[cfarm-users] PowerPC 32 bit abi on gcc110.fsffrance.org

[Bruno Haible]

Hi Paulo,

>   Just testing for hopefully soon new release of GNU Lightning, I noticed
> now when building with -m32 it appears to have a different abi that I
> could not find any description searching on google; probably I did not
> search enough..

Generally you find this info in the GCC sources, more precisely in
gcc/config/rs6000/rs6000.[hc]. But it's easy to misunderstand these
macro definitions. To get a reliable understanding, I compile specific
C functions [1] with "gcc -S -O2" and look at the generated code.

On 32-bit PowerPC, there are essentially two ABIs: "AIX" and "SysV.4".

Argument passing conventions are as follows:

  All arguments, except the first 8 words, are passed on the stack with
  word alignment. Doubles take two words. The first 13 doubles and floats
  are also passed in floating-point-registers.
  To return a structure, the called function copies the value to space
  pointed to by its first argument, and all other arguments are shifted
  down by one.

  Differences between AIX and SysV.4 argument passing conventions:
  - AIX: the first 13 doubles and floats are passed in FP registers,
         and when they do, there is still room allocated for them in the
         argument sequence (integer regs or stack).
    SysV.4: the first 8 doubles and floats are passed in FP registers,
         and no room is allocated for them in the argument sequence.
  - AIX: Structures are passed in the argument sequence, possibly partially
         in registers and partially on the stack. But stack room is
         reserved for the parts that are passed in registers, so that it's
         not too expensive to take the address of such a struct in C.
    SysV.4: Structures are passed by reference: only a pointer appears in
         the argument sequence.
  - AIX: Long longs are only word aligned.
    SysV.4: Long longs are two-word aligned, both when passed in registers
         (pairs: 3/4, 5/6, 7/8, 9/10) and when passed on the stack. (Recall
         that the stack is always 8-byte aligned).

Linux uses the SysV.4 ABI. (Does anyone know of a non-Linux platform that
also uses the SysV.4 ABI?)

AIX and Mac OS X use the AIX ABI, with a difference in a detail:
Small structures (less than a word large) are passed left-adjusted by AIX xlc,
but right-adjusted by GCC on AIX and on Mac OS X.

All this does not take into account varargs calls. Note that for varargs calls,
different compilers have a different notion of what a "varargs call" is. On
HP-PA (I know it doesn't help you directly - just to highlight the potential
issue) for the HP compiler, a "varargs call" is a call done through a function
pointer or directly to a function declared with a varargs prototype; whereas
for GCC, a "varargs call" is when the function's type is a varargs function.
When in doubt, you can put the floating-point values into the general registers
and the floating-point registers (in the AIX ABI).

Bruno

[1] https://git.savannah.gnu.org/gitweb/?p=libffcall.git;a=blob;f=testcases.c