[RFC] PatternMatch for SelectionDAG

[Nadav Rotem]

David, 

I am really happy to see that you are working on this. As part of the discussions on replacing SelectionDAG with something new, a few people mentioned that it would be much easier to migrate to a new infrastructure if the patterns were written in patterns such as the ones that you are proposing.  I like your approach. I look forward to seeing the complete patch. 

Thanks,
Nadav

On Jul 20, 2013, at 6:48 PM, David Majnemer <david.majnemer at gmail.com> wrote:

> I'd like to propose a PatternMatch-like interface for SelectionDAG.
> 
> I have a simple implementation attached, it matches on SDValue types and provides a convenience overload for SDNodes.
> 
> Currently, it uses similar names to PattternMatch.h (match, m_And, etc.).
> I don't really care about the names but I figured to start with something familiar. They shouldn't conflict because they would both exist in their own namespaces.
> 
> The effect is pretty nice on some DAGCombiner code:
> 
>    // (or (and X, C1), (and Y, C2))  -> (and (or X, Y), C3) if possible.
>    if (N0.getOpcode() == ISD::AND &&
>        N1.getOpcode() == ISD::AND &&
>        N0.getOperand(1).getOpcode() == ISD::Constant &&
>        N1.getOperand(1).getOpcode() == ISD::Constant &&
>        // Don't increase # computations.
>        (N0.getNode()->hasOneUse() || N1.getNode()->hasOneUse())) {
>      const APInt &LHSMask =
>        cast<ConstantSDNode>(N0.getOperand(1))->getAPIntValue();
>      const APInt &RHSMask =
>        cast<ConstantSDNode>(N1.getOperand(1))->getAPIntValue();
> 
> becomes:
>    const APInt *LHSMask, *RHSMask;
>    if (match(N0, m_And(m_Value(), m_APInt(LHSMask))) &&
>        match(N1, m_And(m_Value(), m_APInt(RHSMask))) &&
>        // Don't increase # computations.
>        (match(N0, m_OneUse()) || match(N1, m_OneUse()))) {
> 
> Note that the attached implementation is pretty incomplete, I just wanted a proof of concept before I run off and flesh it out further.
> 
> Thanks
> 
> -- 
> David Majnemer
> <ns.h>_______________________________________________
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