ulpwise

Numerical conformance testing for ML code. A Rust core with a Python API and a pytest plugin.

ulpwise answers three questions that come up every time a numerical test goes red on one platform and green on another:

1. Which inputs break first? Named edge values computed from the float format (the largest x with x * x == 0, the first x whose square overflows, the value where 1 / x overflows, binade boundaries, subnormals) and every-float enumerations of a range.
2. What is the right answer, exactly? Rounding oracles that compare the exact result against the candidate floats with integer arithmetic on significands, so the correctly rounded sqrt, reciprocal and quotient, and the distance from the exact result to the rounding midpoint, do not depend on any libm.
3. Where do two implementations disagree? Knife-edge scans: the inputs whose exact result sits within tol ulp of a rounding midpoint, so that two implementations that differ by one ulp return different floats. 8.4 million f32 inputs are scanned in about 0.3 s.

Why this exists

On 2026-09-23 a kornia pull request went red on 21 of 39 CI jobs after a maintainer added a test around the shape (1.0, 0.9235056042671204, 0.8528626561164856). In float32 the exact value of sqrt(0.8528626561164856) lies 0.0004 ulp below the midpoint of its two float32 neighbours. The macOS runners and my sandbox rounded it down, the ubuntu and windows runners rounded it up, and the estimator under test took two different branches. ulpwise finds that input, and the 16,995 others like it in [0.5, 1), before CI does:

$ python -m ulpwise midpoint sqrt 0.8528626561164856 --dtype f32
rounded 0.9235056042671204, exact result lies above it, 3.771e-04 ulp from the rounding midpoint

$ python -m ulpwise knife sqrt --lo 0.85 --hi 0.86 --tol 1e-3 --limit 3
                       x                  rounded  midpoint distance (ulp) exact lies
      0.8500027060508728       0.9219558835029602                2.521e-04      above
      0.8500217199325562       0.9219662547111511                5.452e-04      below
      0.8500407338142395       0.9219765067100525                5.924e-04      above
3 knife edge(s) with distance < 0.001 ulp

Measured with examples/torch_sqrt_conformance.py on one Linux x86_64 machine (AVX512, torch 2.14.0+cpu built with MKL, numpy 2.2.6), at the 16,996 float32 knife edges of [0.5, 1) with tol = 1e-3:

On random inputs the same torch.sqrt differs from correct rounding at 0.70% of float32 values, which is why this goes unnoticed until a test happens to pin one of them. Other platforms will show other numbers; the CI of this repository prints them for ubuntu, macOS and windows on every run.

Install

pip install ulpwise

Wheels on PyPI cover Linux x86_64 and aarch64, macOS arm64 and x86_64, and Windows x64, for Python 3.9 or newer (abi3). To build from source you need a Rust toolchain (1.86 or newer) and maturin:

pip install maturin
pip install .            # or: maturin develop --release  (inside a virtualenv)

The Rust crate is usable on its own (cargo add --git https://github.com/Nicholas022400701/ulpwise).

Use

import ulpwise

# ulp distances, dtype aware: a float32 tensor is measured in float32 ulps.
ulpwise.ulp_distance(0.9235056042671204, 0.9235056638717651, "f32")   # 1
ulpwise.assert_max_ulp(torch_out, reference, max_ulp_=2)               # floats, lists, numpy, torch

# the 29 named edge values of a dtype
dict(ulpwise.special("f32"))["square_underflows_to_zero"]              # 2.6469779601696886e-23, the largest x with x * x == 0

# exact placement of a result relative to its rounding midpoint
ulpwise.midpoint("sqrt", 0.8528626561164856, "f32")   # (0.9235056042671204, 0.000377, True, False)
ulpwise.midpoint("div", 1.0, "f64", 3.0)              # (0.3333333333333333, 0.1666..., True, False)

# knife edges of a function on a range: (x, rounded, distance_ulp, exact_above)
ulpwise.knife_edges("exp", 0.5, 1.0, tol_ulp=1e-3, dtype="f32", limit=100)

# a correctly rounded sqrt that does not depend on the platform
ulpwise.sqrt_cr(0.8528626561164856, "f32")            # 0.9235056042671204

midpoint and knife_edges accept sqrt, recip and div (exact integer oracle, f32 and f64) and rsqrt exp exp2 expm1 log log2 log10 log1p sin cos tan atan tanh sigmoid softplus (f32 only, f64 libm as reference, trust the distance down to about 1e-8 ulp).

pytest plugin

Installed automatically. A test that takes edge_f32 or edge_f64 runs once per named edge value, and assert_max_ulp is available as a fixture:

def test_my_kernel_survives_the_edges(edge_f32, assert_max_ulp):
    x = torch.tensor([edge_f32])
    assert_max_ulp(my_kernel(x), reference(x), max_ulp_=1)

Failures read test_my_kernel_survives_the_edges[square_overflows].

Regression corpus

python/ulpwise/corpus/cases.json holds upstream bugs found by the contribution pipeline this project grew out of, each with a runnable repro, the buggy behaviour quoted from the merged pull request, and the check that tells the two apart. pytest tests/test_corpus.py runs every case whose packages are installed; a case is an expected failure while the installed release is not known to contain the fix, so the run tells you which bugs are present in your environment.

Two ultralytics fixes (#26240, #26246) are not in the corpus yet because their repros need model weights and a dataset layout; they will come with fixtures.

How the exact oracle works

For sqrt(x) the candidate r and the midpoint m between r and its neighbour are written as integers times a power of two, m^2 is formed in u128 (at most 110 bits) and compared with x after aligning exponents. The sign says on which side of m the exact root lies, and |x - m^2| / (sqrt(x) + m) is the distance to the midpoint. If the hardware result turns out to be on the wrong side of a midpoint it is stepped one ulp toward the exact value and checked again, so sqrt_cr is correctly rounded even where the platform sqrt is not. Division uses the same machinery with m * b against a, and there the residual is exact. tests/test_core.py cross-checks both against fractions.Fraction and decimal.Decimal at 80 digits.

Roadmap

• Mutation scoring for numerical tests: single token mutants of the code under test (abs, a dropped sqrt, / 4 for / 16) run against the test suite, reporting which survive.
• float16 and bfloat16 ulps and edge values.
• Exact references for transcendental functions (correctly rounded exp, log, ...) so the f64 ones can be scanned too.
• Zero copy paths for numpy arrays and torch tensors.
• More corpus entries, with fixtures for the cases that need data.

AI disclosure

This project is written with an AI coding agent (Claude) working for 区梓灏 (@Nicholas022400701), who owns the repository and reviews what is published. The same pipeline produced the upstream fixes in the corpus; each of those pull requests carries the same disclosure.

License

Licensed under either of

• the MIT license (LICENSE-MIT), or
• the Apache License, Version 2.0 (LICENSE-APACHE),

at your option. "At your option" means that whoever uses or redistributes this code picks whichever of the two licenses they want to comply with. Nobody has to ask anyone. Unless you say otherwise, a contribution you send is dual licensed the same way, without extra terms.