Re: "small data" in m68k gcc?

[Gunther Nikl <gni at gecko dot de>]

On Mon, Oct 13, 2003 at 01:13:55AM +0200, Bernardo Innocenti wrote:
> > I'm trying to move some code away from a proprietary compiler, but
> > the multitasker depends on having A5-relative global variables, and
> > I'm having a hard time figuring out how to generate them.
> 
> This has been quite common practice on the Amiga to generate residentable
> programs and reduce code and relocs size. Actually, the Amiga uses
> A4-relative addressing because A5 is used as a frame pointer and A6 is
> reserved for calling shared library functions through a jump table.

  Correct. The AmigaOS/m68k patches add two additional pic modes (word
  and long sized) and handle these modes at those places where they work
  differently to the normal pic/PIC modes. These mode do only affect data,
  not function calls.

> The Amiga people always enjoied A5-relative addressing with SAS/C
> (formerly known as Lattice C) and Storm C.

  I suppose that should read "A4-relative", shouldn't it?

> Of course smart A5 addressing requires some support in the startup
> code (loading A5 for the first time).

  There is an attribute for doing just that: __saveds__

> Programmers must be careful when mixing code compiled with small-data
> with code using A5 as a general purpose register.

  Yes, that would cause havoc.

> On the Amiga, you'd use the "__loadds" function attribute for callbacks
> and softints.

  __saveds

> > Even the -l option to gas, which is documented as changing the
> > default absolute addressing mode to .w (rather than .l), doesn't
> > seem to work.
> 
> That's not meant to be used to trick the compiler. It's just
> a compatibility option for assembly source written for other
> assemblers.

  The AmigaOS version of GAS 1.38 had a -l switch which forced the gas to
  emit 16bit references for calls to external symbols. 

> > attribute((pcrel)) -> var(%pc)

  For const data in the text section such code should be generated by GAS
  automatically.

  Gunther