[FIRRTL] Fix when flattening with layerblocks domination issue (#10650)

When flattening a when block, statements like asserts are merged with
ltlAndWithCondition and ltlImplicationWithCondition. In the process of
flattening, flattened ops that are identical become shared as an
optimization strategy. However, an op created inside of a layerblock
should not be shared by sibling layerblocks because an op defined in one
layerblock's region does not dominate a sibling layerblock.

This fixes the issue by saving a copy of cache before processing the layer
block. It then restores the copy after it finishes processing the
layerblock.

Assisted-by: Claude Code
Co-authored-by: Jack Toubes <jtoubes@berkeley.edu>

Author: jack2bs

lib/Dialect/FIRRTL/Transforms/ExpandWhens.cpp

@@ -22,6 +22,7 @@
 #include "mlir/Pass/Pass.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/SaveAndRestore.h"
 
 namespace circt {
 namespace firrtl {
@@ -706,6 +707,15 @@ void WhenOpVisitor::visitStmt(WhenOp whenOp) {
 
 // NOLINTNEXTLINE(misc-no-recursion)
 void WhenOpVisitor::visitStmt(LayerBlockOp layerBlockOp) {
+  // Operations created inside a layerblock are not accessible in sibling
+  // layerblocks. Snapshot the LTL op caches before entering and restore them
+  // afterward so that ops created inside one layerblock are not reused by a
+  // sibling, which would cause dominance violations. Parent-scope cached ops
+  // remain available inside the layerblock since they dominate it.
+  llvm::SaveAndRestore savedLTLAndOps(createdLTLAndOps);
+  llvm::SaveAndRestore savedLTLImplicationOps(createdLTLImplicationOps);
+  llvm::SaveAndRestore savedLTLClockOps(createdLTLClockOps);
+
   process(*layerBlockOp.getBody());
 }
 

test/Dialect/FIRRTL/expand-whens.mlir

@@ -776,4 +776,36 @@ firrtl.module @instance_choice_test(in %a : !firrtl.uint<1>,
   }
 }
 
+
+// Test that sibling layerblocks inside a when each get their own LTL ops and do
+// not share cached values across layerblock boundaries (which would cause
+// dominance violations). See https://github.com/llvm/circt/issues/10104.
+firrtl.layer @LayerBar bind attributes {sym_visibility = "private"} {}
+// CHECK-LABEL: firrtl.module @SiblingLayerblocksImplication
+firrtl.module @SiblingLayerblocksImplication() {
+  %c0_ui1 = firrtl.constant 0 : !firrtl.uint<1>
+  // CHECK: firrtl.layerblock @LayerBar {
+  // CHECK:   firrtl.int.ltl.implication
+  // CHECK:   [[AND1:%.+]] = firrtl.int.ltl.and %c0_ui1, %c0_ui1
+  // CHECK:   [[IMPL1:%.+]] = firrtl.int.ltl.implication [[AND1]], %c0_ui1
+  // CHECK:   firrtl.int.verif.assert [[IMPL1]], %c0_ui1
+  // CHECK: }
+  // CHECK: firrtl.layerblock @LayerBar {
+  // CHECK:   firrtl.int.ltl.implication
+  // CHECK:   [[AND2:%.+]] = firrtl.int.ltl.and %c0_ui1, %c0_ui1
+  // CHECK:   [[IMPL2:%.+]] = firrtl.int.ltl.implication [[AND2]], %c0_ui1
+  // CHECK:   firrtl.int.verif.assert [[IMPL2]], %c0_ui1
+  // CHECK: }
+  firrtl.when %c0_ui1 : !firrtl.uint<1> {
+    firrtl.layerblock @LayerBar {
+      %0 = firrtl.int.ltl.implication %c0_ui1, %c0_ui1 : (!firrtl.uint<1>, !firrtl.uint<1>) -> !firrtl.uint<1>
+      firrtl.int.verif.assert %0, %c0_ui1 : !firrtl.uint<1>, !firrtl.uint<1>
+    }
+    firrtl.layerblock @LayerBar {
+      %0 = firrtl.int.ltl.implication %c0_ui1, %c0_ui1 : (!firrtl.uint<1>, !firrtl.uint<1>) -> !firrtl.uint<1>
+      firrtl.int.verif.assert %0, %c0_ui1 : !firrtl.uint<1>, !firrtl.uint<1>
+    }
+  }
+}
+
 }