__builtin_frame_address vs %esp in pt-machine.h

[Roland McGrath]

> Roland McGrath wrote:
> > 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.
> 
> I've no time to think about the reason I had to make the change in the
> moment.  But I added this code to fix a bug.  It is a necessary change.

Bah!  I searched all the logs and mailing list archives I could find and
didn't see anything that would have explained the problem, like a related
bug report around the time of the change.  This is why you write comments
in the code when there are nonobvious reasons for how it is!  

I still cannot imagine any reason why %esp would not be right for
ptlongjmp.c's use (and any other).