# Passing Data Structures to FPGAs

Next week, my former PhD student and postdoctoral researcher, Felix Winterstein, will present our paper Pass a Pointer: Exploring Shared Virtual Memory Abstractions in OpenCL Tools for FPGAs at the IEEE International Conference on Field-Programmable Technology in Melbourne, Australia.

Before launching his current startup, Xelera, Felix and I worked together on the problem of automating the production of custom memory systems for FPGA-based accelerators. I previously blogged about some highly novel work we’d done during his PhD on high-level synthesis for code manipulating complex data structures like trees and linked lists. Full detail can be found in the book version of his PhD thesis. All this work – as exciting as it is – was based on sequential C code description as the input format to a high-level synthesis tool.

Many readers of this blog will be aware that OpenCL is rapidly becoming viewed as an alternative way to write correctness-portable code for FPGA development, with both Intel and Xilinx offering OpenCL flows based around OpenCL 1.X. However, OpenCL 2.0 offers a number of interesting features around shared virtual memory which could radically simplify programming, at the cost of making the compiler significantly more complex for FPGA-based computation. It is this issue we address in the paper Felix will present next week.

There’s lots of exciting program analysis work that could be built on top of Felix’s framework, and I’m keen to explore this further – if a reader of this blog would like to collaborate in this direction or like to do a PhD in this field, feel free to get in touch.

Perhaps most importantly, Felix’s framework is open source – check it out at https://github.com/constantinides/FPGA-shared-mem and let us know if you use it!

# HLS and Power: Some FPL Contributions

This week sees the IEEE International Conference on Field-Programmable Logic and Applications, in Ghent, Belgium.  Two of my team are attending to present their research papers on High Level Synthesis and on Run-time Power Estimation. In this post, I briefly summarise the key contributions of these papers.

High-Level Synthesis (HLS) is an important technology, which aims to automatically generate hardware designs from high-level (typically software) descriptions of their behaviour. In a previous blog post, I described some work from my PhD student Junyi Liu (joint with Sam Bayliss) on extending a common paradigm for analysis memory dependences – the polyhedral model – to a parametric version, for efficient pipelining in HLS. This week, Junyi presents an alternative use for the same parametric polyhedral HLS framework: automatic loop tiling (joint work with John Wickerson). Loop tiling is a very common compiler transformation – for example it is often used in matrix-matrix multiplication. The key advantage is to make sure that you only have a small set of data you’re working with at any given moment in time (traditionally for cache, in the FPGA context for embedded scratch-pad memories). The size of this working set can be traded off against the amount of off-chip memory traffic by selection of tile sizes. In a multi-dimensional loop, there are many possible options, and navigating this space is non-trivial. Junyi’s work provides a way to produce an explicit formula for both the memory requirement and the amount of off-chip data traffic required for any given tile size. He can then use nonlinear optimisation techniques to explicitly optimise the traffic subject to any given constraint on buffer size. This work is available as an open-source tool at https://github.com/Junyi-Liu/PolyTSS.

Back in 2016, some work I did with Eddie Hung, James Davis, Josh Levine, Ed Stott and Peter Cheung won the best paper prize at FCCM 2016. We showed that it is possible to use an online (recursive least squares) algorithm to learn the instantaneous power consumption of individual components in an FPGA design, with a view to some kind of run-time manager using this information. The solution worked by monitoring certain signal activity at run-time, but the missing part of the puzzle was which signals to monitor. James’s latest paper, STRIPE, with the same co-authors, answers this question. It turns out that the answer to this problem – as with so many in engineering (and life?) – lies in linear algebra. Golub and Van Loan describe in their classic textbook how QR factorisation can be used to heuristically select a subset of “nearly linearly independent” vectors from a larger set, and it’s this approach that tends to win out when given enough data to work with.

# Floating-Point Workshop

This week I attended a very enjoyable workshop at Dagstuhl on Analysis and Synthesis of Floating-Point Programs, organised by Eva Darulova, Ally Donaldson, Zvonimir Rakamarić, and Cindy Rubio González. As I noted in my talk on Algorithm-Architecture Codesign, this is one of the very few meetings I’ve been to at which delegates spanned some of – what I consider to be – the most exciting research areas at the moment: computer architecture, high-level synthesis, computer arithmetic, numerical analysis, programming languages, and formal methods. Hats off to the organisers for getting all these people in the room at once!

