[WebAssembly] EmscriptenEHSjLj: Don't abort if __THREW__ is defined

[sbc100]

Summary

This allows THREW to be defined in the current module, although
it is still required to be a GlobalVariable.

In emscripten we want to be able to compile the source
code that defined this symbol.

Previously we were avoid this by not running this pass when building
that compiler-rt library, but I have change out to build it using the
normal compiler path: https://github.com/emscripten-core/emscripten/pull/8391

[efriedma]

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

625 ↗

(On Diff #193592)

It's not a good idea to use cast<> if you can't prove it will succeed.

[sbc100]

• feedback

[kripken]

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

338 ↗

(On Diff #193596)

I'm a little confused that previously we mentioned the linkage, but no longer do, and getorInsertGlobal doesn't document itself as setting Extrenal as the linkage?

[sbc100]

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

338 ↗

(On Diff #193596)

AFAICT that is what it does: https://github.com/llvm-mirror/llvm/blob/21c63d584f9be3cdc2d045f53bebd3f83cd4d707/lib/IR/Module.cpp#L227

[kripken]

Fair enough. Slightly worries me since it's not obvious from the name or the docs, and so perhaps it might change one day, but that's overly paranoid perhaps.

[sbc100]

In D60232#1454264, @kripken wrote:

Fair enough. Slightly worries me since it's not obvious from the name or the docs, and so perhaps it might change one day, but that's overly paranoid perhaps.

The only thing we care about here is that it exists so I think we should be ok.

llvm/trunk/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

static bool canThrow(const Value *V) {
   // not a function, so an indirect call - can throw, we can't tell
   return true;
 }
 
 // Get a global variable with the given name.  If it doesn't exist declare it,
 // which will generate an import and asssumes that it will exist at link time.
 static GlobalVariable *getGlobalVariableI32(Module &M, IRBuilder<> &IRB,
                                             const char *Name) {
-  if (M.getNamedGlobal(Name))
-    report_fatal_error(Twine("variable name is reserved: ") + Name);
 
-  return new GlobalVariable(M, IRB.getInt32Ty(), false,
-                            GlobalValue::ExternalLinkage, nullptr, Name);
+  auto* GV = dyn_cast<GlobalVariable>(M.getOrInsertGlobal(Name, IRB.getInt32Ty()));
+  if (!GV)
+    report_fatal_error(Twine("unable to create global: ") + Name);
+
+  return GV;
 }
 
 // Simple function name mangler.
 // This function simply takes LLVM's string representation of parameter types
 // and concatenate them with '_'. There are non-alphanumeric characters but llc
 // is ok with it, and we need to postprocess these names after the lowering
 // phase anyway.
 static std::string getSignature(FunctionType *FTy) {