Code Generation for an Application-Specific VLIW Processor With Clustered, Addressable Register Files

International audienceModern compilers integrate recent advances in compiler construction, intermediate representations, algorithms and programming language front-ends. Yet code generation for appli\-cation-specific architectures benefits only marginally from this trend, as most of the effort is oriented towards popular general-purpose architectures. Historically, non-orthogonal architectures have relied on custom compiler technologies, some retargettable, but largely decoupled from the evolution of mainstream tool flows. Very Long Instruction Word (VLIW) architectures have introduced a variety of interesting problems such as clusterization, packetization or bundling, instruction scheduling for exposed pipelines, long delay slots, software pipelining, etc. These have been addressed in the literature, with a focus on the exploitation of Instruction Level Parallelism (ILP). While these are well known solutions already embedded into existing compilers, they rely on common hardware functionalities that are expected to be present in a fairly large subset of VLIW architectures. This paper presents our work on back-end compiler for Mephisto, a high performance low-power application-specific processor, based on LLVM. Mephisto is specialized enough to challenge established code generation solutions for VLIW and DSP processors, calling for an innovative compilation flow. Conversely, even though Mephisto might be seen a somewhat exotic processor, its hardware characteristics such as addressable register files benefit from existing analyses and transformations in LLVM. We describe our model of the Mephisto architecture, the difficulties we encountered, and the associated compilation methods, some of them new and specific to Mephisto

CABAC accelerator architectures for video compression in future multimedida : a survey

The demands for high quality, real-time performance and multi-format video support in consumer multimedia products are ever increasing. In particular, the future multimedia systems require efficient video coding algorithms and corresponding adaptive high-performance computational platforms. The H.264/AVC video coding algorithms provide high enough compression efficiency to be utilized in these systems, and multimedia processors are able to provide the required adaptability, but the algorithms complexity demands for more efficient computing platforms. Heterogeneous (re-)configurable systems composed of multimedia processors and hardware accelerators constitute the main part of such platforms. In this paper, we survey the hardware accelerator architectures for Context-based Adaptive Binary Arithmetic Coding (CABAC) of Main and High profiles of H.264/AVC. The purpose of the survey is to deliver a critical insight in the proposed solutions, and this way facilitate further research on accelerator architectures, architecture development methods and supporting EDA tools. The architectures are analyzed, classified and compared based on the core hardware acceleration concepts, algorithmic characteristics, video resolution support and performance parameters, and some promising design directions are discussed. The comparative analysis shows that the parallel pipeline accelerator architecture seems to be the most promising

HD-VideoBench: A benchmark for evaluating high definition digital video applications

HD-VideoBench is a benchmark devoted to high definition (HD) digital video processing. It includes a set of video encoders and decoders (Codecs) for the MPEG-2, MPEG-4 and H.264 video standards. The applications were carefully selected taken into account the quality and portability of the code, the representativeness of the video application domain, the availability of high performance optimizations and the distribution under a free license. Additionally, HD-VideoBench defines a set of input sequences and configuration parameters of the video Codecs which are appropriate for the HD video domain.Peer ReviewedPostprint (published version

VLSI architecture design approaches for real-time video processing

This paper discusses the programmable and dedicated approaches for real-time video processing applications. Various VLSI architecture including the design examples of both approaches are reviewed. Finally, discussions of several practical designs in real-time video processing applications are then considered in VLSI architectures to provide significant guidelines to VLSI designers for any further real-time video processing design works

A highly scalable parallel implementation of H.264

Developing parallel applications that can harness and efficiently use future many-core architectures is the key challenge for scalable computing systems. We contribute to this challenge by presenting a parallel implementation of H.264 that scales to a large number of cores. The algorithm exploits the fact that independent macroblocks (MBs) can be processed in parallel, but whereas a previous approach exploits only intra-frame MB-level parallelism, our algorithm exploits intra-frame as well as inter-frame MB-level parallelism. It is based on the observation that inter-frame dependencies have a limited spatial range. The algorithm has been implemented on a many-core architecture consisting of NXP TriMedia TM3270 embedded processors. This required to develop a subscription mechanism, where MBs are subscribed to the kick-off lists associated with the reference MBs. Extensive simulation results show that the implementation scales very well, achieving a speedup of more than 54 on a 64-core processor, in which case the previous approach achieves a speedup of only 23. Potential drawbacks of the 3D-Wave strategy are that the memory requirements increase since there can be many frames in flight, and that the frame latency might increase. Scheduling policies to address these drawbacks are also presented. The results show that these policies combat memory and latency issues with a negligible effect on the performance scalability. Results analyzing the impact of the memory latency, L1 cache size, and the synchronization and thread management overhead are also presented. Finally, we present performance requirements for entropy (CABAC) decoding. This work was performed while the fourth author was with NXP Semiconductors.Peer ReviewedPostprint (author's final draft

