Is r29 treated specially / ignored by gccvb? (Bug?)

Does gccvb intentionally ignore the use of r29 when determining how to assign registers? I’m calling the following function inside a loop and gccvb assigns r29 to the loop’s counter in spite of my explicitly assigning r29 in the broken case, and explicitly flagging r29 as clobbered in the latter working case:

#define WHY_DOES_THIS_CRASH
#ifdef WHY_DOES_THIS_CRASH
/* This doesn't work */
void CopyBits(u32 destBitOffset, u32 srcBitOffset, u32 stringLength, u32* dest, u32* src)
{
	register u32 destBitOffsetReg asm ("r26") = destBitOffset;
	register u32 srcBitOffsetReg asm ("r27") = srcBitOffset;
	register u32 stringLengthReg asm ("r28") = stringLength;
	register u32* destReg asm ("r29") = dest;
	register u32* srcReg asm ("r30") = src;
	asm("
		movbsu
		"
		: /* output */
		: "r" (destBitOffsetReg), "r" (srcBitOffsetReg), "r" (stringLengthReg), "r" (destReg), "r" (srcReg) /* input */
		);
}
#else
/* This works */
void CopyBits(u32 destBitOffset, u32 srcBitOffset, u32 stringLength, u32* dest, u32* src)
{
	register u32 destBitOffsetReg asm ("r26") = destBitOffset;
	register u32 srcBitOffsetReg asm ("r27") = srcBitOffset;
	register u32 stringLengthReg asm ("r28") = stringLength;
	register u32* destReg = dest;
	register u32* srcReg asm ("r30") = src;
	asm("
		mov r29,r1
		mov %3,r29
		movbsu
		mov r1,r29
		"
		: /* output */
		: "r" (destBitOffsetReg), "r" (srcBitOffsetReg), "r" (stringLengthReg), "r" (destReg), "r" (srcReg) /* input */
		: "r1", "r29" /* clobbered */
		);
}
#endif

I’ve noticed that DanB’s Hunter engine follows this pattern as well– backing up r29 before executing movbsu. My question is: why is this necessary? Is this a bug in gccvb? I read through NEC’s official documentation for both the v810 and the v830 and with the exception of the bitstring instructions, neither designates r29 as a first-class citizen. I’m kinda stumped and would prefer the compiler to handle register assignments as it tends to be better at that than I am, especially when inlining.