google-research/dex-lang

Do unsigned remainder on unsigned types

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#1,211 opened on Feb 5, 2023

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 (1 comment) (1 reaction) (0 assignees)Haskell (116 forks)batch import
buggood first issue

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Description

Manifestation:

big = 2147483647
rem (big + 1) big
> 4294967295

Expected the answer to be 1.

(Note that big + 1 doesn't overflow the underlying Word32 -- it's just the rem operation messing up).

The cause is https://github.com/google-research/dex-lang/blob/c996e988d07f4bec0495136c9d4ca51bcb0c431c/src/lib/ImpToLLVM.hs#L614, which unconditionally emits a signed remainder instruction, regardless of whether the Dex type is meant to be signed or unsigned.

Contributor guide

Research direction
Review the LLVM IR generation for remainder operations in ImpToLLVM.hs and modify it to emit an unsigned remainder instruction (e.g., 'urem') instead of a signed one when the Dex type is unsigned.
Tech stack
haskell
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
None
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40