Unnecessary sign-extension for some never-negative expressions

描述

nint M1(uint mask) {
    return BitOperations.TrailingZeroCount(mask) / sizeof(ushort);
}

nint M2(uint mask) {
    return (nint)(uint.TrailingZeroCount(mask) / sizeof(ushort));
}

C:M1(uint):nint:this (FullOpts):
       xor      eax, eax
       tzcnt    eax, esi
       shr      eax, 1
       cdqe     
       ret      

C:M2(uint):nint:this (FullOpts):
       xor      eax, eax
       tzcnt    eax, esi
       shr      eax, 1
       ret    

贡献者指南

技术栈

csharp

领域

performance

议题类型

performance

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3

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clear

前置要求

Basic C# knowledgeUnderstanding of assembly language

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。

55

研究方向

This issue concerns an unnecessary sign extension (cdqe) in the JIT compiled code for certain integer divisions when the result is never negative. The fix likely involves modifying the JIT optimizer in src/coreclr/jit/ to recognize that expressions like BitOperations.TrailingZeroCount(mask) / sizeof(ushort) produce a non negative result and thus omit the sign extension. Review the existing JIT handling of sign extension (e.g., in morph.cpp or lsra.cpp) and test the change with the provided code snippets.