Hi, On Tue, 12 Dec 2006, Andrew Haley wrote:
> > > 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. Yes of course it was meant to be used such. The ABI actually only gives a recommendation that %rbp should be zero in the outermost frame, it's not a must. The ABI _requires_ proper .eh_frame descriptors when unwinding is desired; so it's useless (and wrong) for any unwinder to look at %rbp and determine if it should stop. Alternatively (though not sanctioned by the ABI) all functions through which unwinding is desired but for which no unwind info is created _have_ to use %rbp as frame pointer and not as general register. In that case the zeroing of %rbp would be a usable stop condition for functions without unwind info. But that's already outside the ABI. Ciao, Michael.
