8.8. Matrices

A matrix is a two-dimensional array. A matrix in the C language can be statically allocated as a two-dimensional array (M[n][m]), dynamically allocated with a single call to malloc, or dynamically allocated as an array of arrays. Let’s consider the array of arrays implementation. The first array contains n elements (M[n]), and each element M[i] in our matrix contains an array of m elements. The following code snippets each declare matrices of size 4 × 3:

//statically allocated matrix (allocated on stack)
int M1[4][3];

//dynamically allocated matrix (programmer friendly, allocated on heap)
int **M2, i;
M2 = malloc(4 * sizeof(int*));
for (i = 0; i < 4; i++) {
    M2[i] = malloc(3 * sizeof(int));
}

In the case of the dynamically allocated matrix, the main array contains a contiguous array of int pointers. Each integer pointer points to a different array in memory. Figure 1 illustrates how we would normally visualize each of these matrices.

matrices
Figure 1. Illustration of a statically allocated (M1) and a dynamically allocated (M2) 3x4 matrix

For both of the above matrix declarations, element (i,j) can be accessed using the double indexing syntax M[i][j], where M is either M1 or M2. However, these matrices are organized differently in memory. While both store the elements in their primary array contiguously in memory, our statically allocated matrix also stores all the rows contiguously in memory, as shown in Figure 2.

matrixArray
Figure 2. Matrix M1’s memory layout in row-major order

This contiguous ordering is not guaranteed for M2. Recall that to contiguously allocate an n × m matrix on the heap, we should use a single call to malloc that allocates n × m elements:

//dynamic matrix (allocated on heap, memory efficient way)
#define ROWS 4
#define COLS 3
int *M3;
M3  = malloc(ROWS * COLS * sizeof(int));

Recall that with the declaration of M3, element (i,j) cannot be accessed using the M[i][j] notation. Instead, we must index the element using the format M3[i*cols + j].

8.8.1. Contiguous Two-Dimensional Arrays

Consider a function sumMat that takes a pointer to a contiguously allocated (either statically allocated or memory-efficiently dynamically allocated) matrix as its first parameter, along with the numbers of rows and columns, and returns the sum of all the elements inside the matrix.

We use scaled indexing in the code snippet below, since it applies to both statically and dynamically allocated contiguous matrices. Recall that the syntax m[i][j] does not work with the memory-efficient contiguous dynamic allocation previously discussed.

int sumMat(int *m, int rows, int cols) {
    int i, j, total = 0;
    for (i = 0; i < rows; i++){
        for (j = 0; j < cols; j++){
            total += m[i*cols + j];
        }
    }
    return total;
}

Here is the corresponding assembly. Each line is annotated with its English translation:

<sumMat>:
0x08048507 <+0>:  push %ebp                  #save ebp
0x08048508 <+1>:  mov  %esp,%ebp             #update ebp (new stack frame)
0x0804850a <+3>:  sub  $0x10,%esp            #add 4 more spaces to stack frame
0x0804850d <+6>:  movl $0x0,-0xc(%ebp)       #copy 0 to ebp-12 (total)
0x08048514 <+13>: movl $0x0,-0x4(%ebp)       #copy 0 to ebp-4 (i)
0x0804851b <+20>: jmp  0x8048555 <sumMat+78> #goto <sumMat+78>
0x0804851d <+22>: movl $0x0,-0x8(%ebp)       #copy 0 to ebp-8 (j)
0x08048524 <+29>: jmp  0x8048549 <sumMat+66> #goto <sumMat+66>
0x08048526 <+31>: mov  -0x4(%ebp),%eax       #copy i to eax
0x08048529 <+34>: imul 0x10(%ebp),%eax       #multiply i * cols, place in eax
0x0804852d <+38>: mov  %eax,%edx             #copy i*cols to edx
0x0804852f <+40>: mov  -0x8(%ebp),%eax       #copy j to %eax
0x08048532 <+43>: add  %edx,%eax             #add i*cols with j, place in eax
0x08048534 <+45>: lea  0x0(,%eax,4),%edx     #mult (i*cols+j) by 4,put in edx
0x0804853b <+52>: mov  0x8(%ebp),%eax        #copy m pointer to eax
0x0804853e <+55>: add  %edx,%eax             #add m to (i*cols+j)*4,put in eax
0x08048540 <+57>: mov  (%eax),%eax           #copy m[i*cols+j] to eax
0x08048542 <+59>: add  %eax,-0xc(%ebp)       #add eax to total
0x08048545 <+62>: addl $0x1,-0x8(%ebp)       #increment j by 1 (j+=1)
0x08048549 <+66>: mov  -0x8(%ebp),%eax       #copy j to eax
0x0804854c <+69>: cmp  0x10(%ebp),%eax       #compare j with cols
0x0804854f <+72>: jl   0x8048526 <sumMat+31> #if (j < cols) goto <sumMat+31>
0x08048551 <+74>: addl $0x1,-0x4(%ebp)       #add 1 to i (i+=1)
0x08048555 <+78>: mov  -0x4(%ebp),%eax       #copy i to eax
0x08048558 <+81>: cmp  0xc(%ebp),%eax        #compare i with rows
0x0804855b <+84>: jl   0x804851d <sumMat+22> #if (i < rows) goto sumMat+22
0x0804855d <+86>: mov  -0xc(%ebp),%eax       #copy total to eax
0x08048560 <+89>: leave                      #prepare to leave the function
0x08048561 <+90>: ret                        #return total