Below, I just choose a few of the many great talks I heard as some personal highlights of the workshop for me. Presentations and – more importantly! – debates during and after presentations were of uniformly excellent quality.

Rubio González, Hollingsworth, and Rakamarić all presented work on precision tuning. This is a topic I did some of the early work on back in 2001, in the context of fixed-point arithmetic for DSP algorithms in hardware, and have maintained an interest in ever since [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], over the years slowly migrating from the very special class of LTI algorithms implemented in fixed-point arithmetic to much broader classes of algorithm, including proving termination of loops under finite precision floating-point and making forays into real algebraic geometry. The topic is currently exhibiting a resurgence of interest, especially for floating-point software. Of the tools presented, I only personally have some experience with FPTuner.

Damouche discussed a tool for automatically rewriting floating-point code for accuracy improvement (joint work with Martel). I’ve been aware of the interesting work of Martel for a while, and it inspired our own SOAP tool and the associated papers [17,18,19] which extend the capability to hardware where one is concerned with performance, area, and numerical error. This theme was also picked up by Panchekha, who has developed some very interesting tools for diagnosing numerical instability and correcting it.

Another common theme of the workshop was reproducibility. Many researchers (and developers!) are unhappy about the non-reproducible nature of floating-point code: change compiler or platform, and you might get a different result – more insidiously, run again on the same platform and you still might get a different result. My colleague Miriam Leeser, Michaela Taufer, Ganesh Gopalakrishnan and Thomas Wahl all spoke eloquently on this topic. Wahl’s work considered the idea of stabilising programs against platform uncertainty. The work auto-inserts pragmas in order to only determinise certain “key” properties, like ensuring the same control flow path is taken each time the program is run.

Donaldson spoke about detecting compiler bugs by inserting (precise) semantics-preserving transformations, and highlighted several such bugs his group has found. A similar theme was picked up by Nagarakatte, who has found bugs in LLVM floating-point optimisations and is proposing a DSL to specify such optimisations precisely.

Jim Demmel gave an interesting summary of proposed changes being discussed by the IEEE-754 floating-point standards body (a new rounded addition useful as a component within reproducible summation), the BLAS standards body, and progress made since his outstanding paper with Dumitriu and Holtz. This paper, when first written, inspired me to pursue the implications of this research for hardware design with my former PhD student, Theo Drane, now at Cadence. For Theo’s thesis, we used Demmel’s work to develop a design flow for hardware implementation of polynomial evaluation, given desired relative error bounds.

Titolo discussed an abstract interpretation approach to proving numerical properties in floating-point, which is also the conceptual framework utilised by our SOAP tool. Aiming at a similar goal, my former postdoctoral researcher Victor Magron presented the approach we derived together (jointly with Donaldson) for bounding error in floating-point computation, closely aligned with the approach I initially kick-started with my former PhD student David Boland back in 2010. I’ve blogged informally about this approach before – see here. Rakamarić discussed the tool FPTaylor which also targets this problem, within the context of the SMACK toolflow developed at Utah. While she didn’t give a talk at the seminar, Eva Darulova, one of the organisers, has developed an excellent paper and tool in the same area, and it was a pleasure discussing her work with her.

There was much discussion at the workshop on the topic of tool inter-operability. Tatlock and Panchekha presented a format for numerical benchmarking, and are urging the research community to cohere around this – it could be very interesting.

There was industrial representation from both Imagination Technologies and Cadence. In Drane’s talk, he made – I believe – an important observation that the research community should take note of “in my experience, if a customer has the time to do in depth verification of their numerical hardware, they also have the time to customise their hardware.”

I had a very enjoyable few days at Dagstuhl, and I hope that we find a way to keep this community together and talking to each other.

# ARITH24: Some Highlights

Imperial College had the privilege of hosting the 24th IEEE Symposium on Computer Arithmetic (ARITH24) this week, thanks to the local arrangements work of my colleague Dr David Thomas. ARITH is the main conference dedicated solely to questions of computer arithmetic. It’s a small but active and dedicated community with – I would argue – increasing significance in the world of custom and FPGA-based accelerator computation, and I’ve enjoyed being on the Technical Programme Committee of the conference in recent years.

In this post, I describe my personal highlights from the conference.

