ppc405gp and GCC register usage

ppc405gp and GCC register usage

[Juergen Beisert]

Hello all,

does someone know, which register I can use in my own assembler routines
without disturbing code generated by gcc (in kernel space)? I did not found
any spec which registers gcc use, yet. Is such a document available?

-- JB

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Re: ppc405gp and GCC register usage

[Wolfgang Denk]

In message <200307151224.04020.jbeisert@eurodsn.de> you wrote:
>
> does someone know, which register I can use in my own assembler routines
> without disturbing code generated by gcc (in kernel space)? I did not found
> any spec which registers gcc use, yet. Is such a document available?

See the (E)ABI specifications for the PowerPC architecture,  and  GCC
source code.

For PowerPC, the following registers have specific use:

	R1:	stack pointer
	R2:	TOC pointer
	R3-R4:	parameter passing and return values
	R5-R10:	parameter passing
	R13:	small data area pointer
	R30:	GOT pointer
	R31:	frame pointer



Best regards,

Wolfgang Denk

--
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd@denx.de
Life would be so much easier if we could  just  look  at  the  source
code.                                                   -- Dave Olson

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Re: ppc405gp and GCC register usage

[Vladimir A. Gurevich]

Hi Juergen,

There is a standard, called ABI (Application Binary Interface) that
defines this.
ABIs are, obviuosly, CPU-specific. Compilers tend to respect ABI that
is defined for the CPU they generate code for.

The best documents specifying PPC ABI can be found on IBM's website:

PowerPC Compiler Writer's Guide
http://www-3.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF7785256996007558C6/$file/cwg.pdf

Developing PowerPC Embedded Application Binary Interface (EABI)
Compliant Programs
http://www-3.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF77852569970071B0D6/$file/eabi_app.pdf

They not only describe register usage, but stack layout, parameter
passing and many
other things. There are a couple of slightly different ABI's for
PowerPC, that differ
with regards to R2 and R13 usage to access the so-called small data
sections.

Also, depending on what you are doing you have 2 options:
   -- do the whole function in assembly. You are free to do whatever (as
long as you
      obey the ABI), but you've got to watch register allocation
yourself (among other
      thing)
   -- Use __asm__ in your C code. Then the compiler will do most of the
register
       allocation for you and will help you with ABI issues as well.

Happy Hacking,
Vladimir

Juergen Beisert wrote:

>Hello all,
>
>does someone know, which register I can use in my own assembler routines
>without disturbing code generated by gcc (in kernel space)? I did not found
>any spec which registers gcc use, yet. Is such a document available?
>
>-- JB
>
>
>


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Re: ppc405gp and GCC register usage

[Juergen Beisert]

Am Dienstag, 15. Juli 2003 12:42 schrieb Vladimir A. Gurevich:
> Hi Juergen,
>
> PowerPC Compiler Writer's Guide
> http://www-3.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF77852
>56996007558C6/$file/cwg.pdf
>
> Developing PowerPC Embedded Application Binary Interface (EABI)
> Compliant Programs
> http://www-3.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF77852
>569970071B0D6/$file/eabi_app.pdf

Thank you very,very much. That's what I'm searching for.

Best regards,
Juergen Beisert

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Re: ppc405gp and GCC register usage

[Paul Mackerras]

Juergen Beisert writes:

> does someone know, which register I can use in my own assembler routines
> without disturbing code generated by gcc (in kernel space)? I did not found
> any spec which registers gcc use, yet. Is such a document available?

It's defined in the PowerPC ELF ABI document.  Basically a procedure
can use r0 and r3 - r12 without saving and restoring them.  r1 is the
stack pointer, r2 is used by the kernel to hold the pointer to the
task_struct for the current task.  r13 - r31 have to be preserved so
your procedure has to save and restore them if it wants to use them.
Similarly your procedure has to preserve cr2 - cr4 but can use cr0,
cr1, cr5 - cr7, lr, ctr, xer without saving & restoring them.

Unfortunately the PowerPC ELF ABI specification doesn't really have
anyone looking after it at the moment, so I don't know of a public
site where you can get it.  I could send you the pdf if you are
interested.

Paul.

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