# ICFP 2010

# Paper #69

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## #69

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Simon Peyton Jones Stephanie Weirich

## Hoopl: A Modular, Reusable Library for Dataflow Analysis and Transformation

Submitted | 238kB |  | Friday 2 Apr 2010 10:24:41am EDT | 4e8df6a07b4c698ac20a073bf2a3decf1de2467c

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+ Abstract − Abstract
Dataflow analysis and transformation of control-flow graphs is pervasive in optimizing compilers, but it is typically tightly interwoven with the details of a particular [more] Dataflow analysis and transformation of control-flow graphs is pervasive in optimizing compilers, but it is typically tightly interwoven with the details of a particular compiler. We describe Hoopl, a reusable Haskell library that makes it unusually easy to define new analyses and transformations for *any* compiler. Hoopl's interface is modular and polymorphic, and it offers unusually strong static guarantees. The implementation is also far from routine: it encapsulates state-of-the-art algorithms (interleaved analysis and rewriting, dynamic error isolation), and it cleanly separates their tricky elements so that they can be understood independently.

+ Authors − Authors
N. Ramsey, J. Dias, S. Jones [details] Norman Ramsey (Tufts University) <nr@cs.tufts.edu>
João Dias (Tufts University) <dias@cs.tufts.edu>
Simon Peyton Jones (Microsoft Research) <simonpj@microsoft.com>
+ Topics, Documents, and Options − Topics
compilation components and composition
Documents and Options
Submission category: Functional pearl
Supplementary material

| OveMer | Exp
Review #69A |  | B | Y |
Review #69B |  | A | Y |
Review #69C |  | A | X |
Review #69D |  | C | Y |

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| Reviews and comments in plain text

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### Review #69A

Modified Tuesday 18 May 2010 8:33:53am EDT

Overall merit (?) B. OK paper, but I will not champion it. |  | Expertise (?) Y. I am knowledgeable in the area, though not an expert.

Paper summary
Describes the design, interface and implementation of a generic library for dataflow analyses and transformations. The approach followed is Lerner, Grove and Chambers's "analyze and (simultaneously) transform" rather than the more traditional "analyze then transform". GADTs and type families are used in a couple of places to statically enforce some well-formedness properties on control-flow graphs (e.g. maintain a distinction between nodes that start a block, end a block, or are in the middle of a block).

Paper strengths and weaknesses + Very solid engineering. + Good writing for such a technical topic - The examples don't demonstrate the full power of the Lerner-Grove-Chambers approach (composition of analyses) - As a research paper: no breakthrough, conceptual advances are small. - As a functional pearl: not really entertaining, reads too much like documentation for a library.

Comments for author
This is a nice, well-engineered implementation of the "analyze and transform" approach of Lerner et al.
As a research paper, it is very short on conceptual advances, though. The use of GADTs and type families to enforce shape properties on CFGs is cute but not very deep. Everything else is known already, although the paper does a good job of connecting the pieces of knowledge together.
As a programming pearl, I'm afraid it fails the Bird-Gibbons test (http://www.comlab.ox.ac.uk/people/Jeremy.Gibbons/pearls/):
"Bird characterizes them as "polished, elegant, instructive, entertaining". [...] Think more along the lines of short stories --- 6 to 10 pages, brisk, engaging, accessible, surprising."
Polished and instructive it is; elegant and accessible, to some extent; but entertaining, brisk, engaging and surprising, no.
I was disappointed by the very short treatment of compositionality (section 4.5). The ability to compose analyses is the major innovation of Lerner et al's paper. For a single analysis followed by a single transformation, their approach simplifies a bit the transfer function (as explained well in the paper), but doesn't improve the precision of the analysis. Where Lerner et al really shine is the ability to combine multiple analyses and transformations. I really missed an example of such a combination in the paper, and was disappointed to see the "thenFwd" combinator on page 7 left as an exercise for the reader. There is also the question of combining forward and backward analyses that is left open in Lerner et al's paper (as far as I can remember) and would definitely deserve a second look.
Page 11: CIL is not "analysis-only". It provides about as many (or as few?) facilities for code rewriting than for code analysis. The comment about the API being complicated (in both cases) is correct, though.
The paper is generally well written but is somewhat repetitive in emphasizing how hard the authors worked and how nice the resulting API is. I'm not sure all of the following sentences have their place in a scientific paper (some of them perhaps, but all of them is a bit too much):
"... an idea that is elegantly captured by the FwdRes type..."
"What a beautiful type thenFwrdRw has!"
"We have spent six years implementing..."
"This formidable design problem..."
"...have been through dozens of revisions"
"We are proud of using GADTs to track..."

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### Review #69B

Modified Wednesday 19 May 2010 8:22:31am EDT

Overall merit (?) A. Good paper. I will champion it at the PC meeting. |  | Expertise (?) Y. I am knowledgeable in the area, though not an expert.
Paper summary The paper presents Hoopl, a Haskell framework for implementing dataflow analyses and compiler optimizations. Hoopl implements the ideas first presented by Lerner, Grove, and Chambers in their POPL 2002 paper. The paper presents Hoopl, shows how to use it practically on one example (constant folding), and sketches its implementation.
Paper strengths and weaknesses + nicely written + good and convincing illustration of the Haskell type machinery + inspiring paper on compilers architecture - merely a packaging of already published ideas

Comments for author
Section 4.5 vaguely remembers me Expansion Passing Style by Dybvig,
Friedman and Haynes. Don't you think EPS is related to your rewrite
functions that return rewrite functions?
Page 7, why no giving definition of rewriteE?
Page 8, section 4.7. This section is not very convincing. It is not
technically very deep and I have mostly read it as a disrupting digression.
Toward the conclusion, you state that using the fuel monad was one of
your three goals. If, indeed, you consider using the fuel monad that
important, then I think you should emphasize it all along the paper.
Section 5. This section is really for Haskell aficionados. I understand why
it is there and I thank you for not making it too long but I still don't
like it very much. It reads too much as a self contentment Section.

