Repository navigation
On wasm32, half operation results aren't correctly rounded between each operation #96437
Description
Activity
@llvm/issue-subscribers-backend-webassembly
Author: None (beetrees)
Details
The following IR ([compiler explorer](https://godbolt.org/z/TbKnj8n4e)):target triple = "wasm32-unknown-wasi" define half @<!-- -->f(half %x) { start: %y = fmul half %x, 0xH4000 ; 0xH4000 == 2.0 %z = fdiv half %y, 0xH4000 ; 0xH4000 == 2.0 ret half %z } define half @<!-- -->callf() { %res = call half @<!-- -->f(half 0xH7BFF) ; 0xH7BFF = maximum finite `f16` ret half %res }
Is compiled into the following WASM:
f: # @<!-- -->f local.get 0 call __truncsfhf2 call __extendhfsf2 local.tee 0 local.get 0 f32.add f32.const 0x1p-1 f32.mul end_function callf: # @<!-- -->callf f32.const 0x1.ffcp15 call f end_functioncallfshould return positive infinity (0xH7C00), but on WASM it will return0xH7BFFdue to the extra range of thef32used to store the intermediate result.This Rust program, when compiled with
rustc 1.81.0-nightly (3cb521a43 2024-06-22)withrustc --target wasm32-wasip1 code.rsand run withwasmtime, demonstrates the issue.#![feature(f16)] fn f(x: f16) -> f16 { x * 2.0 / 2.0 } fn main() { assert_eq!(f(f16::MAX), 1.0 / 0.0); }
The assertion should succeed, but on WASM it fails.
cc @brendandahl, this seems potentially related to your recent fp16 work.
- added a commit that references this issue
on Jan 8, 2026 - added a commit that references this issue
on Jan 8, 2026 - added a commit that references this issue
on Jan 8, 2026 - added a commit that references this issue
on Jan 9, 2026 - added a commit that references this issue
on Jun 14, 2026 - added a commit that references this issue
on Jul 30, 2026
The following IR (compiler explorer):
Is compiled into the following WASM:
f: # @f local.get 0 call __truncsfhf2 call __extendhfsf2 local.tee 0 local.get 0 f32.add f32.const 0x1p-1 f32.mul end_function callf: # @callf f32.const 0x1.ffcp15 call f end_functioncallfshould return positive infinity (0xH7C00), but on WASM it will return0xH7BFFdue to the extra range of thef32used to store the intermediate result.This Rust program, when compiled with
rustc 1.81.0-nightly (3cb521a43 2024-06-22)withrustc --target wasm32-wasip1 code.rsand run withwasmtime, demonstrates the issue.The assertion should succeed, but on WASM it fails.