[WebAssembly] Fix signature of `main` in FixFunctionBitcasts

[sbc100]

Summary

Also, add a check to ensure that when main has the expected signature
we do not create a wrapper.

[sbc100]

cleanup

[sbc100]

I'm not sure why the logic of this flag is set to true by default and then we check for "!TemporaryWorkarounds".

Assuming this is useful I'd like to remove this flag soon.

[sbc100]

fix

[sbc100]

ping

[sbc100]

-rebase

lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp

for (Function &F : M) {
     // If we have a "main" function, and its type isn't
     // "int main(int argc, char *argv[])", create an artificial call with it
     // bitcasted to that type so that we generate a wrapper for it, so that
     // the C runtime can call it.
     if (!TemporaryWorkarounds && !F.isDeclaration() && F.getName() == "main") {
       Main = &F;
       LLVMContext &C = M.getContext();
       Type *MainArgTys[] = {
-        PointerType::get(Type::getInt8PtrTy(C), 0),
-        Type::getInt32Ty(C)
+        Type::getInt32Ty(C),
+        PointerType::get(Type::getInt8PtrTy(C), 0)
       };
       FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys,
                                                /*isVarArg=*/false);
       if (F.getFunctionType() != MainTy) {
+        LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
+                          << *F.getFunctionType() << "\n");
+        LLVM_DEBUG(dbgs() << "expected: " << *MainTy << "\n");
         Value *Args[] = {
           UndefValue::get(MainArgTys[0]),
           UndefValue::get(MainArgTys[1])
         };
         Value *Casted = ConstantExpr::getBitCast(Main,
                                                  PointerType::get(MainTy, 0));
         CallMain = CallInst::Create(Casted, Args, "call_main");
         Use *UseMain = &CallMain->getOperandUse(2);

test/CodeGen/WebAssembly/main.ll

 ; RUN: llc < %s -asm-verbose=false -wasm-temporary-workarounds=false | FileCheck %s
 
-; Test main functions with alternate signatures.
+; Test main functions with alternate signature. Specifically no arguments.
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
 
-define void @main() {
-  ret void
+define i32 @main() {
+  ret i32 0
 }
 
 ; CHECK-LABEL: .L__original_main:
+; CHECK-NEXT: .result   i32
+; CHECK-NEXT: i32.const 0
 ; CHECK-NEXT: end_function
 
 ; CHECK-LABEL: main:
 ; CHECK-NEXT: .param i32, i32
 ; CHECK-NEXT: .result i32
-; CHECK:      call .L__original_main@FUNCTION
+; CHECK: i32.call .L__original_main@FUNCTION
+; CHECK: end_function

test/CodeGen/WebAssembly/main_with_args.ll

 ; RUN: llc < %s -asm-verbose=false -wasm-temporary-workarounds=false | FileCheck %s
 
-; Test main functions with alternate signatures.
+; Test that main function with expected signature is not wrapped
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
 
-define void @main() {
-  ret void
+define i32 @main(i32 %a, i8** %b) {
+  ret i32 0
 }
 
-; CHECK-LABEL: .L__original_main:
-; CHECK-NEXT: end_function
-
 ; CHECK-LABEL: main:
 ; CHECK-NEXT: .param i32, i32
 ; CHECK-NEXT: .result i32
-; CHECK:      call .L__original_main@FUNCTION
+
+; CHECK-NOT: __original_main: