[Python][Parquet] Silent wrong values decoding a double column on aarch64 SVE (pyarrow 25.0.0) #50605

Describe the bug, including details regarding any error messages, version, and platform.

Summary

With pyarrow==25.0.0 on aarch64 Linux with an SVE-capable CPU (AWS Graviton, ECS Fargate), reading a double column from a Parquet file silently returns wrong values — no error, no warning. The same file read on the same machine can yield either correct or corrupt values across different process invocations, and setting ARROW_USER_SIMD_LEVEL=NONE makes the corruption disappear entirely. This points at the ARM64 SVE runtime dispatch newly added in 25.0.0 (GH-49756), possibly interacting with the reworked CPU detection (GH-49940).

This is a silent data-integrity bug: correct-looking programs read numerically wrong floating-point data with no indication anything went wrong. It only surfaced accidentally in data integrity checks.

Environment

pyarrow: 25.0.0 (manylinux aarch64 wheel)
numpy: 2.4.6, pandas: 3.0.3
OS/arch: Linux aarch64, AWS Graviton (ECS Fargate); /proc/cpuinfo Features include sve
Python: 3.12

Minimal reproduction

import numpy as np, pyarrow as pa, pyarrow.parquet as pq

rng = np.random.default_rng(42)
vals = rng.standard_normal(20000) * 100 + 50
vals[rng.random(20000) < 0.2] = np.nan

pq.write_table(pa.table({"x": vals}), "/tmp/syn.parquet")

ref = float(np.nansum(vals)). # ground truth from the in-memory array
got = float(np.nansum(
    pq.read_table("/tmp/syn.parquet").column("x").to_numpy(zero_copy_only=False)
))
print(f"ref={ref:.3f} got={got:.3f} "
      f"{'OK' if abs(got - ref) <= 1e-6 * abs(ref) else 'CORRUPT'}")

• The corruption is intermittent across process starts / hosts even though all hosts report sve - but it is deterministic within a process (every read in a given process agrees) and the wrong value is stable (always 1393219.089 for this input).
• We first hit this on real data: a byte-identical Parquet file (verified by md5 == S3 ETag) decoded to different double sums on different Fargate tasks; the correct value matched an independent ground truth, the corrupt one did not. ARROW_USER_SIMD_LEVEL=NONE fixed every affected host.
• Downgrading to pyarrow==24.0.0 (no SVE dispatch path) reads correctly on the same hosts.

Workarounds

• ARROW_USER_SIMD_LEVEL=NONE
• Pin pyarrow<25

Component(s)

Python, C++, Parquet