Re: [f-cpu] Call convention resume

["Christophe" <christophe.avoinne@laposte.net>]

----- Original Message -----
From: "Michael Riepe" <michael@stud.uni-hannover.de>
To: <f-cpu@seul.org>
Sent: Monday, June 24, 2002 12:31 AM
Subject: Re: [f-cpu] Call convention resume


> On Sun, Jun 23, 2002 at 02:10:57PM +0000, cedric wrote:
> > Hi every body,
> >
> > I will try to do a resume of what has been say about call convention during
> > the last week, before updating the manual.
> >
> > R0: zero
> > R1 : number of parameter (for debug and security) and return value
>
> I still do not agree with that point. It's a waste.

Not only it is a waste but it is unefficient. Why ? how many arguments for such
function : int f (struct a _a) ? 1 or the number of words necessary for struct
a ? it doesn't make a sense to have one. If you really want a debug info, pass
a parameter signature instead, something like C++ functions signature but just
for parameters. So if you call a function with three integer arguments then
passing the string "iii" (for example) would be more precise and usable for
debug than just passing number of arguments. Personnally i would drop it.

>
> > R2-R15 : functions arguments (call-clobbered)
> >
> > R15-R31 : temporary registers (call-clobbered)
> > R32-R58 : local registers (callee-saver)
> >
> > R59 : return adress
> >
> > R60 : pointer to the Procedure Linkage Table
> > R61 : pointer to the Global Offset Table
> >
> > R62 : Frame pointer (Function Entry Frame)
> > R63 : stack pointer after which the parameters after the 14th are push in
> > reverse order (Function Entry Frame - number of bytes allocated)
>
> Better make r61 (or r63) the return address. Statically linked programs
> don't use the PLT and GOT pointers, and we don't want to have a hole in
> the middle.
>
> The stack pointer is always the end of the allocated area on the stack.
> If a program uses memory from the stack, it MUST move the stack pointer.
> That way, the operating system can tell how big the user mode stack is.
>

I totally aggree with Michael; something like :

R63/62 : return pointer (RP) , mandatory
R62/63 : stack pointer (SP) , mandatory
R61       : frame pointer (FP) , optional
R60       : GOT                        , optional
R59       : PLT                         , optional

R14 should point on the first argument pushed in stack, so we can access it as
an argument pointer (AP)

R14,R61 and R62 all point on the same stack indeed.

Personnally I would have used R60 for AP instead of R14, unless there is a real
reason to choose r14.


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