[FIRRTL] Support StringConcatOp in LowerClasses

Add a conversion pattern to lower FIRRTL `StringConcatOp` to OM
`StringConcatOp` in the LowerClasses pass.

AI-assisted-by: Augment (Sonnet 4.5)
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Author: seldridge

lib/Dialect/FIRRTL/Transforms/LowerClasses.cpp

@@ -1743,6 +1743,18 @@ struct IntegerShlOpConversion
   }
 };
 
+struct StringConcatOpConversion
+    : public OpConversionPattern<firrtl::StringConcatOp> {
+  using OpConversionPattern::OpConversionPattern;
+
+  LogicalResult
+  matchAndRewrite(firrtl::StringConcatOp op, OpAdaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const override {
+    rewriter.replaceOpWithNewOp<om::StringConcatOp>(op, adaptor.getOperands());
+    return success();
+  }
+};
+
 struct PathOpConversion : public OpConversionPattern<firrtl::PathOp> {
 
   PathOpConversion(TypeConverter &typeConverter, MLIRContext *context,
@@ -2220,6 +2232,7 @@ static void populateRewritePatterns(
   patterns.add<IntegerMulOpConversion>(converter, patterns.getContext());
   patterns.add<IntegerShrOpConversion>(converter, patterns.getContext());
   patterns.add<IntegerShlOpConversion>(converter, patterns.getContext());
+  patterns.add<StringConcatOpConversion>(converter, patterns.getContext());
   patterns.add<UnrealizedConversionCastOpConversion>(converter,
                                                      patterns.getContext());
   patterns.add<UnknownValueOpConversion>(converter, patterns.getContext());

test/Dialect/FIRRTL/lower-classes.mlir

@@ -409,6 +409,19 @@ firrtl.circuit "IntegerArithmetic" {
   }
 }
 
+// CHECK-LABEL: firrtl.circuit "StringCat"
+firrtl.circuit "StringCat" {
+  firrtl.module @StringCat() {}
+
+  // CHECK-LABEL: om.class @StringConcatClass
+  firrtl.class @StringConcatClass(in %a: !firrtl.string, in %b: !firrtl.string, out %c: !firrtl.string) {
+    // CHECK: %[[CONCAT:.+]] = om.string.concat %a, %b : !om.string
+    %0 = firrtl.string.concat %a, %b : !firrtl.string
+    // CHECK: om.class.fields %[[CONCAT]]
+    firrtl.propassign %c, %0 : !firrtl.string
+  }
+}
+
 // CHECK-LABEL: firrtl.circuit "AltBasePath"
 firrtl.circuit "AltBasePath" {
   // CHECK: hw.hierpath private [[FOO_NLA:@.+]] [@AltBasePath::[[FOO_SYM:@.+]]]