[llvm-dev] Selection DAG chain question

[Craig Topper via llvm-dev]

Chains represent a dependency between nodes that can't be represented by a
data dependency. For example a load following a store that might alias with
the address of the load. The store must happen before the load. So the
load's chain input is dependent on the store's chain output either
directly or through other intermediate nodes that also have chain inputs
and outputs. There can be multiple chains in parallel in the DAG.
TokenFactor nodes are used to merge separate chains. The InstrEmitter
ensures that the chain dependency is satisfied when emitting the linear
instruction sequence after isel. But nothing guarantees that parallel
chains won't be interleaved. After a node is schedule all of the nodes
dependent on it either through data or chain are checked to see if they are
now ready to schedule. The scheduler will pick from the ready to schedule
nodes without any concern for whether they were on the same chain as the
last node scheduled.

Glue is stricter, it says that two nodes must be scheduled adjacent to each
other in the linear instruction sequence.

~Craig


On Fri, Jul 17, 2020 at 10:46 AM Rotate Right via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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