ARM assembly, take 3

After having considered some comments from Mr./Ms. G I optimized my assembly code a little bit.
Here a better approach:

Output
deadbeef

11011110101011011011111011101111

00000000111111111111111111111111

Code:

.equ STRING_LENGTH, 32
.equ ZERO_CHARACTER, 0x30
.equ ONE_CHARACTER, 0x31
 
.data
Mask:
	.ascii  "%x\n\n"
.align 4 
 
varToPrint:
	.long 0xdeadbeef
.balign 8 
 
varToUnspace:
	.long 0xdeadbeef
.balign 8 
 
unspacedVarToPrint:
	.long 0xdeadbeef
.balign 8 
 
stringToPrint:
	.ascii "                                                                                  \n\n"
len = STRING_LENGTH
.balign STRING_LENGTH
 
unspacedStringToPrint:
	.ascii "                                                                                  \n\n"
len = STRING_LENGTH
.balign STRING_LENGTH
 
.text
.globl main
main:
	stmfd	sp!, {r0, r1, r2, r3, r4, r5, r6, r7, lr}	@ save the registers we use to the stack
 
	ldr	r3, AddrVarToPrint
	ldr	r6, [r3]
	ldr	r3, AddrVarToPrint
	str	r6, [r3]
 
	/*
	Generating string of binary representation:
		Address of variable in r0
		Address of target string in r1
	*/
	ldr	r0, AddrVarToPrint
	ldr	r1, AddrStringToPrint
	bl make_binary_number
	/*------------------------------------------*/
 
	/*
	Removing zeroes between the ones in binary value:
		Address of variable in r0
		Address of target variable in r1
	*/
	ldr	r0, AddrVarToPrint
	ldr	r1, AddrUnspacedVarToPrint
	bl remove_spaces
 
	/*
	Generating string of binary representation:
		Address of variable in r0
		Address of target string in r1
	*/
	ldr	r0, AddrUnspacedVarToPrint
	ldr	r1, AddrUnspacedStringToPrint
	bl make_binary_number
	/*------------------------------------------*/
 
	bl print_out_result
 
	b exit
 
/*-Fetching the values and printing them-----------------------*/
print_out_result:
	push	{r0, r1, r2, r3, r4, lr}
 
	ldr     r3, AddrMask
	movs	r0, r3
	ldr	r3, AddrVarToPrint
	ldr	r1, [r3]
	bl	printf
 
	ldr     r3, AddrStringToPrint 
	movs	r0, r3
	bl	printf
 
	ldr     r3, AddrUnspacedStringToPrint
	movs	r0, r3
	bl	printf
 
	pop	{r0, r1, r2, r3, r4, pc}
 
/*-Generate a binary representation string from integer--------*/
/* Address of variable in r0 */
/* Address of target string in r1 */
make_binary_number:
	push	{r2, r3, r4, r5, r6, r7, lr}
	ldr	r3, [r0] /* integer value of variable */
 
	movs	r4, $0 /* loop counter */
bin_loop:
	movs    r2, $0x1
	lsls	r2, r4 /* r2 = (0x01 >> counter) */
 
	movs	r6, $STRING_LENGTH-1
	subs	r6, r6, r4 /* idx = reg_length-counter */
 
	tst	r3, r2
	beq	no_one
 
	movs	r5, $ONE_CHARACTER
	strb	r5, [r1,r6]
	b end_bin_loop
 
no_one:
	movs	r5, $ZERO_CHARACTER
	strb	r5, [r1,r6]
	b end_bin_loop
 
end_bin_loop:
	adds	r4, r4,$1
	cmp	r4, $STRING_LENGTH
	bne	bin_loop
 
	pop	{r2, r3, r4, r5, r6, r7, pc}
 
/*-Removes spaces between ones---------------------------------*/
/* Taken we have: 0xdeadbeef
	Binary representation: 11011110101011011011111011101111
	Then the ones will be shifted together so that we get
	the following result: 00000000111111111111111111111111
*/
/* Address of variable in r0 */
/* Address of target variable in r1 */
remove_spaces:
	push	{r2, r3, r4, r5, r6, r7, lr}
	ldr	r6, [r0]
 
	movs	r4, $0
	movs	r5, $0
	movs	r7, $0
 
unspace_loop:
	movs    r2, $0x1
 
	lsls	r2, r4
 
	tst	r6, r2
	beq	end_unspace_loop
 
	lsls	r5, r5, $1
	adds	r5, r5, $1
 
end_unspace_loop:
	adds	r4, r4,$1
	cmp	r4, $STRING_LENGTH
	bne	unspace_loop
 
	str	r5, [r1]
 
	pop	{r2, r3, r4, r5, r6, r7, pc}
/*-------------------------------------------------------------*/
 
/*--------EXIT main(void) function -> return 0-----------------*/
exit:
	ldmfd	sp!, {r0, r1, r2, r3, r4, r5, r6, r7, pc}	@ restore registers before exit
	movs	r7, $1                  @ set r7 to 1 - the syscall for exit
	swi	0                       @ then invoke the syscall from linux
 
 
/*-----Variable pointers---------------------------------------*/
AddrMask:
	.word	Mask
 
AddrVarToPrint:
	.long	varToPrint
 
AddrVarToUnspace:
	.long	varToUnspace
 
AddrStringToPrint:
	.word stringToPrint
 
AddrUnspacedStringToPrint:
	.word unspacedStringToPrint
 
AddrUnspacedVarToPrint:
	.long unspacedVarToPrint

6 thoughts on “ARM assembly, take 3”

  1. Well, I ended up cheating and looking up what gcc -O3 would do with it. 🙂

    ~/arm-asm $ cat stripzero.c
    int stripzero(int x) {
    int count = 0;
    while (x) {
    count += x & 1;
    x >>= 1;
    }
    return (1 <>= 1
    bne _continue_counting

    // Huh! Instead of return (1 << count) - 1,
    // which I put into the C code,
    // gcc does return ~(~0 << count).
    // Mad optimizations.
    mvn r0, #0 // r0 := ~0
    mvn r0, r0, asl r3 // r0 := ~(r0 << r3)
    bx lr

  2. This time with code font...

    ~/arm-asm $ cat stripzero.c

    int stripzero(int x) {
    int count = 0;
    while (x) {
    count += x & 1;
    x >>= 1;
    }
    return (1 << count) - 1;
    }

    ~/arm-asm $ cc -S -o stripzero.S -O3 stripzero.c

    stripzero:

    // Count bits in r0, store in r3.
    // Destroys r0 along the way.
    mov r3, #0 // r3 := 0
    _continue_counting:
    and r2, r0, #1 // r2 := r0 & 1
    add r3, r3, r2 // r3 += r2
    movs r0, r0, asr #1 // r0 >>= 1
    bne _continue_counting

    // Huh! Instead of return (1 << count) - 1,
    // which I put into the C code,
    // gcc does return ~(~0 << count).
    // Mad optimizations.
    mvn r0, #0 // r0 := ~0
    mvn r0, r0, asl r3 // r0 := ~(r0 << r3)
    bx lr

    1. I think all ALU instructions can mangle their last argument with a shift like that. It's part of the binary Instruction Format, which is the same for a broad range oft data processing instructions. Check this out: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0489f/Chddgcje.html

      It's the section "operand2 as register with optional shift".

      So if your teaching dialect of ARM doesn't support it, it's giving away a few bits per instruction for free 🙂

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