@ The stack switching logic for the arm thumb instruction set.
@ Written by Richard Tew, 2nd May 2006.
@
@ It is not possible to do this as inline assembler as gcc generates functions
@ where the stack register is preserved, so any changes we make to it will be
@ discarded.  However, any stack copying we have done is left in place, which
@ results in corrupt state.
@
@ This adapts the MASM approach written by Kristjan Jonsson, where early
@ returns from SLP_SAVE_STATE (within slp_save_state) are flagged by setting
@ the low bit of the return value.  This either indicates that there was an
@ error (-1) or that we do not need to restore the stack (1) as we are only
@ here to save the current one.
@
@ I know little arm thumb assembly, so in case anyone else wants to know the
@ approach I took, I will document it here.  The first uncertainty was which
@ registers should be saved.  I took this section of code from the assembler
@ generated by gcc with a command line of:
@     gcc -mthumb -S slp_transfer.c
@ where slp_transfer included the inline version of slp_switch for the arm
@ thumb.  Another initial uncertainty were the instructions needed to:
@     a) obtain the stack pointer.    (mov r0, sp)
@     b) restore the stack pointer.   (add sp, sp, r0)
@ which were used in the original inlined assembly.  The rest of the code was
@ just patched together based on Kristjan's existing masm source code.

@ (int) slp_switch (void);
	.thumb
	.align	2
	.global	slp_switch
	.type	slp_switch, %function		@ Apparently useful for .map files.
	.thumb_func

slp_switch:
	push	{r4, r5, r6, r7, lr}
	mov	r7, fp
	mov	r6, sl
	mov	r5, r9
	mov	r4, r8
	push	{r4, r5, r6, r7}
 
	mov	r0, sp
	bl	slp_save_state			@ diff = slp_save_state([r0]stackref)

	mov	r1, #0				@ r1 = -1
	mvn	r1, r1
	
	cmp	r0, r1				@ if (diff == -1)
	beq	.exit				@     return -1;

	cmp	r0, #1				@ if (diff ==  1)
	beq	.no_restore			@     return  0;

.restore:
	add	sp, sp, r0			@ Adjust the stack pointer for the state we are restoring.

	bl	slp_restore_state		@ slp_restore_state()

.no_restore:
	mov	r0, #0				@ Switch successful (whether we restored or not).

.exit:
	pop	{r2, r3, r4, r5}
	mov	r8, r2
	mov	r9, r3
	mov	sl, r4
	mov	fp, r5
	pop	{r4, r5, r6, r7, pc}

	.size	slp_switch, .-slp_switch	@ Apparently useful for .map files.