Section: New Results

Compiler, vectorization, interpretation

Participants : Erven Rohou, André Seznec, Sylvain Collange, Rabab Bouziane, Arif Ali Ana-Pparakkal, Stefano Cherubin, Byron Hawkins, Arif Ali Ana-Pparakkal, Imane Lasri, Kévin Le Bon.

Improving sequential performance through memoization

Participants : Erven Rohou, Imane Lasri, André Seznec.

Many applications perform repetitive computations, even when properly programmed and optimized. Performance can be improved by caching results of pure functions, and retrieving them instead of recomputing a result (a technique called memoization).

We previously proposed [23] a simple technique for enabling software memoization of any dynamically linked pure function and we illustrate our framework using a set of computationally expensive pure functions – the transcendental functions.

A restriction of the proposed framework was that memoization was restricted only to dynamically linked functions and the functions must be determined beforehand. We extended this work, and we propose function memoization using a compile-time technique thus extending the scope of memoization to user defined functions as well as making it transparently applicable to any dynamically linked functions. Our compile-time technique allows static linking of memoization code and this increases the benefit due to memoization by leveraging the inlining capability for the memoization wrapper. Our compile-time analysis can also handle functions with pointer parameters , and we handle constants more efficiently. Instruction set support can also be considered, and we propose associated hardware leading to additional performance gain.

This work was presented at the Compiler Construction Conference 2017 [50]. It is also described in the PhD thesis of Arjun Suresh [24].

Optimization in the Presence of NVRAM

Participants : Erven Rohou, Rabab Bouziane.

Beyond the fact of generating machine code, compilers play a critical role in delivering high performance, and more recently high energy efficiency. For decades, the memory technology of target systems has consisted in SRAM at cache level, and DRAM for main memory. Emerging non-volatile memories (NVMs) open up new opportunities, along with new design challenges. In particular, the asymmetric cost of read/write accesses calls for adjusting existing techniques in order to efficiently exploit NVMs. In addition, this technology makes it possible to design memories with cheaper accesses at the cost of lower data retention times. These features can be exploited at compile time to derive better data mappings according to the application and data retention characteristics. We reviewed a number of compile-time analysis and optimization techniques, and how they could apply to systems in presence of NVMs [37]. In particular, we consider the case of the reduction of the number of writes, and the analysis of variables lifetime for memory bank assignment of program variables.

Concerning the reduction of writes, we propose a fast evaluation of NVM integration at cache level, together with a compile-time approach for mitigating the penalty incurred by the high write latency of STT-RAM. We implement a code optimization in LLVM for reducing so-called silent stores, i.e., store instruction instances that write to memory values that were already present there. This makes our optimization portable over any architecture supporting LLVM. Then, we assess the possible benefit of such an optimization on the Rodinia benchmark suite through an analytic approach based on parameters extracted from the literature devoted to NVMs. This makes it possible to rapidly analyze the impact of NVMs on memory energy consumption. Reported results show up to 42 % energy gain when considering STT-RAM caches. This work is accepted for publication at RAPIDO'18 [38].

This research is done in collaboration with Abdoulaye Gamatié at LIRMM (Montpellier) within the context the the ANR project CONTINUUM.

Dynamic Binary Optimization

Participants : Erven Rohou, Arif Ali Ana-Pparakkal, Kévin Le Bon, Byron Hawkins.

Dynamic Function Specialization

Participants : Erven Rohou, Arif Ali Ana-Pparakkal, Kévin Le Bon.

Compilers can do better optimization with the knowledge of run-time behavior of the program. Function specialization is a compilation technique that consists in optimizing the body of a function for specific values of an argument. Different versions of a function are created to deal with the most frequent values of the arguments, as well as the default case. Compilers can do a better optimization with the knowledge of run-time behaviour of the program. Static compilers, however, can hardly predict the exact value/behaviour of arguments, and even profiling collected during previous runs is never guaranteed to capture future behaviour. We propose a dynamic function specialization technique, that captures the actual values of arguments during execution of the program and, when profitable, creates specialized versions and include them at runtime. Our approach relies on dynamic binary rewriting. We present [36] the principles and implementation details of our technique, analyze sources of overhead, and present our results.

This research is done within the context of the Nano 2017 PSAIC collaborative project.

Runtime Vectorization of Binary Programs

Participant : Erven Rohou.

In many cases, applications are not optimized for the hardware on which they run. Several reasons contribute to this unsatisfying situation, such as legacy code, commercial code distributed in binary form, or deployment on compute farms. In fact, backward compatibility of ISA guarantees only the functionality, not the best exploitation of the hardware. In this work, we focus on maximizing the CPU efficiency for the SIMD extensions.

We previously proposed [3] a binary-to-binary optimization framework where loops vectorized for an older version of the processor SIMD extension are automatically converted to a newer one. It is a lightweight mechanism that does not include a vectorizer, but instead leverages what a static vectorizer previously did. We showed that many loops compiled for x86 SSE can be dynamically converted to the more recent and more powerful AVX; as well as, how correctness is maintained with regards to challenges such as data dependencies and reductions. We obtained speedups in line with those of a native compiler targeting AVX.

We now focus on runtime vectorization of loops in binary codes that were not originally vectorized [29]. For this purpose, we use open source frameworks that we have tuned and integrated to

1. dynamically lift the x86 binary into the Intermediate Representation form of the LLVM compiler,

2. abstract hot loops in the polyhedral model,

3. use the power of this mathematical framework to vectorize them,

4. and finally compile them back into executable form using the LLVM Just-In-Time compiler.

