HACK: Make __ImageBase point to object base if it's not otherwise defined.

Author: lhames

llvm/lib/ExecutionEngine/JITLink/COFF_x86_64.cpp

@@ -202,7 +202,13 @@ class COFFLinkGraphLowering_x86_64 {
         switch (E.getKind()) {
         case EdgeKind_coff_x86_64::Pointer32NB: {
           auto ImageBase = GetImageBase(G);
-          assert(ImageBase && "__ImageBase symbol must be defined");
+          assert(ImageBase && "__ImageBase esymbol must be defined");
+          if (!ImageBase->getAddress())
+            setImageBaseToGraphBase(G, ImageBase);
+          LLVM_DEBUG({
+              dbgs() << "Lowing IMAGE_REL_AMD64_ADDR32NB to Pointer32 using __ImageBase = "
+                     << formatv("{0:x}", ImageBase->getAddress()) << "\n";
+            });
           E.setAddend(E.getAddend() - ImageBase->getAddress().getValue());
           E.setKind(x86_64::Pointer32);
           break;
@@ -243,6 +249,16 @@ class COFFLinkGraphLowering_x86_64 {
     return It->second;
   }
 
+  void setImageBaseToGraphBase(LinkGraph &G, Symbol *ImageBase) {
+    if (G.defined_symbols().empty())
+      return;
+    orc::ExecutorAddr GraphBase =
+      (*G.defined_symbols().begin())->getAddress();
+    for (auto *Sym : G.defined_symbols())
+      GraphBase = std::min(GraphBase, Sym->getAddress());
+    G.makeAbsolute(*ImageBase, GraphBase);
+  }
+
   GetImageBaseSymbol GetImageBase;
   DenseMap<Section *, orc::ExecutorAddr> SectionStartCache;
 };