[FIRRTL] Sink fstring.time in LowerLayers (#8357)

Avoid an error when trying to create ports of `fstring` type.  We
currently only have one operation which can create these, `fstring.time`,
which has simple semantics.  When we see an op like this, if it is not
already in the right layer block, then clone it.

Signed-off-by: Schuyler Eldridge <[email protected]>

Author: seldridge

lib/Dialect/FIRRTL/Transforms/LowerLayers.cpp

@@ -602,9 +602,28 @@ LogicalResult LowerLayersPass::runOnModuleBody(FModuleOp moduleOp,
             }
 
       // For any other ops, create input ports for any captured operands.
-      for (auto operand : op->getOperands()) {
-        if (!isAncestorOfValueOwner(layerBlock, operand))
-          createInputPort(operand, op->getLoc());
+      for (size_t i = 0, e = op->getNumOperands(); i != e; ++i) {
+        auto operand = op->getOperand(i);
+
+        // If the operand is in this layer block, do nothing.
+        if (isAncestorOfValueOwner(layerBlock, operand))
+          continue;
+
+        // If the operand is "special", e.g., it has no port representation,
+        // then we need to clone it.
+        //
+        // TODO: Change this to recursively clone.  This will matter once
+        // FString operations have operands.
+        if (type_isa<FStringType>(operand.getType())) {
+          OpBuilder::InsertionGuard guard(builder);
+          builder.setInsertionPoint(op);
+          op->setOperand(i,
+                         builder.clone(*operand.getDefiningOp())->getResult(0));
+          continue;
+        }
+
+        // Create a port to capture the operand.
+        createInputPort(operand, op->getLoc());
       }
 
       if (auto verbatim = dyn_cast<sv::VerbatimOp>(op))

test/Dialect/FIRRTL/lower-layers.mlir

@@ -322,6 +322,22 @@ firrtl.circuit "Test" {
     }
   }
 
+  // An FString operation is outside the layer block.  This needs to be cloned.
+  //
+  // CHECK: firrtl.module private @[[A:.+]]() {
+  // CHECK:   %time = firrtl.fstring.time
+  // CHECK:   firrtl.printf %clock, %c1_ui1, "{{.*}}" (%time)
+  // CHECK: }
+  // CHECK: firrtl.module @FStringOp
+  firrtl.module @FStringOp() {
+    %time = firrtl.fstring.time : !firrtl.fstring
+    firrtl.layerblock @A {
+       %clock = firrtl.wire : !firrtl.clock
+      %c1_ui1 = firrtl.constant 1 : !firrtl.uint<1>
+      firrtl.printf %clock, %c1_ui1, "{{}}" (%time) : !firrtl.clock, !firrtl.uint<1>, !firrtl.fstring
+    }
+  }
+
   //===--------------------------------------------------------------------===//
   // Resolving Colored Probes
   //===--------------------------------------------------------------------===//