The local variables i, j, and total are loaded at addresses %ebp-4, %ebp-8, and %ebp-12 on the stack, respectively. The input parameters m, row, and cols are located at locations %ebp+8, %ebp+12, and %ebp+16, respectively. Using this knowledge, let’s zoom in on the component that just deals with the access of element (i,j) in our matrix:

0x08048526 <+31>: mov  -0x4(%ebp),%eax    # copy i to eax
0x08048529 <+34>: imul 0x10(%ebp),%eax    # multiply i with cols, place in eax
0x0804852d <+38>: mov  %eax,%edx          # copy i*cols to edx

The first set of instructions computes i * cols and places the result in register %edx. Recall that for a matrix named matrix, matrix + (i * cols) is equivalent to &matrix[i].


0x0804852f <+40>: mov -0x8(%ebp),%eax   # copy j to eax
0x08048532 <+43>: add %edx,%eax         # add i*cols with j, place in eax
0x08048534 <+45>: lea 0x0(,%eax,4),%edx # multiply (i*cols+j) by 4, put in edx

The next set of instructions computes (i * cols + j) * 4. The compiler multiplies the index (i * cols) + j by four since each element in the matrix is a four-byte integer and this multiplication enables the compiler to calculate the correct offset.


The last set of instructions adds the calculated offset to the matrix pointer and dereferences it to yield the value of element (i,j):

0x0804853b <+52>: mov 0x8(%ebp),%eax    # copy m pointer to eax
0x0804853e <+55>: add %edx,%eax         # add m to (i*cols+j)*4, place in eax
0x08048540 <+57>: mov (%eax),%eax       # copy m[i*cols+j] to eax
0x08048542 <+59>: add %eax,-0xc(%ebp)   # add eax to total

The first instruction loads the address of matrix m into register %eax. The add instruction adds the offset (i*cols + j)*4 to the address of m to correctly calculate the address of element (i,j), and places this address in register %eax. The third instruction dereferences %eax and places the resulting value in register %eax. The last instruction adds the value in %eax to the accumulator total, which is located at stack address %ebp-0xc.

Let’s consider how element (1,2) is accessed in Figure 2.

matrixArray
Figure 3. Matrix M1’s memory layout in row-major order

Element (1,2) is located at address M1 + (1 * COLS) + 2. Since COLS = 3, element (1,2) corresponds to M1+5. To access the element at this location, the compiler must multiply 5 by the size of the int data type (four bytes), yielding the offset M1 + 20, which corresponds to byte x20 in the figure. Dereferencing this location yields element 5, which is indeed element (1,2) in the matrix.

8.8.2. Noncontiguous Matrix

The noncontiguous matrix implementation is a bit more complicated. Figure 4 visualizes how M2 may be laid out in memory.

matrixDynamic
Figure 4. Matrix M2’s noncontiguous layout in memory

Notice that the array of pointers is contiguous, and that each array pointed to by an element of M2 (e.g., M2[i]) is contiguous. However, the individual arrays are not contiguous with each other.

The sumMatrix function below takes an array of integer pointers (called matrix) as its first parameter, and a number of rows and columns as its second and third parameters:

int sumMatrix(int **matrix, int rows, int cols) {

    int i, j, total=0;

    for (i = 0; i < rows; i++) {
        for (j = 0; j < cols; j++) {
            total += matrix[i][j];
        }
    }
    return total;
}

While this function looks nearly identical to the sumMat function shown earlier, the matrix accepted by this function consists of a contiguous array of pointers. Each pointer contains the address of a separate contiguous array, which corresponds to a separate row in the matrix.

The corresponding assembly for sumMatrix follows. Each line is annotated with its English translation.

