except in some rare cases, xor swapping is probably silly

gistfile1.txt

eric@titan:~/src/code-snips/c$ cat swap-em.c 
#include <stdio.h>
#include <stdint.h>

/* this is the way to swap pointers */
void standard_swap(char **left, char **right)
{
	char *tmp;

	tmp = *left;
	*left = *right;
	*right = tmp;
}

/*
 * The xor swapping of pointers is doable, but dumb.
 * Typically bitwise operations do not work on pointers in C.
 * Casting to intptr_t is a work-around, but ugly.
 * Even so, the result does not look a lot more opimized to me.
 * $ gcc -S -O3 swap-em.c
 * $ cat swap-em.s
 */
void xor_swap(char **left, char **right)
{
	*left = (char*) (((intptr_t)*left) ^ ((intptr_t)*right));
	*right = (char*) (((intptr_t)*left) ^ ((intptr_t)*right));
	*left = (char*) (((intptr_t)*left) ^ ((intptr_t)*right));
}

int main(int argc, char **argv)
{
	char *a, *b;
	char *o, *t;

	o = "one";
	t = "two";

	a = o;
	b = t;

	printf("initial state: %s, %s\n", a, b);
	standard_swap(&a, &b);
	printf("standard swap: %s, %s\n", a, b);
	xor_swap(&a, &b);
	printf("xor swap back: %s, %s\n", a, b);

	return 0;
}


From x86:

eric@titan:~/src/code-snips/c$ gcc -S -O3 swap-em.c
eric@titan:~/src/code-snips/c$ cat swap-em.s 
	.file	"swap-em.c"
	.text
	.p2align 4,,15
	.globl	standard_swap
	.type	standard_swap, @function
standard_swap:
.LFB24:
	.cfi_startproc
	pushl	%ebx
	.cfi_def_cfa_offset 8
	.cfi_offset 3, -8
	movl	8(%esp), %edx
	movl	12(%esp), %eax
	movl	(%edx), %ecx
	movl	(%eax), %ebx
	movl	%ebx, (%edx)
	movl	%ecx, (%eax)
	popl	%ebx
	.cfi_restore 3
	.cfi_def_cfa_offset 4
	ret
	.cfi_endproc
.LFE24:
	.size	standard_swap, .-standard_swap
	.p2align 4,,15
	.globl	xor_swap
	.type	xor_swap, @function
xor_swap:
.LFB25:
	.cfi_startproc
	movl	4(%esp), %eax
	movl	8(%esp), %ecx
	movl	(%eax), %edx
	xorl	(%ecx), %edx
	movl	%edx, (%eax)
	xorl	(%ecx), %edx
	movl	%edx, (%ecx)
	xorl	%edx, (%eax)
	ret
	.cfi_endproc
.LFE25:
	.size	xor_swap, .-xor_swap
	.section	.rodata.str1.1,"aMS",@progbits,1
.LC0:
	.string	"two"
.LC1:
	.string	"one"
.LC2:
	.string	"initial state: %s, %s\n"
.LC3:
	.string	"standard swap: %s, %s\n"
.LC4:
	.string	"xor swap back: %s, %s\n"
	.section	.text.startup,"ax",@progbits
	.p2align 4,,15
	.globl	main
	.type	main, @function
main:
.LFB26:
	.cfi_startproc
	pushl	%ebp
	.cfi_def_cfa_offset 8
	.cfi_offset 5, -8
	movl	%esp, %ebp
	.cfi_def_cfa_register 5
	andl	$-16, %esp
	subl	$16, %esp
	movl	$.LC0, 12(%esp)
	movl	$.LC1, 8(%esp)
	movl	$.LC2, 4(%esp)
	movl	$1, (%esp)
	call	__printf_chk
	movl	$.LC1, 12(%esp)
	movl	$.LC0, 8(%esp)
	movl	$.LC3, 4(%esp)
	movl	$1, (%esp)
	call	__printf_chk
	movl	$.LC0, 12(%esp)
	movl	$.LC1, 8(%esp)
	movl	$.LC4, 4(%esp)
	movl	$1, (%esp)
	call	__printf_chk
	xorl	%eax, %eax
	leave
	.cfi_restore 5
	.cfi_def_cfa 4, 4
	ret
	.cfi_endproc
.LFE26:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4"
	.section	.note.GNU-stack,"",@progbits
eric@titan:~/src/code-snips/c$ 


From ARM:

eric@localhost:~/src/code-snips/c$ gcc -S -O3 swap-em.c 
eric@localhost:~/src/code-snips/c$ cat swap-em.s 
	.arch armv7-a
	.eabi_attribute 28, 1
	.fpu vfpv3-d16
	.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	"swap-em.c"
	.text
	.align	2
	.global	standard_swap
	.syntax unified
	.thumb
	.thumb_func
	.type	standard_swap, %function
standard_swap:
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	@ link register save eliminated.
	ldr	r3, [r0]
	ldr	r2, [r1]
	str	r2, [r0]
	str	r3, [r1]
	bx	lr
	.size	standard_swap, .-standard_swap
	.align	2
	.global	xor_swap
	.syntax unified
	.thumb
	.thumb_func
	.type	xor_swap, %function
xor_swap:
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	@ link register save eliminated.
	ldr	r2, [r0]
	ldr	r3, [r1]
	eors	r3, r3, r2
	str	r3, [r0]
	ldr	r2, [r1]
	eors	r3, r3, r2
	str	r3, [r1]
	ldr	r2, [r0]
	eors	r3, r3, r2
	str	r3, [r0]
	bx	lr
	.size	xor_swap, .-xor_swap
	.section	.text.startup,"ax",%progbits
	.align	2
	.global	main
	.syntax unified
	.thumb
	.thumb_func
	.type	main, %function
main:
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	push	{r3, r4, r5, lr}
	movw	r5, #:lower16:.LC0
	movw	r4, #:lower16:.LC1
	movt	r5, #:upper16:.LC0
	movt	r4, #:upper16:.LC1
	movw	r1, #:lower16:.LC2
	mov	r3, r5
	movt	r1, #:upper16:.LC2
	mov	r2, r4
	movs	r0, #1
	bl	__printf_chk
	mov	r3, r4
	mov	r2, r5
	movw	r1, #:lower16:.LC3
	movs	r0, #1
	movt	r1, #:upper16:.LC3
	bl	__printf_chk
	mov	r3, r5
	mov	r2, r4
	movw	r1, #:lower16:.LC4
	movs	r0, #1
	movt	r1, #:upper16:.LC4
	bl	__printf_chk
	movs	r0, #0
	pop	{r3, r4, r5, pc}
	.size	main, .-main
	.section	.rodata.str1.4,"aMS",%progbits,1
	.align	2
.LC0:
	.ascii	"two\000"
.LC1:
	.ascii	"one\000"
.LC2:
	.ascii	"initial state: %s, %s\012\000"
	.space	1
.LC3:
	.ascii	"standard swap: %s, %s\012\000"
	.space	1
.LC4:
	.ascii	"xor swap back: %s, %s\012\000"
	.ident	"GCC: (Ubuntu/Linaro 5.4.0-3ubuntu1~12.04) 5.4.0 20160603"
	.section	.note.GNU-stack,"",%progbits
eric@localhost:~/src/code-snips/c$ cat /proc/cpuinfo