UNDF: UNDF-2026-000000551

tensorflow-0001 — execute.cc IsHostMemoryArg O(N²) per-op dispatch

Location

tensorflow/core/common_runtime/eager/execute.cc, lines 1191–1196 (outer loop) and 295–309 (IsHostMemoryArg).

Defective Code

// IsHostMemoryArg — line 295
bool IsHostMemoryArg(const EagerOperation& op, const NodeDef* node_def,
                     const Device* op_device, const KernelDef* kernel_def,
                     const int port_id) {
  if (op.is_function()) return false;
  if (node_def == nullptr) return false;
  if (kernel_def == nullptr || op_device == nullptr) return false;
  const auto& host_memory_args = kernel_def->host_memory_arg();  // repeated string field ~ vector
  const OpDef& op_def = OpRegistry::Global()->LookUp(op.Name())->op_def;
  const int arg_id = OpPortIdToArgId(*node_def, op_def.input_arg(), port_id);
  if (arg_id < 0) return false;
  return std::find(host_memory_args.begin(), host_memory_args.end(),
                   op_def.input_arg(arg_id).name()) != host_memory_args.end();  // O(H) scan
}

// Calling loop — line 1191
for (int i = 0, end = inputs->size(); i < end; ++i) {  // O(I) outer loop
    TensorHandle* input = (*inputs)[i];
    Device* input_device;
    bool is_host_memory_arg =
        IsHostMemoryArg(*op, node_def, op_device, kernel_def, i);  // O(H) per input
    // ...
}

For each of I inputs, IsHostMemoryArg does a linear scan of H host_memory_arg strings. Total: O(I × H) per op dispatch.

GetDeviceForInputs is called on every eager op execution, making this a per-inference hot path.

Fixed Code

// Build a set before the loop — O(H)
std::unordered_set<std::string> host_memory_arg_set;
if (!op->is_function() && node_def != nullptr &&
    kernel_def != nullptr && op_device != nullptr) {
  for (const auto& arg_name : kernel_def->host_memory_arg()) {
    host_memory_arg_set.insert(arg_name);
  }
}

// Revised IsHostMemoryArg takes the prebuilt set
bool IsHostMemoryArgFast(const EagerOperation& op, const NodeDef* node_def,
                         const std::unordered_set<std::string>& host_memory_arg_set,
                         const int port_id) {
  if (op.is_function()) return false;
  if (node_def == nullptr) return false;
  const OpDef& op_def = OpRegistry::Global()->LookUp(op.Name())->op_def;
  const int arg_id = OpPortIdToArgId(*node_def, op_def.input_arg(), port_id);
  if (arg_id < 0) return false;
  return host_memory_arg_set.count(op_def.input_arg(arg_id).name()) > 0;  // O(1)
}

// Calling loop — build set once, call per input
auto host_memory_arg_set = BuildHostMemoryArgSet(op, node_def, kernel_def, op_device);
for (int i = 0, end = inputs->size(); i < end; ++i) {
    bool is_host_memory_arg = IsHostMemoryArgFast(*op, node_def, host_memory_arg_set, i);  // O(1)
    // ...
}

Complexity Analysis

Path	Complexity
Slow (original)	O(I × H) per op
Fast (fixed)	O(H + I) per op

For a typical Conv2D with I=8 inputs and H=6 host_memory_args: 6× speedup per op. For ops like CombineNDArray with I=100+ inputs: 100× speedup. Called on every eager tensor operation at inference time.

Severity

HIGH — GetDeviceForInputs is on the critical path of every eager mode tensor operation, including every forward pass call in PyTorch-style TF execution. The O(I×H) scan executes O(ops_per_step) times per training step.

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