Re: Unwinding CFI gcc practice of assumed `same value' regs

[Andrew Haley <aph@redhat.com>]

Ian Lance Taylor writes:
 > Andrew Haley <aph@redhat.com> writes:
 > 
 > > In practice, %ebp either points to a call frame -- not necessarily the
 > > most recent one -- or is null.  I don't think that having an optional
 > > frame pointer mees you can use %ebp for anything random at all, but we
 > > need to make a clarification request of the ABI.
 > 
 > I don't see that as feasible.  If %ebp/%rbp may be used as a general
 > callee-saved register, then it can hold any value.

Sure, we already know that, as has been clear.  The question is *if*
%rbp may be used as a general callee-saved register that can hold any
value.

 > And permitting %ebp/%rbp to hold any value is a very useful
 > optimization in a function which does not require a frame pointer,
 > since it gives the compiler an extra register to use.
 > 
 > If you want to require %ebp/%rbp to hold a non-zero value, then you
 > are effectively saying that this optimization is forbidden.  There is
 > no meaningful way to tell gcc "this is a general register, but you may
 > not store zero in it."  It would be a poor tradeoff to forbid that
 > optimization in order to provide better support for exception
 > handling: exception handling is supposed to be unusual.

Sure, that's reasonable: it's a good reason to suggest that the ABI
spec (still in DRAFT state, I note!) might be changed.

Andrew.