Ordonnancement pour processeurs à parallélisme d'instructions en utilisant des techniques de recherche de motifs

Dans le but de satisfaire les différentes contraintes matérielles, une exploration architecturale peut permettre de définir les paramètres optimaux d'un processeur VLIW (Very Long Instruction Word) pour une application donnée tels que le nombre d'unités fonctionnelles, le nombre de registres, etc. Les paramètres du processeur sont ajustés en fonction du niveau de parallélisme d'instructions de l'application. De la même manière, l'utilisation de jeux d'instructions spécifiques à une application est adaptée à une utilisation au sein des systèmes embarqués qui sont, dans la majeure partie des cas, dédiés à un traitement spécifique. Toutes ces spécialisations permettent d'améliorer efficacement le rapport entre performance, surface et consommation. Ce rapport présente un nouvel outil dont le but est de définir les paramètres optimaux d'un processeur de type VLIW pour une application donnée en termes de dimensionnement, d'organisation et de spécialisation de son jeu d'instructions. Cet outil repose sur la modélisation des problèmes à résoudre en utilisant la programmation par contraintes et exploite la technique de couverture de graphe à l'aide de motifs de calculs. Différentes structures de processeurs pourront alors être comparées en termes de performance et de complexité matérielle

Coarse-grained reconfigurable array architectures

Coarse-Grained Reconfigurable Array (CGRA) architectures accelerate the same inner loops that benefit from the high ILP support in VLIW architectures. By executing non-loop code on other cores, however, CGRAs can focus on such loops to execute them more efficiently. This chapter discusses the basic principles of CGRAs, and the wide range of design options available to a CGRA designer, covering a large number of existing CGRA designs. The impact of different options on flexibility, performance, and power-efficiency is discussed, as well as the need for compiler support. The ADRES CGRA design template is studied in more detail as a use case to illustrate the need for design space exploration, for compiler support and for the manual fine-tuning of source code

Performance impact of unaligned memory operations in SIMD extensions for video CODEC applications

Although SIMD extensions are a cost effective way to exploit the data level parallelism present in most media applications, we will show that they had have a very limited memory architecture with a weak support for unaligned memory accesses. In video codec, and other applications, the overhead for accessing unaligned positions without an efficient architecture support has a big performance penalty and in some cases makes vectorization counter-productive. In this paper we analyze the performance impact of extending the Altivec SIMD ISA with unaligned memory operations. Results show that for several kernels in the H.264/AVC media codec, unaligned access support provides a speedup up to 3.8times compared to the plain SIMD version, translating into an average of 1.2times in the entire application. In addition to providing a significant performance advantage, the use of unaligned memory instructions makes programming SIMD code much easier both for the manual developer and the auto vectorizing compilerPeer ReviewedPostprint (published version

A tuneable software cache coherence protocol for heterogeneous MPSoCs

ABSTRACT In a multiprocessor system-on-chip (MPSoC) private caches introduce the cache coherence problem. Here, we target at heterogeneous MPSoCs with a network-on-chip (NoC). Existing hardware cache coherence protocols are less suitable for MPSoCs because many off-the-shelf processors used in MPSoCs do not support these protocols. Furthermore, these protocols typically rely on global visibility and serialization of writes which does not match well with the parallel pointto-point communication provided by a NoC. Therefore, we propose a software cache coherence protocol, which can be applied in a heterogeneous MPSoC with a NoC. The software cache coherence protocol relies on explicit synchronization in the software. More specifically, caches are guaranteed to be coherent according to the Release Consistency model, on top of which we have implemented the standard Pthreads communication library. Heterogeneous MPSoCs with off-the-shelf processors can easily be supported, because processors are only required to provide cache control operations, e.g., clean and invalidate. All cache coherence operations are interruptible and do not impact the execution of tasks on other processors, therefore this protocol is suitable for predictable MPSoCs. Our software cache coherence protocol is implemented on an ARM926EJ-S MPSoC which is mapped on an FPGA. From experiments we conclude that the protocol overhead is low for the applications taken from the SPLASH-2 benchmark set. For these applications we observed a speedup between 1.89 and 2.01 on the two processor MPSoC