[tab] examples/jax_assert.html (html)
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  <d-front-matter>
    <script id='distill-front-matter' type="text/json">{
    "title": "Performance-degradation Free Value Assertions in JAX",
    "description": "Traditional value assertions in jitted JAX lead to performance degredation. A new (not yet public) JAX API fixes this.",
    "published": "March 26, 2025",
    "url": "https://pdoom.org/jax_assert.html",
    "authors": [
      {
        "author":"Franz Srambical",
        "authorURL":"https://srambical.fr/",
        "affiliations": [{"name": "p(doom)", "url": "https://pdoom.org/"}]
      }
    ],
    "katex": {
      "delimiters": [
        {"left": "$$", "right": "$$", "display": false}
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  </d-front-matter>
  <d-title>
    <p>
      Traditional value assertions in jitted JAX lead to performance degredation. A new (not yet public) JAX API fixes this.
    </p>
  </d-title>
  <d-byline></d-byline>
  <d-article>
    <a class="marker" href="#section-1" id="section-1"><span>1</span></a>
    <p>
      Jitted JAX does not support traditional python asserts that access JAX arrays <d-cite key="jax2025checkify, jax2025callbacks, jax2025jit, jax2025key, deepmind2020chex"></d-cite>.
      Chex <d-cite key="deepmind2020chex"></d-cite> and <code>jax.experimental.checkify.check</code> <d-cite key="jax2025checkify"></d-cite> provide ways of wrapping a jitted function
      with decorators to enable value assertions, but they lead to performance-degradation, making them unusable in practical settings.
    </p>
    <p>
      For a performance-degradation free way of using value assertions in jitted JAX, we can use a new (as of today still private) JAX API: <code>error_check</code> <d-cite key="ayaka76822025error"></d-cite>:
    </p>
    <d-code block="" language="python">
      import jax
      from jax._src.error_check import set_error_if, raise_if_error
      import jax.numpy as jnp

      @jax.jit
      def f(x, y):
          set_error_if(x != 0, 'x must be 0')
          return jnp.multiply(x, y)

      f(1, 0)

      raise_if_error()
    </d-code>

    <d-code block="" language="bash">
      Traceback (most recent call last):
        File "/home/ubuntu/code/temp.py", line 12, in <module>
          raise_if_error()
        File "/home/ubuntu/code/.venv/lib/python3.10/site-packages/jax/_src/error_check.py", line 93, in raise_if_error
          raise exc.with_traceback(filtered_traceback)
        File "/home/ubuntu/code/temp.py", line 10, in <module>
          f(1, 0)
        File "/home/ubuntu/code/temp.py", line 7, in f
          set_error_if(x != 0, 'x must be 0')
      jax._src.error_check.JaxValueError: x must be 0
    </d-code>
    <p>
      This pattern exploits that it suffices to raise an assertion error <i>post-hoc</i>, in this case after the computation of the jitted function.
      Thus, the implementation merely conditionally stores the error in JAX-managed context. While purely functional conditional computation is fully supported by JAX and XLA <d-cite key="jax2025control, xla2025conditional"></d-cite>, and thus fully compatible with XLA graph compilation,
      the error is only raised outside of the jitted function, avoiding the typical performance overhead of value assertions.
    </p>
    <aside>
      Error raising is an operation that is not purely functional, but has side-effects.
    </aside>
    </d-article>

  <d-appendix>

    <h3>Contributions</h3>
    <p>FS worked on all aspects of this post, including research, analysis and writing.</p>
    <d-bibliography src="bibliography.bib"></d-bibliography>
    <distill-appendix>
    </distill-appendix>
  </d-appendix>

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