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Lec 8: Machine Level Programming: Data

Array

Multidimensional (Nested) Array

\[ \begin{bmatrix} A[0][0] & \cdots & A[0][C-1] \\ \vdots & & \vdots \\ A[R-1][0] & \cdots & A[R-1][C-1] \\ \end{bmatrix} \]
  • Declaration

T A[R][C]

  • 2D array of data type T
  • \(R\) rows, \(C\) columns

  • Type T elements requires \(K\) bytes

  • Array Size

  • R * C *K

  • Arrangement

  • Row-Major Ordering image-20240119233723666

  • Array Elements

  • \(A[i][j]\) is at address \(A + (C * i * K) + j * K = A + (C * i + j) * K\)
int get_pgh_digit
    (size_t index, size_t digit)
{
    return pgh[index][digit]; // = *(pgh + index * C * 4 + digit * 4) = *(pgh + (index * C + digit) * 4)
}

leaq     (,%rdi,10) %rdi      # (index * 10)
addq     %rdi %rsi            # (index * 10) + digit
movl     pgh(,%rsi,4), %eax      # ret_val = *(pgh + ((index * 10) + digit) * 4)
ret

Multi-Level Array

image-20240119235907354

For Multi-Level Array:

int get_univ_digit
    (size_t index, size_t digit)
{
    return univ[index][digit]; // = *(*(univ + index * 8) + digit * 4)
}

salq     $2, %rsi            # digit * 4
addq     univ(,%rdi,8) %rsi  # *(univ + index * 8) + digit * 4
movl     (%rsi), %eax        # ret_val = *(*(univ + index * 8) + digit * 4)
ret

Note: The code for these two representations are the same, but the assembly codes are different.

Structures & Alignment

规则：

  1. 结构体第一个成员的偏移量（offset）为0，以后每个成员相对于结构体首地址的 offset 都是该成员大小与有效对齐值中较小那个的整数倍，如有需要编译器会在成员之间加上填充字节。
  2. 在Linux环境下，linux下默认#pragma pack(4)，且结构体中最长的数据类型为4个字节，所以有效对齐单位为4字节
  3. 结构体的总大小为 有效对齐值 的整数倍，如有需要编译器会在最末一个成员之后加上填充字节。

e.g.

  • c struct { int x; char y; char z; };

x: 0*sizeof(int) = 0, y: 5*sizeof(char)=5, z: 6*sizeof(char)=6; tot = 2*4=8

  • c struct { char x; int y; char z; }

x: 0*sizeof(char) = 0, y: 1*sizeof(int)=4, z: 9*sizeof(char)=9; tot = 3*4=12

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