Nick Higham from the University of Manchester gave the first keynote talk on the rise of mixed precision algorithms. This was an exciting tour de force. Nick highlighted the various floating-point precisions available in modern machines, as well as the move to low-precision computation in areas such as machine learning and ultra-high precision requirements in other areas. The key problem studied in the talk was how to solve systems of linear equations while taking advantage of the availability of mixed precision. Nick traced the idea of solution via iterative refinement of an initial approximate solution back to Wilkinson, and traced its development since then. Nick’s own recent work on this problem, in collaboration with Erin Carson, has been to introduce a method involving three different floating-point precisions. He went on to show how such an approach can produce numerical results equivalent to high precision while the bulk of the work is done in low precision. The approach uses approximate LU factorisation, followed by GMRES iterations. I found the whole approach – especially the analysis – fascinating. Moreover, it was an outstanding example of the link between algorithm development and data type selection, a link which was the topic of my own invited talk at ARITH, which I summarise below.

Gonzalez-Navarro and Hormigo presented an interesting floating-point arithmetic where normalisation is limited for efficiency reasons, and presented some empirical results from applying this to DSP tasks.

Oscar Gustafsson presented a very interesting fixed-point implementation of complex rotations, avoiding roundoff errors for low-complexity computation.

In my talk (extended abstract available here), I acted as devil’s advocate in a session organised by Martin Langhammer from Intel. Martin had invited me to give a talk putting a different perspective than “faster / better floating point.” I decided to do this by (semi-) formalising the joint problem of algorithm / data type design for numerical computation, in order to draw out the main differences between design for general purpose processors and for custom or FPGA implementations. After highlighting these from an abstract perspective, I gave a concrete example due to my PhD student Juan Jerez from a few years ago, before discussing work that is trying to automate the kind of design problems faced in this context. Such work includes bounding numerical errors, refactoring code, and in general synthesising numerical code.

Rocca, Dang, and Magron had a very interesting paper on Certified Roundoff Error Bounds using Bernstein Expansions and Sparse Krivine-Stengle Representations. This is the latest incarnation of work Magron, Donaldson and myself started together when Magron was a postdoc in my group, based in turn on the early work I did with Boland bringing automated roundoff error analysis and real algebraic geometry together. It is exciting to see this work branching off in new ways.

Pasca (Intel) and Istoan (INSA) presented a very nice approach to fixed-point function generation for FPGAs. Istoan will be joining my research group as a postdoctoral researcher in September, and I’m excited to be welcoming his expertise.

I organised a special session on Arithmetic in Digital Signal Processing on the second day of the conference. This featured interesting papers from Serre and Püschel (ETHZ) on optimal streamed linear permutations – I have been a fan of Püschel’s work since the first days of SPIRAL – as well as from Imagination Technologies (Rovers and Elliott), Linköping (Gustafsson, et al.), Intel (both Langhammer and Pasca from Intel PSG and Krishnamurthy.)

Unfortunately, I had to miss more than half of the presentations at ARITH due to an unscheduled hospital trip. So there are probably a huge number of exciting talks I have missed out on discussing here – my apologies to the authors. I will definitely be sure to prioritise ARITH attendance in future years.

# Arithmetic Workshop at Imperial

This week my research group had the pleasure of hosting Prof Miloš Ercegovac from UCLA, one of the giants of the field of computer arithmetic. Prof Ercegovac had come at my invitation to deliver a short three-day course on computer arithmetic to approx 40 delegates from Imperial College and elsewhere, including a significant delegation from UK industry.

I first encountered Miloš’s work when I was a PhD student reading around my topic, but I didn’t have the opportunity to make use of his work directly until 2014, when I published a paper with my former PhD student Kan Shi (now with Intel), demonstrating that a digit parallel form of Online Arithmetic can be used in the context of approximate computing, to provide an unexplored trade-off between clock frequency and arithmetic error. I also later developed an architecture based on online arithmetic for arbitrary precision with my BEng student Aaron Zhao (now at Cambridge).

The title of Miloš’s lectures was “An Enduring Pillar of Computer Arithmetic: Redundancy in Representation and its Uses in Algorithms and Implementations”.  I hope that in this brief blog post, I can provide you with an overview of the topics that were discussed at our workshop, aimed at a general technical reader. The best reference for most of the material, should you wish to dig further, is the book by Ercegovac and Lang.

