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