[llvm-dev] MachinePipeliner refactoring

[James Molloy via llvm-dev]

Hi Brendan (and friends of MachinePipeliner, +llvm-dev for openness),

Over the past week or so I've been attempting to extend the
MachinePipeliner to support different idioms of code generation. To make
this a bit more concrete, there are two areas where the currently generated
code could be improved depending on architecture:

  1) The epilog blocks peel off the final iterations in reverse order. This
means that the overall execution of loop iterations isn't in a perfectly
pipelined order. For architectures that have hardware constructs that
insist on a first-in-first-out order (queues), the currently generated code
cannot be used.
  2) For architectures (like Hexagon) that have dedicated predicate
register files, we can generate a compact representation of the loop by
predicating stages of the loop kernel independently. In this case we can
either have a prolog, epilog, or neither (wrapping the prolog and epilog
inside the kernel by using PHIs of predicates).

At the moment, a lot of the code generation helper code in MachinePipeliner
is tightly fit to its current code generation strategy ("If we're in the
epilog, to this, else do this"). I'm keen to try and make some of the
complex calculations it does, such as where PHIs should come from, more
abstract so they can be reused and composed.

https://reviews.llvm.org/D64665 is my current best-effort. This generates
perfect code for PowerPC, but causes a load of problems for Hexagon. It's
become apparent that I don't know enough about some of the edge cases in
the MachinePipeliner code to refactor this from scratch. I'm therefore
looking for direction in factoring in an incremental fashion.

I think there are a couple of areas that I'd like to change, and I'd
appreciate your ideas and opinions because I clearly don't know enough
about the edge cases here.

  a) TII->reduceLoopCount() is hard to understand. Understanding the
intended semantics of this hook from the documentation, I've found, is
hard. Its use appears to be strongly fit to Hexagon (there is even a
comment about the removal of LOOP0 in the MachinePipeliner target agnostic
code, which really shouldn't be there). Why it's called multiple times I
don't understand (why can't we just call it once with the total number of
iterations to peel?).
  b) Understanding how loop-carried PHIs are linked together is really
hard. There are two functions dedicated to this with many edge cases, which
are linked with the prolog and epilog schedule. It'd be great to somehow
factor these such that they are independent of the code generation
strategy. Note that this is really important for some of the code gen
strategies I mention at the beginning, because loop-carried PHIs in this
case may actually end up being selects or uses of predicated instructions.
  c) There is a slight conflation of "iteration" and "stage" in the
documentation that makes it hard to follow what VRMap contains and the
invariants between functions.

My intent in D64665 was to create two abstractions: "Stage" and "Block".
Instead of referring to stages by index (VRMap), each Stage would take a
prior Stage as input. Stages are contained inside Blocks, which handles
predecessors and successors. I feel that arranging the code generation in
this CFG-like way will make the flow of data much easier to analyze. Of
course, a Stage doesn't just depend on a prior Stage - their loop carried
inputs could come from any other Stage (and while I think I understand how
this works, I clearly don't get all the edge cases).

What do you think of this abstraction? do you think it's doomed to failure
because it's too simplistic to cover all the cases?

Do you have any suggestions of areas where we can start to factor out
without a large-scale code breakage? I'm finding this hard to get my teeth
into as the proposed code structure is so different from its current form.

Thanks for any opinions or suggestions!

Cheers,

James
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