google-research/dex-lang

Do unsigned remainder on unsigned types

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#1,211 建立於 2023年2月5日

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 (1 留言) (1 反應) (0 負責人)Haskell (1,681 star) (116 fork)batch import
buggood first issue

描述

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.

貢獻者指南

技術棧
無
領域
backend
議題類型
bug
難度面向新貢獻者的預計實作難度，1 表示很小改動，5 表示專家級工作。
3
預計時間有經驗貢獻者完成調查、實作、測試並準備 pull request 的粗略時間範圍。
under 1 hour
活動狀態議題目前的可參與程度：新鮮、活躍、陳舊、阻塞或等待維護者輸入。
fresh
清晰度議題是否清楚說明預期改動、驗收標準和下一步。
clear
前置要求
basic HaskellLLVM IR basicsDex language familiarity
新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。
50
研究方向
Look at the file src/lib/ImpToLLVM.hs line 614, where a signed remainder (SRem) is emitted. For unsigned types, it should use URem. Investigate how Dex represents unsigned types and ensure the fix correctly selects URem. Verify with the example where big+1 is 2147483648 and rem with 2147483647 should yield 1.