Converting my C++ Program to ARM Assembly - c++

Question

I have been assigned to convert this program to arm assembly v8.
int power(int x, int y){
if (x == 0){
return 0;
}
else if (y < 0){
return 0;
}
else if (y == 0){
return 1;
}
else {
return x * power(x, y - 1);
}
}
Although I'm not very familiar with ARM assembly language and would like to know where to start.
I have attempted to research a bit on this but ultimately found very little on the internet about ARM.

Answer 1

The magic command is arm-linux-gnueabi-gcc -S -O2 -march=armv8-a power.c.
I used arm-linux-gnueabi-gcc since I work on an X86-64 machine and gcc does not have ARM targets available. If you are on an arm system, you should be able to use regular gcc instead. If not it will error, but no harm done.
-S tells gcc to output assembly.
The -O2 is optional and just helps to optimize the code slightly and reduce debug clutter from the result.
-march=armv8-a tells it to use the ARM v8 target while compiling. I chose armv8-a somewhat arbitrarily. According to the docs all of the ARM v8 are armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv8-m.base, armv8-m.main, and armv8.1-m.main. I have no idea what the differences are so you may want to choose a different one.
power.c just tells it which file to compile. Since we don't specify an output file (Ex: -o output.asm), the assembly will be outputted to power.s.
If you are not compiling on an arm machine that has provides the desired target with regular gcc, you can use arm-linux-gnueabi-gcc instead. If you do not have it installed, you can install it with:
sudo apt-get update
sudo apt-get install gcc-arm-linux-gnueabi binutils-arm-linux-gnueabi
Output
If anyone is curious, this is the output I received when I tried it on my machine.
.arch armv8-a
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 2
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "testing.c"
.text
.align 2
.global power
.syntax unified
.arm
.fpu softvfp
.type power, %function
power:
# args = 0, pretend = 0, frame = 0
# frame_needed = 0, uses_anonymous_args = 0
# link register save eliminated.
clz r2, r0
mov r3, r0
lsr r2, r2, #5
orrs r2, r2, r1, lsr #31
bne .L4
cmp r1, #0
mov r0, #1
bxeq lr
.L3:
subs r1, r1, #1
mul r0, r3, r0
bne .L3
bx lr
.L4:
mov r0, #0
bx lr
.size power, .-power
.ident "GCC: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0"
.section .note.GNU-stack,"",%progbits
How can I get started.
Here is my thought process for how I was able to solve this problem. Looking at the problem I could tell it would probably be in two parts:
How can that code be compiled to assembly?
For the first part I found this answer which provided the command gcc -S -fverbose-asm -O2 foo.c. After testing it, I decided to remove the -fverbose-asm since only seemed to provide clutter for a program this small.
How can I set the compiler target to ARM v8?
After a quick google search I found that gcc lets you specify the target architecture with -march=xxx. My next step was to find a list of ARM architectures that I could select from. After finding gcc.gnu.org/onlinedocs/gcc/ARM-Options.html, I selected armv8-a since it sounded the most correct. When I tried it out, gcc told me that the target architecture could not be found. This was not really a surprise since I am on x86-64 and usually compilers come with the compatible targets to reduce the space required. I knew this likely meant I would need to identify the apt package which provided the arm targets so I searched around until I found this answer which filled in the rest of the information I needed.

Answer 2

Compiler explorer is the friend in a such simple cases.
ARMv8-a Clang Assembly with the compiler option -O1 for keeping the recursion:
# Compilation provided by Compiler Explorer at https://godbolt.org/
power(int, int): // #power(int, int)
stp x29, x30, [sp, #-32]! // 16-byte Folded Spill
str x19, [sp, #16] // 8-byte Folded Spill
mov x29, sp
mov w19, w0
mov w0, wzr
cbz w19, .LBB0_5
tbnz w1, #31, .LBB0_5
cbz w1, .LBB0_4
sub w1, w1, #1
mov w0, w19
bl power(int, int)
mul w0, w0, w19
b .LBB0_5
.LBB0_4:
mov w0, #1
.LBB0_5:
ldr x19, [sp, #16] // 8-byte Folded Reload
ldp x29, x30, [sp], #32 // 16-byte Folded Reload
ret
ARM GCC (linux) Assembly with the compiler option -O1 for keeping the recursion:
# Compilation provided by Compiler Explorer at https://godbolt.org/
power(int, int):
push {r4, lr}
mov r4, r0
clz r0, r0
lsrs r0, r0, #5
orrs r3, r0, r1, lsr #31
it ne
movne r0, #0
beq .L6
.L1:
pop {r4, pc}
.L6:
movs r0, #1
cmp r1, #0
beq .L1
subs r1, r1, #1
mov r0, r4
bl power(int, int)
mul r0, r4, r0
b .L1
ARM GCC (none) Assembly with the compiler option -O1 for keeping the recursion:
# Compilation provided by Compiler Explorer at https://godbolt.org/
power(int, int):
push {r4, lr}
mov r4, r0
rsbs r0, r0, #1
movcc r0, #0
orrs r3, r0, r1, lsr #31
movne r0, #0
beq .L6
.L1:
pop {r4, lr}
bx lr
.L6:
cmp r1, #0
moveq r0, #1
beq .L1
sub r1, r1, #1
mov r0, r4
bl power(int, int)
mul r0, r4, r0
b .L1
ARM64 GCC Assembly with the compiler option -O1 for keeping the recursion:
# Compilation provided by Compiler Explorer at https://godbolt.org/
power(int, int):
cmp w1, 0
ccmp w0, 0, 4, ge
bne .L9
mov w0, 0
ret
.L9:
stp x29, x30, [sp, -32]!
mov x29, sp
str x19, [sp, 16]
mov w19, w0
mov w0, 1
cbnz w1, .L10
.L1:
ldr x19, [sp, 16]
ldp x29, x30, [sp], 32
ret
.L10:
sub w1, w1, #1
mov w0, w19
bl power(int, int)
mul w0, w19, w0
b .L1