Re: [PATCH] s390: Add -fsplit-stack support

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Marcin KoÅ?cielnicki wrote:

> Fair enough.  Here's what I'm going to implement in gold:
> 
> - any PLT relocation: call
> - PC32DBL on a larl: non-call
> - PC32DBL otherwise: call
> - any other relocation: non-call
> 
> Does that sound right?

Hmm, I'm wondering about the PC32DBL choices.  There are now
a large number of other non-call instructions that use PC32DBL,
including lrl, strl, crl, cgrl, cgfrl, ...

However, these all access *data* at the pointed-to location,
so it is quite unlikely they would ever be used with a
function symbol.  So, assuming that you also check that the
target of the relocation is a function symbol, treating only
larl as non-call might be OK.

Maybe a more conservative approach might be to  make the decision
the other way round: for PC32DBL check for *branch* instructions,
and treat only those are calls.  There's just a few branch
instruction using PC32DBL:

brasl  (call)
brcl   (conditional or unconditional sibcall)
brcth  (???)

where the last one is extremely unlikely (but theorically
possible) to be used as conditional sibcall combined with
a register decrement; I don't think this can ever happen
with current compilers however.

For full completeness, there are also PC16DBL relocations that
*could* target called functions, but only when compiling with
the -msmall-exec flag to assume total executable size is less
than 64 KB.  These are used by the following instructions:

bras
brc
brct
brctg
brxh
brxhg
brxle
brxlg
crj
cgrj
clrj
clgrj
cij
cgij
clij
clgij

Note that those are *all* branch instructions, so it might
make sense to add any PC16DBL targetting a function symbol
to the list of calls, just in case.  (But since basically
nobody ever uses -msmall-exec, it doesn't really matter
much either.)

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com