Page 160 - ARM 64 Bit Assembly Language

146 Chapter 5

Listing 5.38 Initializing an array of structured data in C.

1 #include <stdio.h>
2 #include <stdlib.h>
3
4 /* image size */
5 const int width = 100;
6 const int height = 100;
7
8 /* define structure for a pixel */
9 struct pixel {
10 unsigned char red;
11 unsigned char green;
12 unsigned char blue;
13 };
14
15 int main(void)
16 {
17 // allocate image
18 struct pixel *image = malloc(width * height * sizeof(struct pixel));
19 if (image == NULL) {
20 fprintf(stderr, "Error: Out of memory.\n");
21 return 1;
22 }
23
24 // initialize all pixels in the image to black
25 for (int j = 0; j < height; j++) {
26 for (int i = 0; i < width; i++) {
27 image[j*width+i].red = 0;
28 image[j*width+i].blue = 0;
29 image[j*width+i].green = 0;
30 }
31 }
32
33 // delete the image
34 free(image);
35
36 return 0;
37 }

to zero on each iteration of the loop, why not use a single store instruction to set sixteen bytes
to zero on each iteration? The only problem with this approach is that we must consider the
possibility that the array may end in the middle of the quad-word. However this can be dealt
with by using two consecutive loops. The first loop sets one quad-word of thearraytozeroon
each iteration, and the second loop finishes off any remaining bytes. Listing 5.41 shows the