Re: [RFA] mips_push_arguments(): Make O64 ABI test explicit

[Daniel Jacobowitz <drow at mvista dot com>]

On Wed, Jul 31, 2002 at 06:30:04PM -0700, Kevin Buettner wrote:
> On Jul 31,  6:10pm, Kevin Buettner wrote:
> 
> > So, it seems that cc wants small structs shifted, but that gcc does not.
> > 
> > Time for me to dig into the ABI documents to find out which compiler's
> > right...
> 
> It looks to me like the SGI (cc) compiler is right.  The text below was
> obtained from:
> 
> http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi/srch3@n64/0650/bks/SGI_Developer/books/Mpro_n32_ABI/sgi_html/ch02.html
> 
> 
>     * Structs, unions, or other composite types are treated as a
>       sequence of doublewords, and are passed in integer or floating
>       point registers as though they were simple scalar parameters to
>       the extent that they fit, with any excess on the stack packed
>       according to the normal memory layout of the object.  More
>       specifically:
> 
> 	- Regardless of the struct field structure, it is treated as a
> 	  sequence of 64-bit chunks.  If a chunk consists solely of a
> 	  double float field (but not a double, which is part of a
> 	  union), it is passed in a floating point register.  Any
> 	  other chunk is passed in an integer register.
> 
> 	- A union, either as the parameter itself or as a struct
> 	  parameter field, is treated as a sequence of integer
> 	  doublewords for purposes of assignment to integer parameter
> 	  registers.  No attempt is made to identify floating point
> 	  components for passing in floating point registers.
> 
> 	- Array fields of structs are passed like unions.  Array
> 	  parameters are passed by reference (unless the relevant
> 	  language standard requires otherwise).
> 
> 	- Right-justifying small scalar parameters in their save area
> 	  slots notwithstanding, struct parameters are always
> 	  left-justified.  This applies both to the case of a struct
> 	  smaller than 64 bits, and to the final chunk of a struct
> 	  which is not an integral multiple of 64 bits in size.  The
> 	  implication of this rule is that the address of the first
> 	  chunk's save area slot is the address of the struct, and the
> 	  struct is laid out in the save area memory exactly as if it
> 	  were allocated normally (once any part in registers has been
> 	  stored to the save area).  [These rules are analogous to the
> 	  o32-bit ABI treatment - only the chunk size and the ability
> 	  to pass double fields in floating point registers are
> 	  different.]
> 
> So, if I understand this correctly, gdb ought to be left shifting small
> struct parameters for the o32, n32, and n64 ABIs.  I.e, I think my patch
> was correct, though I do agree with Michael that it might be better to
> list the ABIs that we want to do this struct shifting for instead of
> trying to list the ones that we don't want to shift.

GCC already has at least one known bug in this area on O32.  I am
inclined to go with SGI CC's interpretation; you should complain at
Eric some more to get GCC fixed properly.  Lord knows I haven't had
much luck :P

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer