[llvm-dev] Selection DAG chain question

[Björn Pettersson A via llvm-dev]

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<mailto:arsenm2 at gmail.com>> wrote:


> On Jul 16, 2020, at 17:00, Hendrik Greving <hgreving at google.com<mailto: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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