register scavenger

I have a question about register scavenger.

I am considering using register scavenger for MIPS to free up register AT which is currently reserved to load large immediates.

All targets which currently use register scavenger to search for a scratch register (ARM, CellSPU, PowerPC and XCore)
override function processFunctionBeforeCalleeSavedScan and call RegisterScavenger::setScavengingFrameIndex
if they decide an emergency spill slot is needed. Since processFunctionBeforeCalleeSavedScan is called before it is known
which callee-saved registers need to be spilled, the targets decide whether the spill slot is needed without knowing the exact
stack frame size.

It seems to me that it would be better to wait until calculateFrameObjectOffsets computes the exact stack size
to make the decision and redo the stack size calculation only if the spill slot is needed.

bool PEI::runOnMachineFunction(MachineFunction &Fn) {

TFI->processFunctionBeforeCalleeSavedScan(Fn, RS);

calculateCalleeSavedRegisters(Fn);

calculateFrameObjectOffsets(Fn);

if (target needs spill slot) {
call setScavengingFrameIndex and reserve spill slot
calculateFrameObjectOffsets(Fn); // redo calculation.
}

This seems to be simpler and more efficient since you don’t need to write or run a function like ARM’s estimateStackSize to estimate the stack size.

Does this sound reasonable or is there something I am missing?