Unwinding CFI gcc practice of assumed `same value' regs

[Ulrich Drepper]

Andrew Haley wrote:
> Sure it does.  Not breaking things is an excellent reason, probably
> one of the the best reasons you can have.

Nothing breaks if the responsible tools are updated in unison.


> Really?  Well, that's one interpretation.  I don't believe that,
> though.  It's certainly an inconsistency in the specification, which
> says that null-termination is supported, and this implies that you
> can't put a zero in there.

Again, this is just because the "authors" of the ABI didn't think.  x86 
has the same problem.  ebp is freely used and not just for non-NULL 
values.  Register's a scarce and I doubt  you'll find any support 
introducing a register class which says that the register can only hold 
non-zero value.


> "All of these" might be the right way to go.  That is, keep
> null-terminating the stack, strengthen the rules about what you might
> do with %ebp, and extend debuginfo.

The thread setup and the startup code certainly does initialize the 
register with zero.  But this means nothing, the register can have zero 
values in all kinds of other places.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