Inject a "global pointers array".

This change adds (to the AOT LLVM module) an array containing the
addresses of all global variables in the module.

We can use this to find the addresses of global variables (by index)
without leaning on dlsym(3).

Author: vext01

llvm/lib/YkIR/YkIRWriter.cpp

@@ -751,6 +751,21 @@ class YkIRWriter {
       serialiseGlobal(G);
     }
 
+    // Now that we've finished serialising globals, add a global (immutable, for
+    // now) array to the LLVM module containing pointers to all the global
+    // variables. We will use this to find the addresses of globals at runtime.
+    // The indices of the array correspond with `GlobalDeclIdx`s in the AOT IR.
+    vector<llvm::Constant *> GlobalsAsConsts;
+    for (llvm::GlobalVariable *G : Globals) {
+      GlobalsAsConsts.push_back(cast<llvm::Constant>(G));
+    }
+    ArrayType *GlobalsArrayTy =
+        ArrayType::get(PointerType::get(M.getContext(), 0), Globals.size());
+    GlobalVariable *GlobalsArray = new GlobalVariable(
+        M, GlobalsArrayTy, true, GlobalValue::LinkageTypes::ExternalLinkage,
+        ConstantArray::get(GlobalsArrayTy, GlobalsAsConsts));
+    GlobalsArray->setName("__yk_globalvar_ptrs");
+
     // num_types:
     OutStreamer.emitSizeT(Types.size());
     // types: