define LLVM_ADD_CLOPT_CONSTRUCTOR(F, V) \
__attribute__((section("__DATA,__llvmopts"),visibility("hidden"))) \
void (*V)() = F;
#elif defined(...) // Handle magic for other platforms.
#else
// Create a local symbol with a static initializer that calls F.
#define LLVM_ADD_CLOPT_CONSTRUCTOR(F, V) \
namespace { struct V##Type { V##Type() { F(); } } V; }
#endif
void initOptions();
// Add an options constructor the array of cl::opt initializers.
LLVM_ADD_CLOPT_CONSTRUCTOR(initOptions, OptionsConstructorVariable);
but maybe there's a cleaner downgrade that doesn't add an extra type.
In D105959#2883956 <https://reviews.llvm.org/D105959#2883956>, @mehdi_amini
wrote:
> Something else I'm not sure about is how it works across DSOs: when each LLVM
> library is linked into its own shared library, is the dynamic linker creating
> this array when loading the libraries? (I'm starting to doubt about how this
> would even work on windows)
Yeah, this technique only works within a single final linked image. It'd need
to be off when the library isn't statically linked to LLVMSupport.
I imagine we'd want CMake to just "get this right" for us, but we can also
enforce this at link time to catch configuration errors -- add a
(dead-strippable) symbol to LLVMSupport that's `visibility("hidden")`, and
manufacture a (dead-strippable) reference to it in any TU that uses the macro.
If the TU ends up in a different linked image than LLVMSupport you'll get a
link error.
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