[MLIR][EmitC] Add empty emitc.constant check

[QuqqU]

Summary

Implementing logic to check if the emitc dialect constant Op is empty.

[marbre]

I added @simon-camp and myself as reviewers, as we initial pushed the code and mostly maintain it. We will take a look early next week.

[marbre]

Thanks!

mlir/include/mlir/Dialect/EmitC/IR/EmitC.td

%1 = "emitc.constant"()
         : () -> !emitc.opaque<"char">
     ```
   }];
 
   let arguments = (ins EmitC_OpaqueOrTypedAttr:$value);
   let results = (outs AnyType);
 
   let hasFolder = 1;
-  // TODO: Disallow empty constants.
   let hasVerifier = 1;
 }
 
 def EmitC_IncludeOp
     : EmitC_Op<"include", [HasParent<"ModuleOp">]> {
   let summary = "Include operation";
   let description = [{
     The `include` operation allows to define a source file inclusion via the

mlir/lib/Dialect/EmitC/IR/EmitC.cpp

 // ConstantOp
 //===----------------------------------------------------------------------===//
 
 /// The constant op requires that the attribute's type matches the return type.
 LogicalResult emitc::ConstantOp::verify() {
   if (getValueAttr().isa<emitc::OpaqueAttr>())
     return success();
 
+  // Value must not be empty
+  StringAttr strAttr = getValueAttr().dyn_cast<StringAttr>();
+  if (strAttr && strAttr.getValue().empty())
+    return emitOpError() << "value must not be empty";
+
   auto value = cast<TypedAttr>(getValueAttr());
   Type type = getType();
   if (!value.getType().isa<NoneType>() && type != value.getType())
     return emitOpError() << "requires attribute's type (" << value.getType()
                          << ") to match op's return type (" << type << ")";
   return success();
 }
 

mlir/test/Dialect/EmitC/invalid_ops.mlir

 func.func @const_attribute_return_type_2() {
     // expected-error @+1 {{'emitc.constant' op requires attribute's type ('!emitc.opaque<"char">') to match op's return type ('!emitc.opaque<"mychar">')}}
     %c0 = "emitc.constant"(){value = "CHAR_MIN" : !emitc.opaque<"char">} : () -> !emitc.opaque<"mychar">
     return
 }
 
 // -----
 
+func.func @empty_constant() {
+    // expected-error @+1 {{'emitc.constant' op value must not be empty}}
+    %c0 = "emitc.constant"(){value = ""} : () -> i32
+    return
+}
+
+// -----
+
 func.func @index_args_out_of_range_1() {
     // expected-error @+1 {{'emitc.call' op index argument is out of range}}
     emitc.call "test" () {args = [0 : index]} : () -> ()
     return
 }
 
 // -----