[LLVMdev] Inserting an assembly instruction in the calling sequence of the powerpc target

[Chris Lattner]

On Fri, 12 Jan 2007, Nicolas Geoffray wrote:

> I'm currently implementing a linux/ppc target in llvm. The abis between

cool

> Darwin/ppc and linux/ppc are different and I'm running into problems 
> with vararg calls.

ok

> Before a variadic method is called, an extra instruction must be 
> executed (which is creqv 6, 6, 6). This instruction is not necessary in 
> Darwin/ppc.
> I looked into the PowerPC target implementation and the code generation uses
> Dags (lib/Target/PowerPC/PPCISelLowering.cpp, LowerCALL).


right.

> I need some help on how to insert the creqv instruction in the calling
> sequence.

The basic approach you want to take:
1. Introduce a new target-specific dag node to represent "SETCR", which
    takes a single register operand (in this case, CR6).
2. At call lowering time, flag this into the call sequence.
3. At ISel time, turn this into creq reg,reg,reg


> After this is implemented, I will be able to send a patch for linux/ppc
> support.

Nice.  This will also resolve http://llvm.org/PR1064

> I also need to know what is your preference for a linux/ppc target
> implementation? I can either
> 1) Use macros (#ifdef __MACH__ for darwin or #ifdef __PPC__ for linux)

This won't work for cross compiles.

> 2) Test the target in the code when needed
>   if (os == linux && ppc32) then...
>   else if (os == linux && ppc64) then ...
>   else if (os == darwin && ppc32) then ...
>   else if (os == darwin && ppc64) then ...

This is inefficient :)

> 3) Create a new subtarget (but does it worth it? considering that the
> difference between the 2 abis is minor)

There is already a subtarget.  In the call lowering code, just do 
something like:

if (!PPCSubTarget->isDarwin())
   ... insert creq here ...

-Chris

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