[mlir][EmitC] Fix call ops with zero arguments in func to emitc conversion (#94936)

Author: Simon Camphausen

mlir/lib/Conversion/FuncToEmitC/FuncToEmitC.cpp

@@ -31,7 +31,7 @@ class CallOpConversion final : public OpConversionPattern<func::CallOp> {
   LogicalResult
   matchAndRewrite(func::CallOp callOp, OpAdaptor adaptor,
                   ConversionPatternRewriter &rewriter) const override {
-    // Multiple results func was not converted to `emitc.func`.
+    // Multiple results func cannot be converted to `emitc.func`.
     if (callOp.getNumResults() > 1)
       return rewriter.notifyMatchFailure(
           callOp, "only functions with zero or one result can be converted");

mlir/test/Conversion/FuncToEmitC/func-to-emitc.mlir

@@ -88,3 +88,19 @@ func.func @index_args_only(%i: index) -> f32 {
   %0 = arith.constant 0.0 : f32
   return %0 : f32
 }
+
+// -----
+
+// CHECK-LABEL: emitc.func private @return_void() attributes {specifiers = ["static"]}
+// CHECK-NEXT: emitc.return
+func.func private @return_void() {
+  return
+}
+
+// CHECK-LABEL: emitc.func @call()
+// CHECK-NEXT: emitc.call @return_void() : () -> ()
+// CHECK-NEXT: emitc.return
+func.func @call() {
+  call @return_void() : () -> ()
+  return
+}