[0] Perl> sub steps { my($i,$m,$o) = (0,@_); print( "$i: $m $o" ), ++$i,( $m >= $o ? ($m -= $o) : ($o -= $m )) while $m and $o; };;

[0] Perl> steps( 9, 6 );;
0: 9 6
1: 3 6
2: 3 3

[0] Perl> steps( 17, 9 );;
0: 17 9
1: 8 9
2: 8 1
3: 7 1
4: 6 1
5: 5 1
6: 4 1
7: 3 1
8: 2 1
9: 1 1

##</code><code>##

[0] Perl> sub steps { my($i,$m,$o) = (0,@_); ++$i,( $m >= $o ? ($m -= $o) : ($o -= $m )) while $m and $o; print $i; };;
Subroutine steps redefined at (eval 17) line 1, <STDIN> line 9.

[0] Perl> steps( 2**29, 2**28-1 );;
134217731

##</code><code>##

p$ = 80
size$ = 88
x$ = 96
y$ = 104
swaps$ = 112
_xchgBuffersR PROC

; 53   : U32 _xchgBuffersR( register U64 *p, const U32 size, register U32 x, register U32 y, U32 register swaps ) {

$LN19:
	mov	QWORD PTR [rsp+16], rbx
	mov	QWORD PTR [rsp+24], rbp
	mov	QWORD PTR [rsp+32], rsi
	push	rdi
	push	r12
	push	r13
	push	r14
	push	r15
	sub	rsp, 32					; 00000020H
	mov	rax, rcx
	movsxd	rbx, r9d
	movsxd	r15, r8d
	and	eax, 7
	mov	esi, edx
	mov	r13, rcx
	mov	r14, r15
	mov	rdi, rbx
	mov	QWORD PTR tv246[rsp], rax
	npad	8
$LL12@xchgBuffer@2:                                 ############## **HERE** (see below)

; 54   :     U32 register i;
; 55   :     U32 const savedY = y;

	mov	ebp, ebx
	mov	r12, rdi

; 56   : 
; 57   :     assert( y <= size );

	cmp	ebx, esi
	jbe	SHORT $LN8@xchgBuffer@2
	lea	rdx, OFFSET FLAT:$SG3784
	lea	rcx, OFFSET FLAT:$SG3785
	mov	r8d, 57					; 00000039H
	call	_wassert
	mov	rax, QWORD PTR tv246[rsp]
$LN8@xchgBuffer@2:

; 58   :     assert( ( (U64)p & 7ull ) == 0 );

	test	rax, rax
	je	SHORT $LN9@xchgBuffer@2
	lea	rdx, OFFSET FLAT:$SG3788
	lea	rcx, OFFSET FLAT:$SG3789
	mov	r8d, 58					; 0000003aH
	call	_wassert
	mov	rax, QWORD PTR tv246[rsp]
$LN9@xchgBuffer@2:

; 59   : 
; 60   :     if( y == size ) return swaps;

	cmp	ebx, esi
	je	SHORT $LN18@xchgBuffer@2              ############# recursion end condition jumps to **THERE**

; 61   : 
; 62   :     for( i = x; i < savedY; ++i ) {

	cmp	r15d, ebx
	jae	SHORT $LL12@xchgBuffer@2
	mov	eax, ebx
	sub	eax, r15d
	mov	edx, eax
	add	DWORD PTR swaps$[rsp], edx
	add	r15d, eax
$LL4@xchgBuffer@2:

; 63   :         if( y == size ) y = savedY;

	cmp	ebx, esi
	jne	SHORT $LN1@xchgBuffer@2
	mov	ebx, ebp
	mov	rdi, r12
$LN1@xchgBuffer@2:

; 64   :         swapElems( p, x++, y++ );

	mov	rax, QWORD PTR [r13+rdi*8]
	mov	rcx, QWORD PTR [r13+r14*8]
	inc	rdi
	mov	QWORD PTR [r13+r14*8], rax
	mov	QWORD PTR [r13+rdi*8-8], rcx
	inc	r14
	inc	ebx
	sub	rdx, 1
	jne	SHORT $LL4@xchgBuffer@2

; 65   :         ++swaps;
; 66   :     }
; 67   :     return _xchgBuffersR( p, size, x, y, swaps );

	mov	rax, QWORD PTR tv246[rsp]
	jmp	$LL12@xchgBuffer@2                    ############### and just before it, it unconditionally jumps to **HERE**
$LN18@xchgBuffer@2:                                   ############### **THERE**

; 59   : 
; 60   :     if( y == size ) return swaps;   

	mov	eax, DWORD PTR swaps$[rsp]

; 68   : }

	mov	rbx, QWORD PTR [rsp+88]
	mov	rbp, QWORD PTR [rsp+96]
	mov	rsi, QWORD PTR [rsp+104]
	add	rsp, 32					; 00000020H
	pop	r15
	pop	r14
	pop	r13
	pop	r12
	pop	rdi
	ret	0
_xchgBuffersR ENDP

##</code><code>##

C:\test\C>bufswap 536870912 268435455 2  ### 2^29 2^28-1
size:536870912 offset;268435455
[         0          1 ...  268435453    268435454 ^  268435455    268435456   ...  536870910  536870911 ]
[ 268435455  268435456 ...  536870910    536870911 ^          0            1   ...  268435453  268435454 ]
iterative: swaps:536870912 took 7.359985176 secs.

[         0          1 ...  268435453    268435454 ^  268435455    268435456   ...  536870910  536870911 ]
[ 268435455  268435456 ...  536870910    536870911 ^          0            1   ...  268435453  268435454 ]
recursive: swaps:536870911 took 3.762964774 secs.

[         0          1 ...  268435453    268435454 ^  268435455    268435456   ...  536870910  536870911 ]
[ 268435455  268435456 ...  536870910    536870911 ^          0            1   ...  268435453  268435454 ]
reversive: swaps:536870911 took 4.901475821 secs.