[ORC] Add absoluteSymbolsLinkGraph to expose absolute symbols to platform (#77008)

Adds a function to create a LinkGraph of absolute symbols, and a
callback in dynamic library search generators to enable using it to
expose its symbols to the platform/orc runtime. This allows e.g. using
__orc_rt_run_program to run a precompiled function that was found via
dlsym. Ideally we would use this in llvm-jitlink's own search generator,
but it will require more work to align with the Process/Platform
JITDylib split, so not handled here.

As part of this change we need to handle LinkGraphs that only have
absolute symbols.

1 files changed, 11 insertions, 3 deletions

diff --git a/llvm/lib/ExecutionEngine/Orc/ObjectLinkingLayer.cpp b/llvm/lib/ExecutionEngine/Orc/ObjectLinkingLayer.cpp
index 3d77f82..b828294 100644
--- a/llvm/lib/ExecutionEngine/Orc/ObjectLinkingLayer.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/ObjectLinkingLayer.cpp
@@ -93,15 +93,20 @@ private:
 
     Interface LGI;
 
-    for (auto *Sym : G.defined_symbols()) {
+    auto AddSymbol = [&](Symbol *Sym) {
       // Skip local symbols.
       if (Sym->getScope() == Scope::Local)
-        continue;
+        return;
       assert(Sym->hasName() && "Anonymous non-local symbol?");
 
       LGI.SymbolFlags[ES.intern(Sym->getName())] =
           getJITSymbolFlagsForSymbol(*Sym);
-    }
+    };
+
+    for (auto *Sym : G.defined_symbols())
+      AddSymbol(Sym);
+    for (auto *Sym : G.absolute_symbols())
+      AddSymbol(Sym);
 
     if (hasInitializerSection(G))
       LGI.InitSymbol = makeInitSymbol(ES, G);
@@ -705,6 +710,9 @@ Error ObjectLinkingLayer::notifyEmitted(MaterializationResponsibility &MR,
   if (Err)
     return Err;
 
+  if (!FA)
+    return Error::success();
+
   return MR.withResourceKeyDo(
       [&](ResourceKey K) { Allocs[K].push_back(std::move(FA)); });
 }