将X-many位设置为整数X.

Question

如何缩短以下C片段？ 基本上，给定一个整数X，我想设置X多位。

for(i=0;i<NUM; i++){
    u32Mask |= (0x1 << i);
}

Answer 1

    if(i == 32)                   // X86 ignores shift count of 32
        u32mask = 0xffffffffu;
    else
        u32mask = (1u << i) - 1;

或由BeeOnRope评论

    uint32_t tbl[33] =
       {0x00000000u,0x00000001u,0x00000003u,0x00000007u,
        0x0000000fu,0x0000001fu,0x0000003fu,0x0000007fu,
        0x000000ffu,0x000001ffu,0x000003ffu,0x000007ffu,
        0x00000fffu,0x00001fffu,0x00003fffu,0x00007fffu,
        0x0000ffffu,0x0001ffffu,0x0003ffffu,0x0007ffffu,
        0x000fffffu,0x001fffffu,0x003fffffu,0x007fffffu,
        0x00ffffffu,0x01ffffffu,0x03ffffffu,0x07ffffffu,
        0x0fffffffu,0x1fffffffu,0x3fffffffu,0x7fffffffu,
        0xffffffff};
    // ...
    u32mask = tbl[i];