[WebAssembly] Strip threadlocal attribute from globals in single thread mode

[dschuff]

Summary

The default thread model for wasm is single, and in this mode thread-local
global variables can be lowered identically to non-thread-local variables.

Fixes PR36807

[sbc100]

Thanks for fixing this!

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

189 ↗

(On Diff #139198)

Does LLVM style allow for this kind of bracketing where one side of a conditions uses braces and the other doesn't?

[sbc100]

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

188 ↗

(On Diff #139198)

What do the other passes all have create functions but this one use just using new?

[dschuff]

lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

188 ↗

(On Diff #139198)

I assume it's so that you can forward-declare FunctionPass *createFooPass() without including the full class definition (e.g. in include/llvm/Analysis/Passes.h). It's not necessary when you define the pass in the same file in an anonymous namespace. The implementations of createFooPass() are all just return new FooPass().

189 ↗

(On Diff #139198)

I don't know but I don't like it, and it wasn't my intention :) I'll fix.

[dschuff]

• add braces in else

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

if (!I) {
     // function that reside in TargetOptions.
     resetTargetOptions(F);
     I = llvm::make_unique<WebAssemblySubtarget>(TargetTriple, CPU, FS, *this);
   }
   return I.get();
 }
 
 namespace {
+class StripThreadLocal final : public ModulePass {
+  // The default thread model for wasm is single, where thread-local variables
+  // are identical to regular globals and should be treated the same. So this
+  // pass just converts all GlobalVariables to NotThreadLocal
+  static char ID;
+
+ public:
+  StripThreadLocal() : ModulePass(ID) {}
+  bool runOnModule(Module &M) override {
+    for (auto &GV : M.globals())
+      GV.setThreadLocalMode(GlobalValue::ThreadLocalMode::NotThreadLocal);
+    return true;
+  }
+};
+char StripThreadLocal::ID = 0;
+
 /// WebAssembly Code Generator Pass Configuration Options.
 class WebAssemblyPassConfig final : public TargetPassConfig {
 public:
   WebAssemblyPassConfig(WebAssemblyTargetMachine &TM, PassManagerBase &PM)
       : TargetPassConfig(TM, PM) {}
 
   WebAssemblyTargetMachine &getWebAssemblyTargetMachine() const {
     return getTM<WebAssemblyTargetMachine>();

 }
 
 //===----------------------------------------------------------------------===//
 // The following functions are called from lib/CodeGen/Passes.cpp to modify
 // the CodeGen pass sequence.
 //===----------------------------------------------------------------------===//
 
 void WebAssemblyPassConfig::addIRPasses() {
-  if (TM->Options.ThreadModel == ThreadModel::Single)
+  if (TM->Options.ThreadModel == ThreadModel::Single) {
     // In "single" mode, atomics get lowered to non-atomics.
     addPass(createLowerAtomicPass());
-  else
+    addPass(new StripThreadLocal());
+  } else {
     // Expand some atomic operations. WebAssemblyTargetLowering has hooks which
     // control specifically what gets lowered.
     addPass(createAtomicExpandPass());
+  }
 
   // Lower .llvm.global_dtors into .llvm_global_ctors with __cxa_atexit calls.
   addPass(createWebAssemblyLowerGlobalDtors());
 
   // Fix function bitcasts, as WebAssembly requires caller and callee signatures
   // to match.
   addPass(createWebAssemblyFixFunctionBitcasts());
 

llvm/trunk/test/CodeGen/WebAssembly/tls.ll

+; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -disable-wasm-explicit-locals -thread-model=single | FileCheck --check-prefix=SINGLE %s
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown-wasm"
+
+; SINGLE-LABEL: address_of_tls:
+define i32 @address_of_tls() {
+  ; SINGLE: i32.const $push0=, tls
+  ; SINGLE-NEXT: return $pop0
+  ret i32 ptrtoint(i32* @tls to i32)
+}
+
+; SINGLE: .type tls,@object
+; SINGLE-NEXT: .section .bss.tls,"",@
+; SINGLE-NEXT: .p2align 2
+; SINGLE-NEXT: tls:
+; SINGLE-NEXT: .int32 0
+@tls = internal thread_local global i32 0