[mlir] Add a parameter for passing default values to `StringRefParameter`

[fmorac]

Summary

For an explanation of these patches see D154153.

Commit message:
Currently the StringRefParameter parameter doesn't support default values,
this patch allows the usage of them, eg:
StringRefParameter<"description", [{"default_value"}]>

[fmorac]

Rebasing.

[fmorac]

Rebasing.

[mehdi_amini]

Could you use this in the test dialect to showcase that it works as intended?

[fmorac]

In D154097#4464072, @mehdi_amini wrote:

Could you use this in the test dialect to showcase that it works as intended?

Yes, I'll add it.

[fmorac]

Added test to the Test dialect.

mlir/include/mlir/IR/AttrTypeBase.td

 
 // A parameter with a default value.
 class DefaultValuedParameter<string type, string value, string desc = ""> :
     AttrOrTypeParameter<type, desc> {
   let defaultValue = value;
 }
 
 // For StringRefs, which require allocation.
-class StringRefParameter<string desc = ""> :
+class StringRefParameter<string desc = "", string value = ""> :
     AttrOrTypeParameter<"::llvm::StringRef", desc> {
   let allocator = [{$_dst = $_allocator.copyInto($_self);}];
   let printer = [{$_printer << '"' << $_self << '"';}];
   let cppStorageType = "std::string";
+  let defaultValue = value;
 }
 
 // For APFloats, which require comparison.
 class APFloatParameter<string desc> :
     AttrOrTypeParameter<"::llvm::APFloat", desc> {
   let comparator = "$_lhs.bitwiseIsEqual($_rhs)";
 }
 

mlir/test/lib/Dialect/Test/TestTypeDefs.td

 
 def TestTypeCustomString : Test_Type<"TestTypeCustomString"> {
   let parameters = (ins StringRefParameter<>:$foo);
   let mnemonic = "custom_type_string";
   let assemblyFormat = [{ `<` custom<FooString>($foo)
                               custom<BarString>(ref($foo)) `>` }];
 }
 
+def TestTypeOptionalString : Test_Type<"TestTypeOptionalString"> {
+  let parameters = (ins StringRefParameter<"description", [{"default"}]>:$str);
+  let mnemonic = "optional_type_string";
+  let assemblyFormat = [{ (`<` $str^ `>`)? }];
+}
+
 def TestTypeElseAnchor : Test_Type<"TestTypeElseAnchor"> {
   let parameters = (ins OptionalParameter<"std::optional<int>">:$a);
   let mnemonic = "else_anchor";
   let assemblyFormat = "`<` (`?`) : ($a^)? `>`";
 }
 
 def TestTypeElseAnchorStruct : Test_Type<"TestTypeElseAnchorStruct"> {
   let parameters = (ins OptionalParameter<"std::optional<int>">:$a,
                         OptionalParameter<"std::optional<int>">:$b);
   let mnemonic = "else_anchor_struct";
   let assemblyFormat = "`<` (`?`) : (struct($a, $b)^)? `>`";
 }
 
 #endif // TEST_TYPEDEFS

mlir/test/mlir-tblgen/attr-or-type-format-roundtrip.mlir

 // CHECK: !test.ap_float<>
 // CHECK: !test.default_valued_type<(i64)>
 // CHECK: !test.default_valued_type<>
 // CHECK: !test.custom_type<-5>
 // CHECK: !test.custom_type<2 0 1 5>
 // CHECK: !test.custom_type_spacing<1 2>
 // CHECK: !test.custom_type_string<"foo" foo>
 // CHECK: !test.custom_type_string<"bar" bar>
+// CHECK: !test.optional_type_string
+// CHECK: !test.optional_type_string
+// CHECK: !test.optional_type_string<"non default">
 
 func.func private @test_roundtrip_default_parsers_struct(
   !test.no_parser<255, [1, 2, 3, 4, 5], "foobar", 4>
 ) -> (
   !test.struct_capture_all<v3 = 3, v1 = 1, v2 = 2, v0 = 0>,
   !test.optional_param<, 6>,
   !test.optional_param<5, 6>,
   !test.optional_param<5, 6, "foo">,
) -> (
   !test.ap_float<5.0>,
   !test.ap_float<>,
   !test.default_valued_type<(i64)>,
   !test.default_valued_type<>,
   !test.custom_type<-5>,
   !test.custom_type<2 9 9 5>,
   !test.custom_type_spacing<1 2>,
   !test.custom_type_string<"foo" foo>,
-  !test.custom_type_string<"bar" bar>
+  !test.custom_type_string<"bar" bar>,
+  !test.optional_type_string,
+  !test.optional_type_string<"default">,
+  !test.optional_type_string<"non default">
 )