Any ideas, Michael?

For what it's worth, I haven't yet seen such output when I run Benchmark_O directly.
It does occur a handful of times in every Benchmark_Driver-generated log file that I've looked at so far.

This is bizarre since the code that calculates the minimum is:

 var samples = [UInt64](count: c.numSamples, repeatedValue: 0)

  let sampler = SampleRunner()
  for s in 0..<c.numSamples {
    let time_per_sample: UInt64 = 1_000_000_000 * UInt64(c.iterationScale)

    var scale : UInt
    var elapsed_time : UInt64 = 0
    // Compute the scaling factor if a fixed c.fixedNumIters is not specified.
    scale = c.fixedNumIters

    // Rerun the test with the computed scale factor.
    elapsed_time = sampler.run(name, fn: fn, num_iters: scale)
    ...
    // save result in microseconds or k-ticks
    samples[s] = elapsed_time / UInt64(scale) / 1000
    ...
  }

  ...

  // Return our benchmark results.
  return BenchResults(delim: c.delim, sampleCount: UInt64(samples.count),
                      min: samples.minElement()!, max: samples.maxElement()!,
                      mean: mean, sd: sd, median: internalMedian(samples))
}

If the min element is greater than the maximum element then it would suggest that Array.minElement/Array.maxElement are incorrect.

EDITED: Formatted the pasted code.

It may be higher than the raw swift driver though. I will add some asserts that checks bounds to the swift driver. If it doesn't trigger it means that a script on top is misunderstanding the output of the low level driver.

Gotcha! It's because Benchmark_Driver uses string comparison to calculate the minimum/maximum:

    avg_test_output[NUM_SAMPLES_INDEX] = num_samples
    avg_test_output[MIN_INDEX] = min(test_outputs,
                                     key=lambda x: x[MIN_INDEX])[MIN_INDEX]
    avg_test_output[MAX_INDEX] = max(test_outputs,
                                     key=lambda x: x[MAX_INDEX])[MAX_INDEX]
    avg_test_output = map(str, avg_test_output)

The cases I've seen confirm that a lexical comparison caused them.

I'll create a pull request in a few moments. (My Python is a bit rusty.)

Simple unit testing of the script would have caught this bug. I filed:

https://bugs.swift.org/browse/SR-873

Submitted fix as PR #1530