Re: MIPS 32-bit ABI stack pointer

[Mason <mpeg dot blue at free dot fr>]

Mason wrote:

> I'm having problems mixing MIPS 32-bit object files compiled
> with two different compilers.
>
> 1) gcc version 4.4.1 (Sourcery G++ Lite 4.4-305)
> 2) Green Hills Software (exact version unknown)
>
> Before describing my problem, I'd like to ask a few questions.
>
> GCC supports several MIPS ABIs. Where are they documented?
> http://gcc.gnu.org/onlinedocs/gcc/MIPS-Options.html
> -mabi=32
> -mabi=o64
> -mabi=n32
> -mabi=64
> -mabi=eabi
>
> ( Links for self reference )
> http://gcc.gnu.org/gcc-3.4/mips-abi.html
> http://www.cygwin.com/ml/binutils/2003-06/msg00436.html
> http://math-atlas.sourceforge.net/devel/assembly/mipsabi32.pdf
>
> Next question, when a function starts, is the stack pointer
> (r29) supposed to point to
> A) the last live value in the previous frame
> OR
> B) the first free/available word in the current frame
> ???
>
> Do different compilers have different conventions/ABIs?
>
> The reason I ask is because, in my default invocation,
> gcc seems to consider case B:
>
> $ cat foo.c
> extern void bar(void *p);
> void foo(void *p) { bar(p); }
>
> $ mips-sde-elf-gcc -S foo.c
>   .file 1 "foo.c"
>   .section .mdebug.abi32
>   .previous
>   .gnu_attribute 4, 1
>   .text
>   .align 2
>   .globl foo
>   .set nomips16
>   .set nomicromips
>   .ent foo
>   .type foo, @function
> foo:
>   .frame $fp,24,$31 # vars= 0, regs= 2/0, args= 16, gp= 0
>   .mask 0xc0000000,-4
>   .fmask 0x00000000,0
>   .set noreorder
>   .set nomacro
>
>   addiu $sp,$sp,-24
>   sw $31,20($sp)
>   sw $fp,16($sp)
>   move $fp,$sp
>   sw $4,24($fp)
>   lw $4,24($fp)
>   jal bar
>   nop
>
>   move $sp,$fp
>   lw $31,20($sp)
>   lw $fp,16($sp)
>   addiu $sp,$sp,24
>   j $31
>   nop
>
>   .set macro
>   .set reorder
>   .end foo
>   .size foo, .-foo
>   .ident "GCC: (Sourcery G++ Lite 4.4-305) 4.4.1"
>
> gcc stores r4 at fp+24 (fp+24 being the initial value of sp)

I tried targeting every ABI documented by gcc.
$ for abi in 32 o64 n32 64 eabi; do gcc -S -mabi=$abi -o foo-$abi.s foo.c; done

I got errors for o64, n32, 64
foo.c:1: error: '-march=mips32r2' is not compatible with the selected ABI
foo.c:1: error: '-march=mips32r2' is not compatible with the selected ABI
foo.c:1: error: '-march=mips32r2' is not compatible with the selected ABI

abi=32 was the default, abi=eabi looks "better"
(i.e. compatible with the GHS compiler)

foo:
  .frame  $fp,16,$31    # vars= 8, regs= 2/0, args= 0, gp= 0
  .mask  0xc0000000,-4
  .fmask  0x00000000,0
  .set  noreorder
  .set  nomacro

  addiu  $sp,$sp,-16
  sw  $31,12($sp)
  sw  $fp,8($sp)
  move  $fp,$sp
  sw  $4,0($fp)
  lw  $4,0($fp)
  jal  bar
  nop

  move  $sp,$fp
  lw  $31,12($sp)
  lw  $fp,8($sp)
  addiu  $sp,$sp,16
  j  $31
  nop

$ objdump -x foo-eabi.o

foo-eabi.o:     file format elf32-tradbigmips
foo-eabi.o
architecture: mips:isa32r2, flags 0x00000011:
HAS_RELOC, HAS_SYMS
start address 0x00000000
private flags = 70003001: [abi=EABI32] [mips32r2] [not 32bitmode] [noreorder]


In this ABI, gcc does preserve the value pointed to by the
inital value of sp.

Does this mean all I need to do is compile everything with -mabi=eabi ?
And it will all auto-magically work?  :-)

--
Regards.