llvm dev - Jan 2007 - [LLVMdev] Inserting an assembly instruction in the calling sequence of the powerpc target

Hi all,

I'm currently implementing a linux/ppc target in llvm. The abis between 
Darwin/ppc and
linux/ppc are different and I'm running into problems with vararg calls.

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).

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

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

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

Thx for your help.

Cheers,
Nicolas

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

-- 
http://nondot.org/sabre/
http://llvm.org/

Hi Crhis,

Chris Lattner wrote:
> 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).
>   
That I did. I introduced the creqv instruction in PPCInstrInfo.td and
inserted a SETCR in the PPCISD::NodeType enum, file
PPCSelLowering.h.
> 2. At call lowering time, flag this into the call sequence.
>   
I'm still confused on how flags operate into the call sequence. I tried
to do :

  if (isVarArg)
    {
      InFlag = DAG.getNode(PPCISD::SETCR, MVT::i32,
                                DAG.getRegister(PPC::CR6, MVT::i32));
    }

in PPCISelLowering.cpp after the copy-to-reg sequence (line 1462 in 
llvm-cvs), but it does not
work (I always get wrong assertions)
> 3. At ISel time, turn this into creq reg,reg,reg
>
>   
If I understand correctly, all I have to do for this is to add in 
PPCISelDagToDag.cpp::Select a switch case where:

  case PPCISD::SETCR : {
    SDOperand InFlag = N->getOperand(0);
    return CurDAG->getTargetNode(PPC::CREQV, MVT::i32, InFlag, InFlag);
  }
>> 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 :)
>   
>
> There is already a subtarget.  In the call lowering code, just do 
> something like:
>
> if (!PPCSubTarget->isDarwin())
>    ... insert creq here ...
>
>   
OK, that's what I meant with my second proposition.
> -Chris
>
>   
Thx,
Nicolas