[LLVMdev] Modifying DAG in TargetLowering::ReplaceNodeResults()

[Eli Friedman]

On Mon, Jun 13, 2011 at 8:21 AM, Roman Divacky <rdivacky at freebsd.org> wrote:
> Hi!
>
> I am trying to implement va_arg() on ppc32. Everything went smooth, except
> implementing va_arg() of 64bit int. Since i64 is not a legal type on ppc32
> DAGTypeLegalizer::ExpandRes_VAARG() splits the va_arg(i64) into two i32
> va_args.
>
> The problem with ppc32 va_arg is that it needs special "alignment" of its
> gpr pointer when the argument is i64. Ie. I need to know if I am lowering
> va_arg of i64.
>
> I tried to tackle this by providing custom lowering for va_arg(i64) that
> just aligns stuff properly. I do this by adding ISD::VA_ARG handling to
> PPCTargetLowering::ReplaceNodeResults() where I need to read/increse/write
> a number in memory while providing nothing in &Results.
>
> The problem is that the Chain of the va_arg node needs to be updated but
> I dont know how to do that.
>
> I tried stuff like:
>
>
>      SDValue Ops[] = { InChain, VAListPtr, N->getOperand(2), N->getOperand(3) };
>      SDValue NewNode = DAG.getNode(ISD::VAARG, dl, DAG.getVTList(VT, MVT::Other), Ops, 4);
>      DAG.ReplaceAllUsesWith(N, NewNode.getNode());
>      // or this variant
>      //DAG.ReplaceAllUsesWith(SDValue(N, 1), NewNode);
>
>
> but that does not work for various reasons (N having more than one value in the first case
> and N operand 2 not having legal type (i64)).
>
>
> Any idea how to fix this? Or am I doing it completely wrong?

ReplaceNodeResults returns multiple SDValues; you should just be able
to return both the new value and new chain, and have everything just
work.

-Eli