Puzzle about macro MIPS_PROLOGUE_TEMP_REGNUM

["Amker.Cheng" <amker dot cheng at gmail dot com>]

Hi :
   I found the temp register used for saving registers when expanding
prologue is defined by
macro MIPS_PROLOGUE_TEMP_REGNUM on mips target, like:

#define MIPS_PROLOGUE_TEMP_REGNUM \
  (cfun->machine->interrupt_handler_p ? K0_REG_NUM : GP_REG_FIRST + 3)

I don't understand why using registers starting from $3?
in my application, I have to save DFmode fpu regs through gpr regs,
that is $3,$4 in this case,
just like :
    mfc1  $3,  $fpr
    sw     $3,  addr
    mfc1  $4,  $fpr+1
    sw     $4,  addr+4

apparently this would crush the argument in $4.

Here is question,
why don't use $8 for MIPS_PROLOGUE_TEMP_REGNUM like EPILOGUE_TEMP?
Or have I done something wrong?

So, any clarification? Thanks in advance.
-- 
Best Regards.