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    

貢獻者指南

技術棧

csharp

領域

performance

議題類型

performance

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

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1-3 hours

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fresh

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clear

前置要求

Basic C# knowledgeUnderstanding of assembly language

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。

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.