[WebAssembly] Create synthetic __dso_handle symbol

[sbc100]

Summary

This is used by __cxa_ataxit to determine the currently
executing DLL. Once we fully support DLLs this will need
to be set to some address within the DLL.

The ELF linker added support for this symbol here:
https://reviews.llvm.org/D33856

[dschuff]

Couldn't this just be in crtbegin.c? Then there wouldn't have to be custom linker logic for it.

[sbc100]

For background on this see:
https://reviews.llvm.org/D33856
http://lists.llvm.org/pipermail/llvm-dev/2017-June/113648.html

[ruiu]

I'm not opposing to this patch, but I wonder if this is the best way of doing what you want to do in the new file format. We define __dso_handle symbol for ELF because the existing code expects the symbol. But since we don't have such existing code in wasm (am I correct?) I wonder if there's a different way.

[sbc100]

In D42024#977637, @ruiu wrote:

I'm not opposing to this patch, but I wonder if this is the best way of doing what you want to do in the new file format. We define __dso_handle symbol for ELF because the existing code expects the symbol. But since we don't have such existing code in wasm (am I correct?) I wonder if there's a different way.

We do have existing code that depends on dso_handle in that clang will generate calls to cxa_atexit() passing __dso_handle as the third arg (see clang/lib/CodeGen/ItaniumCXXABI.cpp).

[ruiu]

LGTM

lld/trunk/wasm/Driver.cpp

if (!Config->Relocatable) {
     static WasmSignature Signature = {{}, WASM_TYPE_NORESULT};
     if (!Config->Entry.empty())
       addSyntheticUndefinedFunction(Config->Entry, &Signature);
 
     // Handle the `--undefined <sym>` options.
     for (auto* Arg : Args.filtered(OPT_undefined))
       addSyntheticUndefinedFunction(Arg->getValue(), nullptr);
 
+    // Create linker-synthetic symbols
+    // __wasm_call_ctors:
+    //    Function that directly calls all ctors in priority order.
+    // __stack_pointer:
+    //    Wasm global that holds the address of the top of the explict
+    //    value stack in linear memory.
+    // __dso_handle;
+    //    Global in calls to __cxa_atexit to determine current DLL
     Config->CtorSymbol = Symtab->addDefinedFunction(
         "__wasm_call_ctors", &Signature, WASM_SYMBOL_VISIBILITY_HIDDEN);
     Config->StackPointerSymbol = Symtab->addDefinedGlobal("__stack_pointer");
+    Symtab->addDefinedGlobal("__dso_handle")->setVirtualAddress(0);
   }
 
   createFiles(Args);
   if (errorCount())
     return;
 
   // Add all files to the symbol table. This will add almost all
   // symbols that we need to the symbol table.