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Posted to github@arrow.apache.org by "miguelcorderopamphile (via GitHub)" <gi...@apache.org> on 2026/10/01 07:17:14 UTC
[I] i256::to_f32 double-rounds through f64 [arrow-rs]
miguelcorderopamphile opened a new issue, #11316:
URL: https://github.com/apache/arrow-rs/issues/11316
### Describe the bug
`i256` only implements `ToPrimitive::to_f64`, so `ToPrimitive::to_f32` falls back to the default `to_f64() as f32`. When the exact value is within half an `f64` ULP of an `f32` midpoint, the intermediate rounding moves it across the midpoint and the result differs from a correctly rounded `f32`.
Verified on `main` (Rust):
| value | correctly rounded (`i128 as f32`) | `i256::to_f32()` |
| --- | --- | --- |
| `2^53 + 2^29 + 1` | `0x5a000001` | `0x5a000000` |
| `-(2^53 + 2^29 + 1)` | `0xda000001` | `0xda000000` |
| `2^54 + 2^30 + 1` | `0x5a800001` | `0x5a800000` |
For `v = 9007199791611905`, the expected `f32` is `9007200328482816.0` and `to_f32()` returns `9007199254740992.0`.
### To Reproduce
```rust
use arrow_buffer::i256;
use num_traits::ToPrimitive;
let v: i128 = (1 << 53) + (1 << 29) + 1;
let value = i256::from_i128(v);
assert_eq!(value.to_f32().unwrap().to_bits(), (v as f32).to_bits()); // fails
```
### Expected behavior
For values in the `i128` range, `i256::to_f32` should match `i128 as f32` (correctly rounded), the same expectation that `to_f64` now satisfies after #11315.
### Additional context
Found while validating #11315. Similar values exist across f32 exponents (the double-rounding window is around 2^-29, so they are rare in practice). There are no in-tree callers of `i256::to_f32`; `Decimal256 -> Float32` goes through `to_f64` plus scaling. Low impact, but a correctness gap in the public conversion.
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Re: [I] i256::to_f32 double-rounds through f64 [arrow-rs]
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb commented on issue #11316:
URL: https://github.com/apache/arrow-rs/issues/11316#issuecomment-6174476477
What were you doing when you hit this error? I am trying to figure out how you found this
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Re: [I] i256::to_f32 double-rounds through f64 [arrow-rs]
Posted by "RanaPriyansh (via GitHub)" <gi...@apache.org>.
RanaPriyansh commented on issue #11316:
URL: https://github.com/apache/arrow-rs/issues/11316#issuecomment-5953785837
I am preparing a direct `i256::to_f32` conversion for this issue. The change will leave the `to_f64` helper in #11315 unchanged.
Codex used.
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