llvm dev - Apr 2021 - How to get started with VLIW scheduler?

Hi,
I am building a toy target to teach at a university but I am not well aware
of LLVM's scheduler infrastructure. My target is VLIW, 1 Load, 1 Store and
2 ALUs. ALUs can perform almost all common mathematical operations.

Where do I start building a scheduler for it? I watched a couple of videos
from the LLVM dev meeting but what is the current way of building a new
scheduler? Is there a .rst page out there which I missed reading?

Thanks.
Jeni.
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The way we do it for Hexagon is that we have our own specialization of the
MachineScheduler pass, followed by a packetizer pass.  Actually, we run two
schedulers: machine scheduler, and post-RA scheduler, but that doesn’t change
much conceptually.  In the scheduler passes we don’t form instruction packets,
at least not explicitly.  We do some sort of a simulation of “what if we were
forming packets right now” type of thing, maximizing packetization opportunities
ahead of time, before the packetization itself, hoping that enough of it will
prevail.

Finally, near the end of the post-RA passes we do run the DFAPacketizer pass
which forms the actual packets (i.e. “bundles” in LLVM terminology).  The DFA
packetizer relies on tables that are generated out of your target’s td files
describing your processor’s resources (some of which the standard scheduler uses
as well).

If you’re starting from scratch, I’d get the packetizer working first.

--
Krzysztof Parzyszek  kparzysz at quicinc.com<mailto:kparzysz at
quicinc.com>   AI tools development

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of jeniffer
lesley via llvm-dev
Sent: Monday, April 26, 2021 1:33 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] How to get started with VLIW scheduler?

Hi,
I am building a toy target to teach at a university but I am not well aware of
LLVM's scheduler infrastructure. My target is VLIW, 1 Load, 1 Store and 2
ALUs. ALUs can perform almost all common mathematical operations.

Where do I start building a scheduler for it? I watched a couple of videos from
the LLVM dev meeting but what is the current way of building a new scheduler? Is
there a .rst page out there which I missed reading?

Thanks.
Jeni.
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