$ta0 .. $ta3 registers in O32 on MIPS

$ta0 .. $ta3 registers in O32 on MIPS

[Ralf Baechle]

The register names $ta0 .. $ta3 were added by SGI for N32 / N64 code.
Because these reference $8 .. $11 just like $t0 .. $t3 in the O32 ABI
their availability in O32 as well appears dangerous, if not a bug:

$ cat s.s 
	addu	$ta0, $ta0
$ mips-linux-as -o s.o s.s
$ file s.o
s.o: ELF 32-bit MSB relocatable, MIPS, MIPS-I version 1 (SYSV), not stripped
$

I was expecting an error message and I'm wondering, was this intentional?

  Ralf

RE: $ta0 .. $ta3 registers in O32 on MIPS

[Paul_Koning]

>The register names $ta0 .. $ta3 were added by SGI for N32 / N64 code.
>Because these reference $8 .. $11 just like $t0 .. $t3 in the O32 ABI their availability in O32 as well appears dangerous, if not a bug:
>
>$ cat s.s 
>	addu	$ta0, $ta0
>$ mips-linux-as -o s.o s.s
>$ file s.o
>s.o: ELF 32-bit MSB relocatable, MIPS, MIPS-I version 1 (SYSV), not stripped $
>
>I was expecting an error message and I'm wondering, was this intentional?

I would say so.  I call this a feature.  It makes it easier to write assembly code that assembles without change in both O32 and N32/N64.  Consider a function that has 4 or fewer arguments, but needs a pile of scratch registers.  It can use ta0-ta3 as four scratch registers, which is correct in all the ABIs.

	paul

RE: $ta0 .. $ta3 registers in O32 on MIPS

[Paul_Koning]

>The register names $ta0 .. $ta3 were added by SGI for N32 / N64 code.
>Because these reference $8 .. $11 just like $t0 .. $t3 in the O32 ABI their availability in O32 as well appears dangerous, if not a bug:
>
>$ cat s.s 
>	addu	$ta0, $ta0
>$ mips-linux-as -o s.o s.s
>$ file s.o
>s.o: ELF 32-bit MSB relocatable, MIPS, MIPS-I version 1 (SYSV), not stripped $
>
>I was expecting an error message and I'm wondering, was this intentional?

I would say so.  I call this a feature.  It makes it easier to write assembly code that assembles without change in both O32 and N32/N64.  Consider a function that has 4 or fewer arguments, but needs a pile of scratch registers.  It can use ta0-ta3 as four scratch registers, which is correct in all the ABIs.

	paul

Re: $ta0 .. $ta3 registers in O32 on MIPS

[Ralf Baechle]

On Wed, Sep 28, 2011 at 10:11:40AM -0500, Paul_Koning@Dell.com wrote:

> >I was expecting an error message and I'm wondering, was this intentional?
> 
> I would say so.  I call this a feature.  It makes it easier to write assembly code that assembles without change in both O32 and N32/N64.  Consider a function that has 4 or fewer arguments, but needs a pile of scratch registers.  It can use ta0-ta3 as four scratch registers, which is correct in all the ABIs.

Turns out that later IRIX version also retroactively introduced the
ta registers for O32 and I just never noticed.  So I'm going to change the
Linux kernel headers for consistence and compatibility with everybody else.

  Ralf