Day 1 kicked off with a discussion of redundancy in representation (the idea that you can have more than one representation of a given number), its realisation in positional radix number systems, and the implications for addition. As a simple example, imagine working in decimal, but allowing your digits to range from -9 to 9 instead of 0 to 9 as usual. This is clearly redundant because, for example, $18 = 2\bar{2}$ where the bar over a digit indicates a negative digit. But it also has the remarkable property that addition can be performed without inducing carries, leading to the potential for very significant parallelism. I particularly enjoyed that Miloš was able to trace this idea back to the 1700s, in a paper by John Colson, published in the Philosophical Transactions of the Royal Society, entitled “A Short Account of Negativo-affirmative Arithmetick”.

Day 2 covered Miloš’s Online Arithmetic, referred to earlier. As I tell my students: when adding or multiplying two numbers, if the two numbers are big, you expect a big result, right? So why not use that information – why wait until you’ve added all the partial products from least-significant digits before producing the most significant digit. Usually, we must wait because a carry from the least significant digits can throw out the most significant. Not so with online arithmetic – taking advantage of redundancy allows one to produce multiplication and addition (and other) results most-significant-digit-first.

Day 3, the final day of our workshop, ended with a discussion of high-radix division, explaining some interesting ways that performance can be squeezed out of division by performing the division in high radices.

I would recommend anyone interested in arithmetic to get hold of Ercegovac and Lang and explore these ideas in more depth. With the rise of custom and accelerator architectures, there hasn’t been a better time to begin to re-explore some of the assumptions underlying our arithmetic hardware. Inspired readers, feel free to contact me about PhD or postdoc opportunities in this area!

# DATE 2017: Some Personal Highlights

This week I was away at the 2017 Design, Automation and Test in Europe Conference (DATE) in Lausanne, Switzerland, where my collaborator and former staff member Kevin Murray was presenting our joint work with Andrea Suardi and Vaughn Betz.

In this blog post I note a couple of personal highlights from the conference.

Apart from Kevin’s paper, which I describe in accessible form in a previous blog post, Jie Han and Marc Riedel‘s tutorial on stochastic computation was thought provoking. It’s an old idea, which I’ve seen a few times, but it was interesting to catch up with the recent thinking in this area: results on synthesis using Bernstein polynomials and a de-randomisation of the logic.

The theme of approximate computing pervaded the conference. Newcastle had an interesting paper on approximate multiplication where multiple partial products are compressed by replacing (implicit) summation with Boolean or operations. This complements more traditional approaches like I’ve worked on, where we just throw away partial products. It seems like there could be a few interesting generalisations of this approach.

It was also, of course, great to see the many colleagues I often meet at these conferences, and to spend some time in the laboratory of Paolo Ienne and meeting his PhD student Lana Josipovic, who is doing some very interesting work on high-level synthesis.

EPFL also has possibly the best underpass in the world:

DATE remains one of the best conferences to catch up with the various diverse directions in which EDA researchers have gone in recent years.

# Overclocking For Fun and Profit

This week at the Design, Automation and Test in Europe (DATE) conference, Kevin Murray is presenting some exciting work I’ve done in collaboration with Kevin, his supervisor Vaughn Betz at the University of Toronto, and Andrea Suardi at Imperial College.

I’ve been working for a while on the idea that one form of approximate computing derives from circuit overclocking. The idea is that if you overclock a circuit then this may induce some error. However the error may be small or rare, despite a very significant performance enhancement. We’ve shown, for example, that such tradeoffs make sense for image processing hardware and – excitingly – that the tradeoffs themselves can be improved by adopting “overclocking-friendly” number representations.

In the work I’ve done on this topic to date, the intuition that a given circuit is “overclocking friendly” for a certain set of input data has been a human one. In this latest paper we move to an automated approach.

Once we accept the possibility of overclocking, our circuit timing analysis has to totally change – we can’t any longer be content with bounding the worst-case delay of a circuit, because we’re aiming to violate this worst case by design. What we’re really after is a histogram of timing critical paths – allowing us to answer questions like “what’s the chance that we’ll see a critical path longer than this in any given clock period?” Different input values and different switching activities give rise to the sensitization of different paths, leading to different timing on each clock cycle.

This paper’s fundamental contribution is to show that the #SAT probem can be efficiently used to quantify these probabilities, giving rise to the first method for determining at synthesis time the affinity of a given circuit to approximation-by-overclocking.