[WebAssembly] Mangle the argc/argv `main` as `__wasm_argc_argv`.

WebAssembly enforces a rule that caller and callee signatures must
match. This means that the traditional technique of passing `main`
`argc` and `argv` even when it doesn't need them doesn't work.

Currently the backend renames `main` to `__original_main`, however this
doesn't interact well with LTO'ing libc, and the name isn't intuitive.
This patch allows us to transition to `__main_argc_argv` instead.

This implements the proposal in
https://github.com/WebAssembly/tool-conventions/pull/134
with a flag to disable it when targeting Emscripten, though this is
expected to be temporary, as discussed in the proposal comments.

Differential Revision: https://reviews.llvm.org/D70700

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 652efd857064..faff623a2973 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -448,6 +448,10 @@ void CodeGenModule::Release() {
     CodeGenFunction(*this).EmitCfiCheckStub();
   }
   emitAtAvailableLinkGuard();
+  if (Context.getTargetInfo().getTriple().isWasm() &&
+      !Context.getTargetInfo().getTriple().isOSEmscripten()) {
+    EmitMainVoidAlias();
+  }
   emitLLVMUsed();
   if (SanStats)
     SanStats->finish();
@@ -5600,6 +5604,17 @@ void CodeGenModule::EmitDeferredUnusedCoverageMappings() {
   }
 }
 
+void CodeGenModule::EmitMainVoidAlias() {
+  // In order to transition away from "__original_main" gracefully, emit an
+  // alias for "main" in the no-argument case so that libc can detect when
+  // new-style no-argument main is in used.
+  if (llvm::Function *F = getModule().getFunction("main")) {
+    if (!F->isDeclaration() && F->arg_size() == 0 && !F->isVarArg() &&
+        F->getReturnType()->isIntegerTy(Context.getTargetInfo().getIntWidth()))
+      addUsedGlobal(llvm::GlobalAlias::create("__main_void", F));
+  }
+}
+
 /// Turns the given pointer into a constant.
 static llvm::Constant *GetPointerConstant(llvm::LLVMContext &Context,
                                           const void *Ptr) {