On Thu, 19 Nov 2009, Frederic Weisbecker wrote:
>
> > That way the lr would have the current function, and the parent would
> > still be at 8(%sp)
>
> Yeah right, we need at least such very tiny prologue for
> archs that store return addresses in a reg.
Well, it will be architecture-dependent.
For example, alpha can store the return value in _any_ register if I
recall correctly, so you can do the "call to __fentry__" by just picking
another register than the default one as the return address.
And powerpc has two special registers: link and ctr, but iirc you can only
load 'link' with a branch instruction. Which means that you could do
something like
mflr 0
bl __fentry__
in the caller (I forget if R0 is actually a call-trashed register or not),
and then __fentry__ could do something like
mflr 12 # save _new_ link
mtlr 0 # restore original link
mtctr 12 # move __fentry__ link to ctr
.. do whatever ..
bctr # return to __fentry__ caller
to return with 'link' restored (but ctr and r0/r12 trashed - I don't
recall the ppc calling conventions any more, but I think that is ok).
Saving to stack seems unnecessary and pointless.
Linus