0x080484ad <+0>:  push %ebp                     # save ebp
0x080484ae <+1>:  mov  %esp,%ebp                # update ebp (new stack frame)
0x080484b0 <+3>:  sub  $0x10,%esp               # add 4 spaces to stack frame
0x080484b3 <+6>:  movl $0x0,-0xc(%ebp)          # copy 0 to %ebp-12 (total)
0x080484ba <+13>: movl $0x0,-0x4(%ebp)          # copy 0 to %ebp-4 (i)
0x080484c1 <+20>: jmp  0x80484fa <sumMatrix+77> # goto <sumMatrix+77>
0x080484c3 <+22>: movl $0x0,-0x8(%ebp)          # copy 0 to %ebp-8 (j)
0x080484ca <+29>: jmp  0x80484ee <sumMatrix+65> # goto <sumMatrix+65>
0x080484cc <+31>: mov  -0x4(%ebp),%eax          # copy i to %eax
0x080484cf <+34>: lea  0x0(,%eax,4),%edx        # mult i by 4, place in %edx
0x080484d6 <+41>: mov  0x8(%ebp),%eax           # copy matrix to %eax
0x080484d9 <+44>: add  %edx,%eax                # put (i * 4) + matrix in %eax
0x080484db <+46>: mov  (%eax),%eax              # copy matrix[i] to %eax
0x080484dd <+48>: mov  -0x8(%ebp),%edx          # copy j to %edx
0x080484e0 <+51>: shl  $0x2,%edx                # mult j by 4, place in %edx
0x080484e3 <+54>: add  %edx,%eax                # put (j*4)+matrix[i] in %eax
0x080484e5 <+56>: mov  (%eax),%eax              # copy matrix[i][j] to %eax
0x080484e7 <+58>: add  %eax,-0xc(%ebp)          # add matrix[i][j] to total
0x080484ea <+61>: addl $0x1,-0x8(%ebp)          # add 1 to j (j+=1)
0x080484ee <+65>: mov  -0x8(%ebp),%eax          # copy j to %eax
0x080484f1 <+68>: cmp  0x10(%ebp),%eax          # compare j with cols
0x080484f4 <+71>: jl   0x80484cc <sumMatrix+31> # if j<cols goto<sumMatrix+31>
0x080484f6 <+73>: addl $0x1,-0x4(%ebp)          # add 1 to i (i+=1)
0x080484fa <+77>: mov  -0x4(%ebp),%eax          # copy i to %eax
0x080484fd <+80>: cmp  0xc(%ebp),%eax           # compare i with rows
0x08048500 <+83>: jl   0x80484c3 <sumMatrix+22> # if i<rows goto<sumMatrix+22>
0x08048502 <+85>: mov  -0xc(%ebp),%eax          # copy total to %eax
0x08048505 <+88>: leave                         # prepare to leave function
0x08048506 <+89>: ret                           # return total

Once again, the variables i, j, and total are at stack addresses %ebp-4, %ebp-8, and %ebp-12 respectively. The input parameters m, row, and cols are located at stack addresses %ebp+8, %ebp+12, and %ebp+16 respectively.

Let’s zoom in on the section that deals specifically with an access to element (i,j), or matrix[i][j]:

0x080484cc <+31>: mov -0x4(%ebp),%eax      # copy i to %eax
0x080484cf <+34>: lea 0x0(,%eax,4),%edx    # multiply i by 4, place in %edx
0x080484d6 <+41>: mov 0x8(%ebp),%eax       # copy matrix to %eax
0x080484d9 <+44>: add %edx,%eax            # add i*4 to matrix, place in %eax
0x080484db <+46>: mov (%eax),%eax          # copy matrix[i] to %eax

The five instructions between <sumMatrix+31> and <sumMatrix+46> compute matrix[i], or *(matrix+i). Note that the compiler needs to multiply i by four prior to adding it to matrix to calculate the correct offset (recall that pointers are four bytes in size). The instruction at <sumMatrix+46> then dereferences the calculated address to get the element matrix[i].

Since matrix is an array of int pointers, the element located at matrix[i] is itself an int pointer. The jth element in matrix[i] is located at offset j×4 in the matrix[i] array.


The next set of instructions extract the jth element in array matrix[i]:

0x080484dd <+48>: mov -0x8(%ebp),%edx    # copy j to %edx
0x080484e0 <+51>: shl $0x2,%edx          # multiply j by 4, place in %edx
0x080484e3 <+54>: add %edx,%eax          # add j*4 to matrix[i], place in %eax
0x080484e5 <+56>: mov (%eax),%eax        # copy matrix[i][j] to %eax
0x080484e7 <+58>: add %eax,-0xc(%ebp)    # add matrix[i][j] to total

The first instruction in this snippet loads variable j into register %edx. The compiler uses the left shift (shl) instruction to multiply j by four and stores the result in register %edx. The compiler then adds the resulting value to the address located in matrix[i] to get the address of matrix[i][j].

Let’s revisit Figure 4 and consider an example access to M2[1][2]. For convenience, the figure is reproduced below.

matrixDynamic
Figure 5. Matrix M2’s noncontiguous layout in memory

Note that M2 starts at memory location x0. The compiler first computes the address of M2[1] by multiplying 1 by 4 (sizeof(int *)) and adding it to the address of M2 (x0), yielding the new address x4. A dereference of this address yields the address associated with M2[1], or x36. The compiler then multiplies index 2 by 4 (sizeof(int)), and adds the result (8) to x36, yielding a final address of x44. The address x44 is dereferenced, yielding the value 5. Sure enough, the element in Figure 5 that corresponds to M2[1][2] has the value 5.