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

Abstract—Multi-threaded applications execute their threads on different cores with their own local caches and need to share data among the threads. Shared caches are used to avoid lengthy and costly main memory accesses. The degree of cache sharing is a balance between reducing misses and increased hit latency. Dynamic caches have been proposed to adapt this balance to the workload type. Similarly, dynamic processors aim to execute workloads as efficient as possible to being able to balance between exploiting Instruction-level parallelism (ILP) and Thread-level parallelism (TLP). To support this, they consist of multiple processing components and caches that have adaptable interconnects between them. Depending on the workload characteristics, these can connect them together to form a large core that exploits ILP, or split them up to form multiple cores that can run multiple threads (exploiting TLP). In this paper, we propose a cache system that is able to further exploit this additional connectivity of a dynamic VLIW processor by being able to forward cache accesses to multiple cache blocks while the processor is running in multi-threaded (‘split’) mode. Additionally, only requests to global data are broadcasted, while accesses to local data are kept private. This will improve the hit rates similar to existing cache sharing schemes, but reduce the penalty due to stalling the other subcores. Local accesses are recognized by distinguishing memory accesses relative to the stack frame pointer. Results show that our cache exhibits similar miss rate reductions as shared caches (up to 90% and on average 26%), and reduces the number of broadcasted accesses by 21%.@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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