dotnet/runtime

CSE seems to fail when writing `x / cns` and `x % cns` for unsigned integers

Open

#119,131 建立於 2025年8月27日

在 GitHub 查看
 (2 留言) (2 反應) (0 負責人)C# (17,886 star) (5,445 fork)batch import
area-CodeGen-coreclrhelp wantedtenet-performance

描述

Description

CSE seems to fail if I write something like:

uint Natural(uint index)
{
    uint arrIndex = index / 3;
    uint bitIndex = index % 3;

    return arrIndex + bitIndex;
}

(it does the division twice).

This seems to only be an issue for unsigned integers not using DivRem.

Godbolt.

Configuration

Seems to reproduce on any .NET version on godbolt - I've only checked the issue on x64.

Regression?

Probably not.

Analysis

@tannergooding was saying it seems that UMOD might be missing early expansion altogether & it's getting carried down to rationalisation.

貢獻者指南

技術棧
csharp
領域
backendperformance
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。
5
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。
over 1 week
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
C#JIT compiler conceptsx64 assembly basics
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。
5
研究方向
Investigate the JIT compiler's handling of unsigned modulo (UMOD) operations in the early expansion phase, particularly in the context of common subexpression elimination. Look at the code generation for division and modulo of unsigned integers to see why CSE fails. Check if UMOD is expanded early compared to signed modulo, and consider adding early expansion for UMOD to enable CSE.