Re: [patch] Pushing Inferior Function Arguments onto Stack on PowerPC64 machines

[Daniel Jacobowitz <drow at false dot org>]

On Wed, Oct 04, 2006 at 02:44:56PM -0500, Janani Janakiraman wrote:
> 
> Sorry about the email address goofup. Was trying to use a new account so
> that HTML tags don't get in the way.
> Have to figure that out later.
> 
> From reading the PPC64 Platform ABI  at
> http://www.freestandards.org/spec/ELF/ppc64/PPC-elf64abi-1.9.html
> ( see section 3.1.7)  it appears that on Big Endian machines,
> values are  left aligned. But looks like GCC has different rules and right
> aligns the values which are put on the
> registers. This matches  Andrew Cagney  comment in the code, that says that
> ABI specifies that the values
> should be left aligned. But like I said in my earlier note, GCC appears to
> want the values to be
> right aligned.  Would appreciate another set of eyes looking at it as I
> don't have too much experience
> in this area.

I'm going to CC a couple of people who are hopefully much more familiar
with PowerPC64 ABI issues than I am.  Alan, David, could one of you
take a quick look at this?

The problematic code is in gdb/ppc-sysv-tdep.c, in the function
ppc64_sysv_abi_push_dummy_call.  It has to do with the passing of
aggregates with non-word-sized tails.

> Daniel Jacobowitz <drow@false.org> wrote on 10/04/2006 02:10:56 PM:
> 
> > On Wed, Oct 04, 2006 at 03:00:34PM -0400, janani@linux.ibm.com wrote:
> > > I am new to this, so my interpretation might not be completely
> > > accurate, but the way I read the note below ( a snippet from the GNU
> > > GCC Manual about passing function arguments in registers) is that
> > > since PPC64 is big endian, even though the default is to pad downward
> > > (i.e. right align), if the size if greater than the size of an int,
> > > you need to pad upward (left align).
> >
> > You're trying to answer the wrong question :-)
> >
> > It's not "what does GCC do", but "what does the platform ABI say we
> > should do".  Is GCC conforming to the ABI?  Is the ABI wrong, or out of
> > date, or was Andrew's reading of it wrong, or...
> >
> > There could be a real problem here, so it's important that we
> > understand what is _supposed_ to happen before we make a change.
> > If GCC is violating the ABI, then either GCC or the ABI may need to be
> > updated.  If GDB is misinterpreting the ABI, then just GDB needs to be
> > changed.
> >
> > --
> > Daniel Jacobowitz
> > CodeSourcery
> 
> > !                            /* WARNING: cagney/2003-09-21: As best I can
> > !                               tell, the ABI specifies that the value
> should
> > !                               be left aligned.  Unfortunately, GCC
> doesn't
> > !                               do this - it instead right aligns even
> sized
> > !                               values and puts odd sized values on the
> > !                               stack.  Work around that by putting both
> a
> > !                               left and right aligned value into the
> > !                               register (hopefully no one notices :-^).
> > !                               Arrrgh!  */
> > !                            /* Left aligned (8 byte values such as
> pointers
> > !                               fill the buffer).  */
> > !                            memcpy (regval, val + byte, len);
> > !                            /* Right aligned (but only if even).  */
> > !                            if (len == 1 || len == 2 || len == 4)
> > !                                  memcpy (regval + tdep->wordsize - len,
> 
> Janani Janakiraman
-- 
Daniel Jacobowitz
CodeSourcery