__builtin_frame_address vs %esp in pt-machine.h

[Roland McGrath]

> The problem is the ptlongjmp.c code.  For its use of CURRENT_STACK_FRAME
> the value must be as accurate as possible since otherwise the detection
> of frames which have to be handled might be wrong.

I don't understand this.  Its use is to check whether another frame's auto
variable address might be below (off the top) of the current stack.
Another frame's variable could never be between the cleanup function's %ebp
and %esp--that's its own frame.  So using %esp is fine there.