[FIRRTL] Add folder for add (#8200)

Author: mtsokol

include/circt/Dialect/FIRRTL/FIRRTLExpressions.td

@@ -1393,6 +1393,8 @@ def IntegerAddOp : IntegerBinaryPrimOp<"integer.add", [Commutative]> {
                                          !firrtl.integer
     ```
   }];
+
+  let hasFolder = true;
 }
 
 def IntegerMulOp : IntegerBinaryPrimOp<"integer.mul", [Commutative]> {

lib/Dialect/FIRRTL/FIRRTLFolds.cpp

@@ -935,8 +935,20 @@ LogicalResult NEQPrimOp::canonicalize(NEQPrimOp op, PatternRewriter &rewriter) {
 }
 
 OpFoldResult IntegerAddOp::fold(FoldAdaptor adaptor) {
-  // TODO: implement constant folding, etc.
-  // Tracked in https://github.com/llvm/circt/issues/6696.
+  if (auto rhsCst = getConstant(adaptor.getRhs())) {
+    // Constant folding
+    if (auto lhsCst = getConstant(adaptor.getLhs())) {
+      auto resultWidth =
+          std::max(lhsCst->getBitWidth(), rhsCst->getBitWidth()) + 1;
+      APSInt lhsCstExt = (*lhsCst).extend(resultWidth);
+      APSInt rhsCstExt = (*rhsCst).extend(resultWidth);
+      return IntegerAttr::get(IntegerType::get(getContext(), resultWidth),
+                              lhsCstExt + rhsCstExt);
+    }
+    // x + 0 -> x
+    if (rhsCst->isZero())
+      return getLhs();
+  }
   return {};
 }
 

test/Dialect/FIRRTL/canonicalization.mlir

@@ -3576,19 +3576,28 @@ firrtl.module private @Issue7562(in %sel : !firrtl.uint<1>, in %a : !firrtl.cons
 }
 
 // CHECK-LABEL: firrtl.class @PropertyArithmetic
-firrtl.class @PropertyArithmetic(in %in: !firrtl.integer, out %out0: !firrtl.integer, out %out1: !firrtl.integer) {
+firrtl.class @PropertyArithmetic(in %in: !firrtl.integer, out %out0: !firrtl.integer, out %out1: !firrtl.integer, out %out2: !firrtl.integer, out %out3: !firrtl.integer) {
+  // CHECK: [[C3:%.+]] = firrtl.integer 3
   // CHECK: [[C4:%.+]] = firrtl.integer 4
   %0 = firrtl.integer 0
   %1 = firrtl.integer 1
   %2 = firrtl.integer 2
 
-  %3 = firrtl.integer.shl %1, %2 : (!firrtl.integer, !firrtl.integer) -> !firrtl.integer
-  %4 = firrtl.integer.shl %in, %0 : (!firrtl.integer, !firrtl.integer) -> !firrtl.integer
+  %res0 = firrtl.integer.shl %1, %2 : (!firrtl.integer, !firrtl.integer) -> !firrtl.integer
+  %res1 = firrtl.integer.shl %in, %0 : (!firrtl.integer, !firrtl.integer) -> !firrtl.integer
 
   // CHECK: firrtl.propassign %out0, [[C4]]
   // CHECK: firrtl.propassign %out1, %in
-  firrtl.propassign %out0, %3 : !firrtl.integer
-  firrtl.propassign %out1, %4 : !firrtl.integer
+  firrtl.propassign %out0, %res0 : !firrtl.integer
+  firrtl.propassign %out1, %res1 : !firrtl.integer
+
+  %res2 = firrtl.integer.add %1, %2 : (!firrtl.integer, !firrtl.integer) -> !firrtl.integer
+  %res3 = firrtl.integer.add %in, %0 : (!firrtl.integer, !firrtl.integer) -> !firrtl.integer
+
+  // CHECK: firrtl.propassign %out2, [[C3]]
+  // CHECK: firrtl.propassign %out3, %in
+  firrtl.propassign %out2, %res2 : !firrtl.integer
+  firrtl.propassign %out3, %res3 : !firrtl.integer
 }
 
 // CHECK-LABEL: firrtl.module private @LayerBlocks