[RuntimeDyld][Orc][MCJIT] Add partial weak-symbol support to RuntimeDyld.

This patch causes RuntimeDyld to check for existing definitions when it
encounters weak symbols. If a definition already exists then the new weak
definition is discarded. All symbol lookups within a "logical dylib" should now
agree on the address of any given weak symbol. This allows the JIT to better
match the behavior of the static linker for C++ code.

This support is only partial, as it does not allow strong definitions that
occur after the first weak definition (in JIT symbol lookup order) to override
the previous weak definitions. Support for this will be added in a future
patch.

llvm-svn: 278065

2 files changed, 22 insertions, 2 deletions

diff --git a/llvm/lib/ExecutionEngine/Orc/OrcMCJITReplacement.h b/llvm/lib/ExecutionEngine/Orc/OrcMCJITReplacement.h
index 805852b4887..db9c519b826 100644
--- a/llvm/lib/ExecutionEngine/Orc/OrcMCJITReplacement.h
+++ b/llvm/lib/ExecutionEngine/Orc/OrcMCJITReplacement.h
@@ -116,11 +116,13 @@ class OrcMCJITReplacement : public ExecutionEngine {
     LinkingResolver(OrcMCJITReplacement &M) : M(M) {}
 
     JITSymbol findSymbol(const std::string &Name) override {
-      return M.findMangledSymbol(Name);
+      return M.ClientResolver->findSymbol(Name);
     }
 
     JITSymbol findSymbolInLogicalDylib(const std::string &Name) override {
-      return M.ClientResolver->findSymbol(Name);
+      if (auto Sym = M.findMangledSymbol(Name))
+        return Sym;
+      return M.ClientResolver->findSymbolInLogicalDylib(Name);
     }
 
   private:
diff --git a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
index 1770525879e..0c69b1a5160 100644
--- a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
+++ b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
@@ -226,6 +226,24 @@ RuntimeDyldImpl::loadObjectImpl(const object::ObjectFile &Obj) {
       // Compute JIT symbol flags.
       JITSymbolFlags JITSymFlags = JITSymbolFlags::fromObjectSymbol(*I);
 
+      // If this is a weak definition, check to see if there's a strong one.
+      // If there is, skip this symbol (we won't be providing it: the strong
+      // definition will). If there's no strong definition, make this definition
+      // strong.
+      if (JITSymFlags.isWeak()) {
+        // First check whether there's already a definition in this instance.
+        // FIXME: Override existing weak definitions with strong ones.
+        if (GlobalSymbolTable.count(Name))
+          continue;
+        // Then check the symbol resolver to see if there's a definition
+        // elsewhere in this logical dylib.
+        if (auto Sym = Resolver.findSymbolInLogicalDylib(Name))
+          if (Sym.getFlags().isStrongDefinition())
+            continue;
+        // else
+        JITSymFlags &= ~JITSymbolFlags::Weak;
+      }
+
       if (Flags & SymbolRef::SF_Absolute &&
           SymType != object::SymbolRef::ST_File) {
         uint64_t Addr = 0;