[Synth][FunctionalReduction] Fold constants (#10487)

Concerns #10484

Author: okekayode

integration_test/circt-synth/functional-reduction-cadical.mlir

@@ -20,3 +20,13 @@ hw.module @functional_reduction_sat(in %a: i1, in %b: i1, in %c: i1, in %d: i1,
   %4 = synth.aig.and_inv %a, not %b, not %c, not %d : i1
   hw.output %2, %3, %4 : i1, i1, i1
 }
+
+// CHECK-LABEL: hw.module @fold_constant_reduction
+hw.module @fold_constant_reduction(in %a: i1, in %b: i1, in %c: i1, out result: i1) {
+  // CHECK: %[[FALSE:.+]] = hw.constant false
+  // CHECK: %[[CHOICE:.+]] = synth.choice %[[FALSE]]
+  // CHECK: hw.output %[[CHOICE]] : i1
+  %0 = synth.aig.and_inv %a, not %a : i1
+  %1 = synth.aig.and_inv %0 : i1
+  hw.output %1 : i1
+}

lib/Dialect/Synth/Transforms/FunctionalReduction.cpp

@@ -348,6 +348,14 @@ void FunctionalReductionSolver::initializeSATState() {
 //===----------------------------------------------------------------------===//
 
 void FunctionalReductionSolver::collectValues() {
+
+  // Seed zero constants so nodes can be merged
+  // if input IR does not contain constants already.
+  OpBuilder builder(module.getContext());
+  builder.setInsertionPointToStart(module.getBodyBlock());
+  auto i1Type = builder.getIntegerType(1);
+  hw::ConstantOp::create(builder, module.getLoc(), i1Type, 0);
+
   // Collect block arguments (primary inputs) that are i1
   for (auto arg : module.getBodyBlock()->getArguments()) {
     if (arg.getType().isInteger(1)) {