[x86] Eliminate `movzx` when upper part of register is irrelevant or overwritten · llvm/llvm-project#168828

(16 评论) (0 反应) (1 负责人)C++ (10,782 fork)batch import

backend:X86good first issuellvm:SelectionDAGmissed-optimization

仓库指标

export fn foo(x: u8) u32 {
    return @ctz(x >> 1);
}

foo:
        shr     dil
        movzx   eax, dil
        or      eax, 256
        tzcnt   eax, eax
        ret

Could be:

foo:
        shr     dil
        or      edi, 256
        tzcnt   eax, edi
        ret

The compiler could also look for things like or eax, -256 and eliminate movzx eax, al since the relevant bits are overwritten anyway.

研究方向: 调查LLVM的x86后端，找到生成movzx的地方，判断高位是否无关或被覆盖。查看DAG或ISel降低中movzx后跟or的模式，看看当目标被覆盖时是否可以消除movzx。
技术栈: cpp
领域: backend
议题类型: 性能
难度: 3
预计时间: 1-3 小时
活动状态: 活跃
清晰度: 清晰
前置要求: GitLLVM
新手友好度: 60