var-tracking and s390's LM(G)

[Ulrich Weigand]

Jakub Jelinek wrote:
> On Wed, Feb 05, 2014 at 10:26:16PM +0100, Ulrich Weigand wrote:
> > Actually, now I think the problem originally described there is still
> > valid: on s390 the CFA is *not* equal to the value at function entry,
> > but biased by 96/160 bytes.  So setting the SP to the CFA is wrong ...
> 
> Such biasing happens on most of the targets, e.g. x86_64 has upon function
> entry CFA set to %rsp + 8, i?86 to %esp + 4.

Ah, but that's a different bias.  In those cases it is still correct
to *unwind* SP to the CFA, since that's the value SP needs to have
back in the caller immediately after the call site.

On s390, the call/return instructions do not modify the SP so the
SP at function entry is equal to the SP in the caller after return,
but neither of these is equal to the CFA.  Instead, SP points to
96/160 bytes below the CFA ...   Therefore you cannot simply
unwind SP to the CFA.

Bye,
Ulrich

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