[llvm-dev] Understanding Chains

[Dilan Manatunga via llvm-dev]

Hi,

I am working on writing a backend in LLVM and I am having a little trouble
completely understanding chains. As far as I can tell, chains are used to
represent dependencies that can't be expressed by the typical def-use
dependency. But I am uncertain where and how I should be using chains.

In the Code Generator
<http://releases.llvm.org/4.0.0/docs/CodeGenerator.html> documentation,
they describe that chains should be used with instructions that have side
effects loads, stores, calls, etc. I guess I am confused to as why those
instructions need chains. For example, won't loads have a dependency on
address to ensure they are only scheduled after address generation. And
stores will have dependencies and addresses and results to ensure they only
occur after those values are created. Is it because of memory
disambiguation causing us to be conservative and needing to ensure that
certain store/loads aren't re-ordered respect to each other?

I guess the real question is when I include chains for operations and when
I want to return chains from operations (especially intrinsics). For
example, with the backend, when I am inserting Loads from the stacks,  I
don't want to chain them all together for performance, right, since it
doesn't matter when the loads are scheduled (as long as it is before the
use). But I should chain them to the incoming chain from
LowerFormalArguments parameter.

On somewhat related note, take this example DAG
https://i.stack.imgur.com/RQRpq.gif, why does copyfromreg need a chain
input?

Thanks
-Dilan
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