Unnecessary sign-extension for some never-negative expressions

Description

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    

Contributor guide

Tech stack

csharp

Domain

performance

Issue type

performance

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Basic C# knowledgeUnderstanding of assembly language

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

55

Research direction

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.