[f-cpu] Another proposition for a call convention

[Cedric BAIL <cedric.bail@free.fr>]

An other call convention for call convention :

R0 : zero
R1 : number of parameter and return value
R2-R13 : functions arguments (call-clobbered)
R14 : pointer to pre allocated stack for all arguments and arguments
after the 12th registers.

R15-R31 : temporary registers (call-clobbered)
R32-R59 : local registers (callee-saver)

R60 : return adress
R61 : global pointer
R62 : Frame pointer
R63 : stack pointer

(We can add a R59 = static link, for language like caml, but I think that
specifing that their are callee-saved, will be good enough for a call
convention between every language and the system)

For va_start we only do this :
	storem R2, [R14], R13 // only store the register from R2 to R13

and it's all. For normal function we have nothing to do !
If we have more parameters than 12, we only need to do something like this :

ex: printf("a %i %i %i ... %i %i %i", 1, 2, 3, ..., 12, 13, 14)

we will do something like :
	loadconsx 14, R1
	loadcons STRINGADDR, R2
	loadconsx 1, R3
	loadconsx 2, R4
	loadconsx 3, R5
	...
	loadconsx 12, R13
	move R63, R14
	subi 14 * 8, R63, R63 // allocated the needed space in stack
	subi 12 * 8, R14, R31 // use a temporary register to start transfering
			      // other parameters
	loadconsx 13, R30
	store -8, [R31], R30
	loadconsx 14, R30
	store -8, [R31], R30
	jump "printf", R60    // do the call and save return address.

if you need less that 12 registers, like a printf("a") you only have to
do :
	loadconsx 1, R1
	loadcons STRINGADDR, R2

	move R63, R14
	subi 8, R63, R63 // allocated the needed space in stack

	jump "printf", R60    // do the call and save return address.

I know we first need to store the address label in a register and then use it,
but that more easy to understand in that way (and in general, we can write
the same code without the need of temporary registers).

I think that we have an error in the manual, the second register in a jump
give normally PC and not PC+4, I don't see the usage of this feature (but
for a call PC+4 is really a good idea).

So what is the problem of this solution ?

A+
  Cedric
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