[llvm-dev] Selection DAG chain question

[Rotate Right via llvm-dev]

newbee here. What's the difference between glue and chain?
Why can't we add chains to any node we want?

On Fri, Jul 17, 2020, 10:25 PM Björn Pettersson A via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Still sounds to me as Glue might help (as already proposed by Craig), but
> maybe I’ve misunderstood something.
>
>
>
> Another option is to do a simple lowering into pseudo instructions that
> you expand after ISel.
>
> (might be easier than doing something before ISel and then having to
> bother about chains, glue etc)
>
>
>
> Regards,
>
> Björn
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Hendrik
> Greving via llvm-dev
> *Sent:* den 16 juli 2020 23:35
> *To:* Matt Arsenault <arsenm2 at gmail.com>
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Selection DAG chain question
>
>
>
> Yea. I think AMD chains the node they're expanding into, but they don't
> chain it into an _existing_ chain. e.g. adding A->B to the DAG is ok. But
> adding A->B and next C->D with B->C is the problem. I appreciate the input
>
>
>
> On Thu, Jul 16, 2020 at 2:04 PM Matt Arsenault <arsenm2 at gmail.com> wrote:
>
>
>
> > On Jul 16, 2020, at 17:00, Hendrik Greving <hgreving at google.com> wrote:
> >
> > > No, non-sideeffecting operations can be legalized as compiler-rt calls
> >
> > Right, but not as "regular" nodes with side-effects? I guess you could
> search and analyze the DAG manually but that seems hacky. Maybe something
> that one day LLVM could support natively.
>
>
> You can’t add arbitrary chains or glue to the regular nodes, but you can
> define a custom node you select the same way with your chain/glue. You
> don’t need to preprocess the IR and can do in the custom lowering. This is
> what AMDGPU does for FDIV (see AMDGPUISD::FMA_W_CHAIN). GlobalISel avoids
> these complications by not having nodes or chains, and just instructions
> with side effects, so in that sense this is a solved problem.
>
> -Matt
>
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