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### Review #69C

Modified Friday 21 May 2010 10:52:35am EDT

Overall merit (?) A. Good paper. I will champion it at the PC meeting. |  | Expertise (?) X. I am an expert in the subject area of this paper.
Paper summary This paper describes a dataflow analysis and transformation framework written in Haskell. A client of this library provides the representation of nodes (i.e., instructions), data-flow facts, and transformations, and the library provides the analysis and transformations. Hoopl makes heavy use of advanced Haskell type-system features (GADTs, etc.) to enforce static correctness.

Paper strengths and weaknesses
Pros:
While it does not introduce a new analysis algorithm or transformation, this
work bridges the gap between the theoretical presentations and actual compilers.
Demonstrates the utility of functional programming for algorithms on
control-flow graphs, which are an aspect of compilers that was _not_ obviously
suited to functional programming languages.
One might argue that the paper does not introduce any new analyses or optimizations,
but I think that problem of how to actually build such applications is important
and this paper does a very good job of addressing that topic in a principled way.
Cons:
Some minor quibbles described below.

Comments for author
On page 4, the claim that middle nodes cannot refer to labels is not
really true (e.g., LoadAddress); what you mean is that middle nodes
do not use labels for their control flow.
In footnote 4, you say that program points correspond to edges, but edges
only occur between blocks and not between nodes. Surely nodes are program
points too?
The mkIfThenElse function in Figure 4 looks like it has a type error:
t and e are AGraphs, but gCat is defined to work on Graphs.
How do you expect function calls to be treated? Would then be open/closed
nodes with a closed/open return node?
I would like to see a bit more discussion about use cases. For example,
would you use it on an SSA representation? Do you support other control-flow
algorithms, such as determining dominators?
You do not mention the MLRisc library, but it also supports some higher-order
programming of optimizations (mostly through the heavy use of functors).
I don't think that there was ever a write up of this, other than the
documentation
http://www.cs.nyu.edu/leunga/www/MLRISC/Doc/html/index.html
One issue that is not addressed is how fast/slow are the resulting optimization
passes? Do you take a big hit because of the abstractions?
Another piece of related work that is missing is Lee and Colby's implementation
of Parameterized Partial Evaluation using SML functors (the idea is based
on Consel's Parameterized Partial). It is similar in that the client
supplies an abstract domain and operators and the library takes care of the
rest. It also supports combining analyses and transformations automatically.

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### Review #69D

Modified Friday 21 May 2010 12:18:00pm EDT

Overall merit (?) C. Weak paper, though I will not fight strongly against it. |  | Expertise (?) Y. I am knowledgeable in the area, though not an expert.
Paper summary The paper descripes Hoopl, a library for dataflow analysis mixed with transformations. Hoopl is implemented in Haskell in a purely functional way, using GADT to enforce invariants on internal data structures.

Paper strengths and weaknesses
Strengths:
- Having this code exported as a library rather than keeping it internal to
the GHC compiler is useful for others.
- This paper may serve as a reference for people using the library or for
other people implementing a similar library in other languages.
Weaknesses:
- This remains the description of code, without much algorithmic novelty,
and however useful the work and smart the implementation are, this remains
a kind of boring paper to read. (Despite the constant efforts of the
authors to transmit their enthusiasm to the reader.)
- One originality of the library compared to other existing implementations
of similar libraries is that it is purely functional. This is presented
as a great advantage for the design and I can believe it. Still, it
would have been interesting to have a discussion and some figures about
the efficiency of the implementation, to convince us that this is not
simultaneously a dropback.

Comments for author
- Section 3.3, col 1, last sentence: "Using GADTs to enforce..." Since you
given an interface in a library, you could use smart constructors and
phantom types to enfore the invariants from the client's point of view,
i.e. to force the client to write only meaningful graphs. The GADTs helps
you "take advantage" of these invariants knowing that other cases cannot
occur.
My impression is that (here and below) you are slightly exagerating the
importance of GADTs---form the clients point of view.
- Section 4.8: col 1, last two items: I do not understand what you mean
here. It seems that you throw away the transformation at each
iteration, so what would be the point of iterating? Please detail.
You call the original graph "virgin". In what sense? Since you transform
graphs rather than annotate them, each graph should have the same status as
another.
- Section 4, last item: "The algorithm is sound". Is it so obvious? Can the
transfer function be wrong, e.g. say that nothing never flows out of
anything and the rewrite eliminated significant stuff because it is not
assume to flow in (this rewrite is sound) so that the resulting
transformation is wrong? You should assume some form of soundness for the
transfer function as well.
- Section 5, one before last paragraph: "clever trick". This seems a rather
obvious thing to do, not even a trick...

### Response

The authors' response is intended to address reviewer concerns and correct misunderstandings. The response should be addressed to the program committee, who will consider it when making their decision. Don't try to augment the paper's content or form—the conference deadline has passed. Please keep the response short and to the point.

This response should be sent to the reviewers.

Overall merit choices are:
A. Good paper. I will champion it at the PC meeting.
B. OK paper, but I will not champion it.
C. Weak paper, though I will not fight strongly against it.
D. Serious problems. I will argue to reject this paper.
Expertise choices are:
X. I am an expert in the subject area of this paper.
Y. I am knowledgeable in the area, though not an expert.
Z. I am not an expert. My evaluation is that of an informed outsider.