In most cases, the obtained speedups are close to the number of elements that can be simultaneously processed by the SIMD unit. The re-vectorizer and auto-vectorizer are implemented inside a dynamic optimization platform; it is completely transparent to the user, does not require any rewriting of the binaries, and operates during program execution.

This work is done in collaboration with Philippe Clauss (Inria CAMUS), it is part of the PhD work of Nabil Hallou [26].

Hardware/Software JIT Compiler

Participant : Erven Rohou.

Dynamic Binary Translation (DBT) is often used in hardware/software co-design to take advantage of an architecture model while using binaries from another one. The co-development of the DBT engine and of the execution architecture leads to architecture with special support to these mechanisms. We proposed [46] a hardware accelerated dynamic binary translation where the first steps of the DBT process are fully accelerated in hardware. Results showed that using our hardware accelerators leads to a speed-up of 8$\times$ and a cost in energy 18$\times$ lower, compared with an equivalent software approach.

Single ISA-Heterogeneous multi-cores such as the ARM big.LITTLE have proven to be an attractive solution to explore different energy/performance trade-offs. Such architectures combine Out of Order cores with smaller in-order ones to offer different power/energy profiles. They however do not really exploit the characteristics of workloads (compute-intensive vs. control dominated). In our recent work, we propose to enrich these architectures with runtime configurable VLIW cores, which are very efficient at compute-intensive kernels. To preserve the single ISA programming model, we resort to Dynamic Binary Translation, and use this technique to enable dynamic code specialization for Runtime Reconfigurable VLIWs cores. Our proposed DBT framework targets the RISC-V ISA, for which both OoO and in-order implementations exist. Our experimental results show that our approach can lead to best-case performance and energy efficiency when compared against static VLIW configurations.

This work has been accepted for publication at DATE 2018 [53].

This research is done in collaboration with Steven Derrien and Simon Rokicki from the CAIRN team.

Customized Precision Computing

Participants : Erven Rohou, Stefano Cherubin, Imane Lasri.

Error-tolerating applications are increasingly common in the emerging field of real-time HPC. Proposals have been made at the hardware level to take advantage of inherent perceptual limitations, redundant data, or reduced precision input, as well as to reduce system costs or improve power efficiency. At the same time, works on floating-point to fixed-point conversion tools allow us to trade-off the algorithm exactness for a more efficient implementation. In this work [39], we aim at leveraging existing, HPC-oriented hardware architectures, while including in the precision tuning an adaptive selection of floating-and fixed-point arithmetic. Our proposed solution takes advantage of the application domain knowledge of the programmers by involving them in the first step of the interaction chain. We rely on annotations written by the programmer on the input file to know which variables of a computational kernel should be converted to fixed-point. The second stage replaces the floating-point variables in the kernel with fixed-point equivalents. It also adds to the original source code the utility functions to perform data type conversions from floating-point to fixed-point, and vice versa. The output of the second stage is a new version of the kernel source code which exploits fixed-point computation instead of floating-point computation. As opposed to typical custom-width hardware designs, we only rely on the standard 16-bit, 32-bit and 64-bit types. We also explore the impact of the fixed-point representation on auto-vectorization. We discuss the effect of our solution in terms of time-to-solutions, error and energy-to-solution.

This is done within the context of the ANTAREX project in collaboration with Stefano Cherubin, and Giovanni Agosta from Politecnico di Milano, and Olivier Sentieys from the CAIRN team.

SPMD Function Call Re-Vectorization

Participant : Sylvain Collange.

SPMD programming languages for SIMD hardware such as C for CUDA, OpenCL or ISPC have contributed to increase the programmability of SIMD accelerators and graphics processing units. However, SPMD languages still lack the flexibility offered by low-level SIMD programming on explicit vectors. To close this expressiveness gap while preserving the SPMD abstraction, we introduce the notion of Function Call Re-Vectorization (CREV). CREV allows changing the dimension of vectorization during the execution of an SPMD kernel, and exposes it as a nested parallel kernel call. CREV affords a programmability close to dynamic parallelism, a feature that allows the invocation of kernels from inside kernels, but at much lower cost. We defined a formal semantics of CREV, and implemented it on the ISPC compiler. To validate our idea, we have used CREV to implement some classic algorithms, including string matching, depth first search and Bellman-Ford, with minimum effort. These algorithms, once compiled by ISPC to Intel-based vector instructions, are as fast as state-of-the-art implementations, yet much simpler. As an example, our straightforward implementation of string matching beats the Knuth-Morris-Pratt algorithm by 12%. This work was presented at the ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (PPoPP) 2017 [45].

This work was done in collaboration with Rubens Emilio and Fernando Pereira at UFMG, as part of the Inria PROSPIEL Associate Team.

Qubit allocation for quantum circuit compilers

Participant : Sylvain Collange.

Quantum computing hardware is becoming a reality. For instance, IBM Research makes a quantum processor available in the cloud to the general public. The possibility of programming an actual quantum device has elicited much enthusiasm. Yet, quantum programming still lacks the compiler support that modern programming languages enjoy today. To use universal quantum computers like IBM's, programmers must design low-level circuits. In particular, they must map logical qubits into physical qubits that need to obey connectivity constraints. This task resembles the early days of programming, in which software was built in machine languages. We have formally introduced the qubit allocation problem and provided an exact solution to it. This optimal algorithm deals with the simple quantum machinery available today; however, it cannot scale up to the more complex architectures scheduled to appear. Thus, we also provide a heuristic solution to qubit allocation, which is faster than the current solutions already implemented to deal with this problem.

This paper is accepted for publication at the Code Generation and Optimization (CGO) conference [49].

This work was done in collaboration with Vinícius Fernandes dos Santos, Fernando Pereira and Marcos Yukio Siraichi at UFMG, Brazil.