Re: [PATCH 0/4] s390 improvements

["Ulrich Weigand" <uweigand@de.ibm.com>]

Richard Henderson wrote:

> This is relative to Dominik's patch from the 16th.  The complete
> tree can be found at
> 
> 	git://github.com/rth7680/libffi.git s390
> 
> Mostly relevant is patch 3, which converts the s390 port to the
> more modern arrangement where there's no callback into ffi_prep_args.

This is a bit confusing to me.  The assembler routine now does:

        lg      %r15,120(%r2)                   # Set up outgoing stack

without ever restoring the initial stack pointer before returning
to its caller.  This probably works right now since the value loaded
here is determined like that:

  /* Pass the outgoing stack frame in the r15 save slot.  */
  frame->gpr_save[8] = (unsigned long)(stack - sizeof(struct call_frame));

and since "stack" was allocated via alloca and ffi_call_int does not
require an argument save area when calling any of its subroutines,
the stack pointer value computed here should always in fact be
identical to the value %r15 already has at the above location.

Using the 160 bytes below "stack" as register save area for use of the
target function called by ffi_call_SYSV is also only safe if those bytes
are in fact the register save area ffi_call_int provides for its caller,
e.g. again if the value is already identical to %r15.  (If this were
any other value, we might clobber parts of ffi_call_int's stack frame
that it conceivably might still access.)

However, if the procedure only works if the "lg" is a nop, why is it
even done?   Also, the whole setup seems a bit fragile since changes
to ffi_call_int might cause it to need an argument save area ...

Bye,
Ulrich

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