diff --git a/compiler/utils/arm/assembler_thumb2.cc b/compiler/utils/arm/assembler_thumb2.cc
index 26f7d0d..2c73fb8 100644
--- a/compiler/utils/arm/assembler_thumb2.cc
+++ b/compiler/utils/arm/assembler_thumb2.cc
@@ -256,7 +256,10 @@
     for (JumpTable& table : jump_tables_) {
       // Bulk ensure capacity, as this may be large.
       size_t orig_size = buffer_.Size();
-      buffer_.ExtendCapacity(orig_size + table.GetSize());
+      size_t required_capacity = orig_size + table.GetSize();
+      if (required_capacity > buffer_.Capacity()) {
+        buffer_.ExtendCapacity(required_capacity);
+      }
 #ifndef NDEBUG
       buffer_.has_ensured_capacity_ = true;
 #endif
diff --git a/compiler/utils/assembler.cc b/compiler/utils/assembler.cc
index c2aa574..e6c3a18 100644
--- a/compiler/utils/assembler.cc
+++ b/compiler/utils/assembler.cc
@@ -47,7 +47,7 @@
 AssemblerBuffer::AssemblerBuffer(ArenaAllocator* arena)
     : arena_(arena) {
   static const size_t kInitialBufferCapacity = 4 * KB;
-  contents_ = arena_->AllocArray<uint8_t>(kInitialBufferCapacity);
+  contents_ = arena_->AllocArray<uint8_t>(kInitialBufferCapacity, kArenaAllocAssembler);
   cursor_ = contents_;
   limit_ = ComputeLimit(contents_, kInitialBufferCapacity);
   fixup_ = nullptr;
@@ -94,6 +94,7 @@
 void AssemblerBuffer::ExtendCapacity(size_t min_capacity) {
   size_t old_size = Size();
   size_t old_capacity = Capacity();
+  DCHECK_GT(min_capacity, old_capacity);
   size_t new_capacity = std::min(old_capacity * 2, old_capacity + 1 * MB);
   new_capacity = std::max(new_capacity, min_capacity);
 
diff --git a/compiler/utils/assembler.h b/compiler/utils/assembler.h
index f70fe04..5267dc3 100644
--- a/compiler/utils/assembler.h
+++ b/compiler/utils/assembler.h
@@ -178,8 +178,8 @@
   class EnsureCapacity {
    public:
     explicit EnsureCapacity(AssemblerBuffer* buffer) {
-      if (buffer->cursor() >= buffer->limit()) {
-        buffer->ExtendCapacity();
+      if (buffer->cursor() > buffer->limit()) {
+        buffer->ExtendCapacity(buffer->Size() + kMinimumGap);
       }
       // In debug mode, we save the assembler buffer along with the gap
       // size before we start emitting to the buffer. This allows us to
@@ -219,7 +219,9 @@
   class EnsureCapacity {
    public:
     explicit EnsureCapacity(AssemblerBuffer* buffer) {
-      if (buffer->cursor() >= buffer->limit()) buffer->ExtendCapacity();
+      if (buffer->cursor() > buffer->limit()) {
+        buffer->ExtendCapacity(buffer->Size() + kMinimumGap);
+      }
     }
   };
 
@@ -233,7 +235,14 @@
   // Returns the position in the instruction stream.
   int GetPosition() { return  cursor_ - contents_; }
 
-  void ExtendCapacity(size_t min_capacity = 0u);
+  size_t Capacity() const {
+    CHECK_GE(limit_, contents_);
+    return (limit_ - contents_) + kMinimumGap;
+  }
+
+  // Unconditionally increase the capacity.
+  // The provided `min_capacity` must be higher than current `Capacity()`.
+  void ExtendCapacity(size_t min_capacity);
 
  private:
   // The limit is set to kMinimumGap bytes before the end of the data area.
@@ -255,10 +264,6 @@
 
   uint8_t* cursor() const { return cursor_; }
   uint8_t* limit() const { return limit_; }
-  size_t Capacity() const {
-    CHECK_GE(limit_, contents_);
-    return (limit_ - contents_) + kMinimumGap;
-  }
 
   // Process the fixup chain starting at the given fixup. The offset is
   // non-zero for fixups in the body if the preamble is non-empty.