diff --git a/src/alloy/arena.cc b/src/alloy/arena.cc
index c708fecf5..1649771dd 100644
--- a/src/alloy/arena.cc
+++ b/src/alloy/arena.cc
@@ -35,6 +35,14 @@ void Arena::Reset() {
   }
 }
 
+void Arena::DebugFill() {
+  auto chunk = head_chunk_;
+  while (chunk) {
+    memset(chunk->buffer, 0xCD, chunk->capacity);
+    chunk = chunk->next;
+  }
+}
+
 void* Arena::Alloc(size_t size) {
   if (active_chunk_) {
     if (active_chunk_->capacity - active_chunk_->offset < size) {
diff --git a/src/alloy/arena.h b/src/alloy/arena.h
index 4375a12b2..2041f1375 100644
--- a/src/alloy/arena.h
+++ b/src/alloy/arena.h
@@ -22,6 +22,7 @@ public:
   ~Arena();
 
   void Reset();
+  void DebugFill();
 
   void* Alloc(size_t size);
   template<typename T> T* Alloc() {
diff --git a/src/alloy/backend/ivm/ivm_assembler.cc b/src/alloy/backend/ivm/ivm_assembler.cc
index ec75bcf5a..58053a950 100644
--- a/src/alloy/backend/ivm/ivm_assembler.cc
+++ b/src/alloy/backend/ivm/ivm_assembler.cc
@@ -69,9 +69,12 @@ int IVMAssembler::Assemble(
   ctx.scratch_arena = &scratch_arena_;
   ctx.label_ref_head = NULL;
 
+  // Reset label tags as we use them.
+  builder->ResetLabelTags();
+
   // Function prologue.
 
-  Block* block = builder->first_block();
+  auto block = builder->first_block();
   while (block) {
     Label* label = block->label_head;
     while (label) {
diff --git a/src/alloy/hir/hir_builder.cc b/src/alloy/hir/hir_builder.cc
index ff058ae45..5b2032012 100644
--- a/src/alloy/hir/hir_builder.cc
+++ b/src/alloy/hir/hir_builder.cc
@@ -42,6 +42,10 @@ void HIRBuilder::Reset() {
   next_value_ordinal_ = 0;
   block_head_ = block_tail_ = NULL;
   current_block_ = NULL;
+#if XE_DEBUG
+  arena_->DebugFill();
+#endif
+  arena_->Reset();
 }
 
 int HIRBuilder::Finalize() {
@@ -319,6 +323,19 @@ void HIRBuilder::InsertLabel(Label* label, Instr* prev_instr) {
   }
 }
 
+void HIRBuilder::ResetLabelTags() {
+  // TODO(benvanik): make this faster?
+  auto block = block_head_;
+  while (block) {
+    auto label = block->label_head;
+    while (label) {
+      label->tag = 0;
+      label = label->next;
+    }
+    block = block->next;
+  }
+}
+
 Block* HIRBuilder::AppendBlock() {
   Block* block = arena_->Alloc<Block>();
   block->arena = arena_;
diff --git a/src/alloy/hir/hir_builder.h b/src/alloy/hir/hir_builder.h
index 356e73b4d..050d398ec 100644
--- a/src/alloy/hir/hir_builder.h
+++ b/src/alloy/hir/hir_builder.h
@@ -13,6 +13,7 @@
 #include <alloy/core.h>
 #include <alloy/hir/block.h>
 #include <alloy/hir/instr.h>
+#include <alloy/hir/label.h>
 #include <alloy/hir/opcodes.h>
 #include <alloy/hir/value.h>
 
@@ -47,6 +48,7 @@ public:
   Label* NewLabel();
   void MarkLabel(Label* label, Block* block = 0);
   void InsertLabel(Label* label, Instr* prev_instr);
+  void ResetLabelTags();
 
   // static allocations:
   // Value* AllocStatic(size_t length);