dotnet/runtime

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

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#119,131 创建于 2025年8月27日

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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.

贡献者指南

技术栈
csharp
领域
backendperformance
议题类型
bug
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
5
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over 1 week
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fresh
清晰度议题是否清楚说明期望改动、验收标准和下一步。
clear
前置要求
C#JIT compiler conceptsx64 assembly basics
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
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.