Adaptive processors can dynamically change their hardware configuration by tuning several knobs that optimize a given metric, according to the current application. However, the complexity of choosing the best setup at runtime increases exponentially as more adaptive resources become available. Therefore, we propose a polymorphic VLIW processor coupled to a machine learning-based decision mechanism that quickly and accurately delivers the best trade-off in terms of energy, performance, and reliability. The proposed system predicts the best processor configuration in 97.37% of the test cases and achieves an efficiency that is close to an oracle (more than 93.30% on all benchmarks).

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Embedded processors must efficiently deliver performance at low energy consumption. Both configurable and reconfigurable techniques can be used to fulfill such constraints, although applied in different situations. In this work, we propose DIM-VEX, a configurable processor coupled with a reconfigurable fabric, which can leverage both design time configurability and runtime reconfigurability. We show that, on average, such system can improve performance by up to 1.41X and reduce energy by up to 60% when compared to a configurable processor at the cost of additional area.

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In the design of modern-day processors, energy consumption and fault tolerance have gained significant importance next to performance. This is caused by battery constraints, thermal design limits, and higher susceptibility to errors as transistor feature sizes are decreasing. However, achieving the ideal balance among them is challenging due to their conflicting nature (e.g., fault-tolerance techniques usually influence execution time or increase energy consumption), and that is why current processor designs target at most two of these axes. Based on that, we propose a new VLIW-based processor design capable of adapting the execution of the application at run-time in a totally transparent fashion, considering performance, fault tolerance, and energy consumption altogether, in which the weight (priority) of each one can be defined a priori. This is achieved by a novel decision module that dynamically controls the application's ILP to increase the possibility of replicating instructions or applying power gating. For an energy-oriented configuration, it is possible, on average, to reduce energy consumption by 37.2% with an overhead of only 8.2% in performance, while maintaining low levels of failure rate, when compared to a fault-tolerant design.@en

Many modern FPGA-based soft-processor designs must include dedicated hardware modules to satisfy the requirements of a wide range of applications. Not seldom they all do not fit in the FPGA target, so their functionalities must be mapped into the much slower software domain. However, many complex soft-core processors usually underuse the available Block RAMs (BRAMs) when comparing to LUTs and registers. By taking advantage of this fact, we propose a generic low-cost BRAM-based function reuse mechanism (the BRAM-FR) that can be easily configured for precise or approximate modes to accelerate execution. The BRAM-FR was implemented in HDL and coupled to a configurable 4-issue VLIW processor. It was used to optimize different applications that use a soft-float library to emulate a Floating-Point Unit (FPU), and an image processing filter that tolerates a certain level of error. We show that our technique can accelerate the former by 1.23x and the latter by 1.52x, with a Reuse Table that fits in the BRAMs (that would otherwise be idle) of five tested FPGA targets with a marginal increase in the number of slice registers and LUTs.

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The well-known Triple Modular Redundancy (TMR), when applied to processors to mitigate the occurrence of faults, implies that all applications have the same level of criticality (since they are all equally protected) and are executed in a homogeneous environment, which naturally would waste precious resources in terms of area and energy. However, many current systems are composed of heterogeneous cores that implement the same ISA (e.g., ARM’s big.LITTLE or DynamIQ), executing some applications that may be more critical than others and that would require different levels of protection. With that in mind, we propose ISA-DTMR, a non-intrusive approach that, taking advantage of heterogeneous systems, can protect applications at different levels in a totally transparent fashion. By using heterogeneous multicore configurations composed of configurable processors that implement the same Instruction Set Architecture (ISA), we will show that it is possible to adapt the level of protection for each application according to its reliability requirements. When compared to homogeneous processors, ISA-DTMR reduces area by up to 54.9%, and energy consumption by 30.35%, with negligible overhead on performance, for a configuration that balances performance and energy consumption. ISA-DTMR is able to provide the same level of protection for critical applications and even improve the reliability for non-critical applications.

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Because of technology scaling, the soft error rate has been increasing in digital circuits, which affects system reliability. Therefore, modern processors, including VLIW architectures, must have means to mitigate such effects to guarantee reliable computing. In this scenario, our work proposes three low overhead fault tolerance approaches based on instruction duplication with zero latency detection, which uses a rollback mechanism to correct soft errors in the pipelanes of a configurable VLIW processor. The first uses idle issue slots within a period of time to execute extra instructions considering distinct application phases. The second works at a finer grain, adaptively exploiting idle functional units at run-time. However, some applications present high instruction-level parallelism (ILP), so the ability to provide fault tolerance is reduced: less functional units will be idle, decreasing the number of potential duplicated instructions. The third approach attacks this issue by dynamically reducing ILP according to a configurable threshold, increasing fault tolerance at the cost of performance. While the first two approaches achieve significant fault coverage with minimal area and power overhead for applications with low ILP, the latter improves fault tolerance with low performance degradation. All approaches are evaluated considering area, performance, power dissipation, and error coverage.@en

Very Long Instruction Word (VLIW) processors are commonplace in embedded systems due to their inherent lowpower consumption as the instruction scheduling is performed by the compiler instead by sophisticated and power-hungry hardware instruction schedulers used in their RISC counterparts. This is achieved by maximizing resource utilization by only targeting a certain application domain. However, when the inherent application ILP (instruction-level parallelism) is low, resources are under-utilized/wasted and the encoding of NOPs results in large code sizes and consequently additional pressure on the memory subsystem to store these NOPs. To address the resource-utilization issue, we proposed a dynamic VLIW processor design that can merge unused resources to form additional cores to execute more threads. Therefore, the formation of cores can result in issue widths of 2, 4, and 8. Without sacrificing the possibility of code interruptability and resumption, we proposed a generic binary scheme that allows a single binary to be executed on these different issue-width cores. However, the code size issue remains as the generic binary scheme even slightly further increases the number NOPS. Therefore, in this paper, we propose to apply a well-known stop-bit code compression technique to the generic binaries that, most importantly, maintains its code compatibility characteristic allowing it to be executed on different cores. In addition, we present the hardware designs to support this technique in our dynamic core. For prototyping purposes, we implemented our design on a Xilinx Virtex-6 FPGA device and executed 14 embedded benchmarks. For comparison, we selected a nondynamic/ static VLIW core that incorporates a similar stop-bit technique for its code compression. We demonstrate, while maintaining code compatibility on top of a flexible dynamic VLIW processor, that the code size can be significantly reduced (up to 80%) resulting in energy savings, and that the performance can be increased (up to a factor of three). Finally, our experimental results show that we can use smaller caches (2 to 4 times as small), which will further help in decreasing energy consumption.

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