[WebAssembly] Set threadmodel during LTO

[ruiu]

Do you think you can add a test for this?

[sbc100]

• add test

[sbc100]

Good to go now?

[sbc100]

This change is need for emscripten to start using LTO, so I'm going to assume this test is good and land it now. Happy to re-visit if there is more feedback.

test/wasm/lto/atomics.ll

+; RUN: llvm-as %s -o %t.o
+; RUN: wasm-ld %t.o -o %t.wasm -lto-O0
+; Atomic operations with fail to compile if the ThreadModel is not
+; correctly set to Single (i.e. if atomics are not lowered to regular ops).
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown-wasm"
+
+@foo = hidden global i32 1
+
+define void @_start() {
+  %1 = load atomic i32, i32* @foo unordered, align 4
+  ret void
+}

wasm/LTO.cpp

 
 using namespace lld;
 using namespace lld::wasm;
 
 static std::unique_ptr<lto::LTO> createLTO() {
   lto::Config C;
   C.Options = InitTargetOptionsFromCodeGenFlags();
 
-  // Always emit a section per function/datum with LTO.
+  // Always emit a section per function/data with LTO.
   C.Options.FunctionSections = true;
   C.Options.DataSections = true;
 
+  // Wasm currently only supports ThreadModel::Single
+  C.Options.ThreadModel = ThreadModel::Single;
+
   C.DisableVerify = Config->DisableVerify;
   C.DiagHandler = diagnosticHandler;
   C.OptLevel = Config->LTOO;
 
   if (Config->SaveTemps)
     checkError(C.addSaveTemps(Config->OutputFile.str() + ".",
                               /*UseInputModulePath*/ true));