Use of getOffsetOfLocalArea on x86?

What is the space between the incoming stack pointer and (incoming
stack pointer + getOffsetOfLocalArea) used for? getOffsetOfLocalArea
is -$(WordSize) on x86.

I was under the impression that it was used to store %rbp, but it
seems %rbp is independently assigned a fixed frame index in
X86FrameLowering::processFunctionBeforeCalleeSavedScan, and assigned its
own slot in PEI::calculateFrameObjectOffsets.

-- Sanjoy