Page 178 - ARM 64 Bit Assembly Language

Abstract data types 165

There are many other possibilities for re-ordering the list. Regardless of which method is
chosen, the main program and the interface (header file) need not be changed. Different im-
plementations of the sorting function can be substituted without affecting any other code,
because the implementation of the ADT is hidden from the code that uses it. This separation
of inteface and implementation provides many benefits.

The wl_print_numerical function can be implemented in assembly as shown in List-
ing 6.8. The function operates by re-ordering the linked list using an insertion sort as de-
scribed above. Listing 6.9 shows the change that must be made to the makefile. Now, when
make is run, it compiles the two C files and the assembly file into object files, then links them
all together. The C implementation of wl_print_numerical in list.c must be deleted
or commented out or the linker will emit an error indicating that it found two versions of
wl_print_numerical.

Listing 6.8 AArch64 assembly implementation of wl_print_numerical.

1 ### Definitions for the wordlistnode type
2 .equ wln_word, 0 // word pointer field
3 .equ wln_count, 8 // count field and padding
4 .equ wln_next, 16 // next pointer field
5 .equ wln_size, 24 // sizeof(wordlistnode)
6 ### Definitions for the wordlist type
7 .equ wl_nwords, 0 // number of words in list and padding
8 .equ wl_head, 8 // head of linked list
9 .equ wl_size, 16 // sizeof(wordlist)
10 ### Define NULL
11 .equ NULL,0
12
13 ### ----------------------------------------------------------
14 ### The sort_numerical function sorts the list of words in
15 ### reverse by number of occurrences, and returns a
16 ### pointer to the head of the re-ordered list.
17 ### Records with identical counts will maintain their
18 ### original ordering with respect to each other.
19 ### x0 holds head of source list (head)
20 ### x1 holds destination list (dest)
21 ### x2 holds pointer to node currently being moved (node)
22 ### x3 holds pointer to current node in destination list (curr)
23 ### x4 holds pointer to previous node in destination list (prev)
24 ### w5 holds count of current node in destination list
25 ### w6 holds count of node currently being moved
26 .type sort_numerical, %function
27 sort_numerical:
28 stp x29, x30, [sp, #-16]!
29 mov x1, #NULL // initialize new list to NULL
30 ## loop until source list is empty
31 loopa: cmp x0, #NULL