Introduce std.varargs attribute to mark variadic arguments functions

This is only teaching the LLVM converter to propagate the attribute onto
    the function type. MLIR will not recognize this arguments, so it would only
    be useful when calling for example `printf` with the same arguments across
    a module. Since varargs is part of the ABI lowering, this is not NFC.

--

PiperOrigin-RevId: 242382427

mlir/lib/Target/LLVMIR/ConvertToLLVMIR.cpp

@@ -83,8 +83,8 @@ private:
 // MLIR LLVM IR dialect types.  Use `loc` as a location when reporting errors.
 // Return nullptr on errors.
 static llvm::FunctionType *convertFunctionType(llvm::LLVMContext &llvmContext,
-                                               FunctionType type,
-                                               Location loc) {
+                                               FunctionType type, Location loc,
+                                               bool isVarArgs) {
   assert(type && "expected non-null type");
 
   auto context = type.getContext();
@@ -105,14 +105,14 @@ static llvm::FunctionType *convertFunctionType(llvm::LLVMContext &llvmContext,
 
   if (type.getNumResults() == 0)
     return llvm::FunctionType::get(llvm::Type::getVoidTy(llvmContext), argTypes,
-                                   /*isVarArg=*/false);
+                                   isVarArgs);
 
   auto wrappedResultType = type.getResult(0).dyn_cast<LLVM::LLVMType>();
   if (!wrappedResultType)
     return context->emitError(loc, "non-LLVM function result"), nullptr;
 
   return llvm::FunctionType::get(wrappedResultType.getUnderlyingType(),
-                                 argTypes, /*isVarArg=*/false);
+                                 argTypes, isVarArgs);
 }
 
 // Create an LLVM IR constant of `llvmType` from the MLIR attribute `attr`.
@@ -417,8 +417,12 @@ bool ModuleTranslation::convertFunctions() {
   // call graph with cycles.
   for (Function &function : mlirModule) {
     Function *functionPtr = &function;
-    llvm::FunctionType *functionType = convertFunctionType(
-        llvmModule->getContext(), function.getType(), function.getLoc());
+    mlir::BoolAttr isVarArgsAttr =
+        function.getAttrOfType<BoolAttr>("std.varargs");
+    bool isVarArgs = isVarArgsAttr && isVarArgsAttr.getValue();
+    llvm::FunctionType *functionType =
+        convertFunctionType(llvmModule->getContext(), function.getType(),
+                            function.getLoc(), isVarArgs);
     if (!functionType)
       return true;
     llvm::FunctionCallee llvmFuncCst =

mlir/test/Target/llvmir.mlir

@@ -793,3 +793,7 @@ func @cond_br_arguments(%arg0: !llvm.i1, %arg1: !llvm.i1) {
 func @llvm_noalias(%arg0: !llvm<"float*"> {llvm.noalias: true}) {
   llvm.return
 }
+
+// CHECK-LABEL: @llvm_varargs(...) 
+func @llvm_varargs()
+  attributes {std.varargs: true}