The performance tradeoff between hardware complexity and clock speed is studied. First, a generic superscalar pipeline is defined. Then the specific areas of register renaming, instruction window wakeup and selection logic, and operand bypassing are analyzed. Each is modeled and Spice simulated for feature sizes of 0:8 m, 0:35 m, and0:18 m. Performance results and trends are expressed in terms of issue width and window size. Our analysis indicates that window wakeup and selection logic as well as operand bypass logic are likely to be the most critical in the future. A microarchitecture that simplifies wakeup and selection logic is proposed and discussed. This implementation puts chains of dependent instructions into queues, and issues instructions from multiple queues in parallel. Simulation shows little slowdown as compared with a completely flexible issue window when performance is measured in clock cycles. Furthermore, because only instructions at queue heads need to be awakened and selected, issue logic is simplified and the clock cycle is faster – consequently overall performance is improved. By grouping dependent instructions together, the proposed microarchitecture will help minimize performance degradation due to slow bypasses in future wide-issue machines. 1

The multicluster architecture that we introduce offers a decentralized, dynamically-scheduled architecture, in which the register files, dispatch queue, and functional units of the architecture are distributed across multiple clusters, and each cluster is assigned a subset of the architectural registers. The motivation for the multicluster architecture is to reduce the clock cycle time, relative to a single-cluster architecture with the same number of hardware resources, by reducing the size and complexity of components on critical timing paths. Resource partitioning, however, introduces instruction-execution overhead and may reduce the number of concurrently executing instructions. To counter these two negative by-products of partitioning, we developed a static instruction scheduling algorithm. We describe this algorithm, and using tracedriven simulations of SPEC92 benchmarks, evaluate its effectiveness. This evaluation indicates that for the configurations considered, the multicluste...

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Recently, there has been a trend towards clustered microarchitectures to reduce the cycle time for wideissue microprocessors. In such processors, the register file and functional units are partitioned and grouped into clusters. Instruction scheduling for a clustered machine requires assignment and scheduling of operations to the clusters. In this paper, a new scheduling algorithm named unified-assign-and-schedule (UAS) is proposed for clustered, statically-scheduled architectures. UAS merges the cluster assignment and instruction scheduling phases in a natural and straightforward fashion. We compared the performance of UAS with various heuristics to the well-known Bottom-up Greedy (BUG) algorithm and to an optimal cluster scheduling algorithm, measuring the schedule lengths produced by all of the schedulers. Our results show that UAS gives better performance than the BUG algorithm and is quite close to optimal. 1 Introduction Many high performance processors are designed with wide is...

Clustered microarchitectures are an effective approach to reducing the penalties caused by wire delays inside a chip. Current superscalar processors have in fact a two-cluster microarchitecture with a naive code partitioning approach: integer instructions are allocated to one cluster and floating-point instructions to the other. This partitioning scheme is simple and results in no communications between the two clusters (just through memory) but it is in general far from optimal because the workload is not evenly distributed most of the time. In fact, when the processor is running integer programs, the workload is extremely unbalanced since the FP cluster is not used at all. In this work we investigate run-time mechanisms that dynamically distribute the instructions of a program among these two clusters. By optimizing the trade-off between inter-cluster communication penalty and workload balance, the proposed schemes can achieve an average speed-up of 36 % for the SpecInt95 benchmark suite.

Accurate branch prediction is essential for obtaining high performance in pipelined superscalar processors that execute instructions speculatively. Some of the best current predictors combine a part of the branch address with a fixed amount of global history of branch outcomes in order to make a prediction. These predictors cannot perform uniformly well across all workloads because the best amount of history to be used depends on the code, the input data and the frequency of context switches. Consequently, all predictors that use a fixed history length are therefore unable to perform up to their maximum potential. We introduce a method —called DHLF — that dynamically determines the optimum history length during execution, adapting to the specific requirements of any code, input data and system workload. Our proposal adds an extra level of adaptivity to two-level adaptive branch predictors. The DHLF method can be applied to any one of the predictors that combine global branch history with the branch address. We apply the DHLF method to gshare (dhlf-gshare) and obtain near-optimal results for all SPECint95 benchmarks, with and without context switches. Some results are also presented for gskewed (dhlf-gskewed), confirming that other predictors can benefit from our proposal. 1.

The key issues for register file design in high-performance processors are access time and energy. While previous work has focused on reducing the number of registers, we propose to reduce the number of register ports through two proposals, one for reads and the other for writes. For reads, we propose bypass hint to reduce register port requirements by avoiding unnecessary register file reads for cases where values are bypassed. Current processors are unable to avoid these unnecessary reads due to timing constraints. For writes, we use register file banking. Current banking schemes assign different banks to instructions that are renamed together, which does not necessarily avoid conflicts among instructions that writeback together. We use decoupled rename, a technique which separates dependence and physical tagging of register operands. Decoupled rename allows us to perform physical register allocation just before writeback, avoiding bank conflicts. Our results show that combining bypass hint and write banking, our 1cycle register file with 6 read ports, and two 4-write-ported banks achieves a 9 % processor energy-delay savings over a system using a perfectly-pipelined, 2-cycle register file with 16 read ports and 8 write ports. 1

Vector Microprocessors by Krste Asanovic Doctor of Philosophy in Computer Science University of California, Berkeley Professor John Wawrzynek, Chair Most previous research into vector architectures has concentrated on supercomputing applications and small enhancements to existing vector supercomputer implementations. This thesis expands the body of vector research by examining designs appropriate for single-chip full-custom vector microprocessor implementations targeting a much broader range of applications. I present the design, implementation, and evaluation of T0 (Torrent-0): the first single-chip vector microprocessor. T0 is a compact but highly parallel processor that can sustain over 24 operations per cycle while issuing only a single 32-bit instruction per cycle. T0 demonstrates that vector architectures are well suited to full-custom VLSI implementation and that they perform well on many multimedia and human-machine interface tasks. The remainder of the thesis contains ...

Lower threshold voltages in deep sub-micron technologies cause more leakage current, increasing static power dissipation. This trend, combined with the trend of larger/more cache memories dominating die area, has prompted circuit designers to develop SRAM cells with low-leakage operating modes (e.g., sleep mode). Sleep mode reduces static power dissipation but data stored in a sleeping cell is unreliable or lost. So, at the architecture level, there is interest in exploiting sleep mode to reduce static power dissipation while maintaining high performance. Current

Many emerging processor microarchitectures seek to manage technological constraints (e.g., wire delay, power, and circuit complexity) by resorting to nonuniform designs that provide resources at multiple quality levels (e.g., fast/slow bypass paths, multi-speed functional units, and grid architectures). In such designs, the constraint problem becomes a control problem, and the challenge becomes designing a control policy that mitigates the performance penalty of the non-uniformity. Given the increasing importance of non-uniform control policies, we believe it is appropriate to examine them in their own right. To this end, we develop slack for use in creating control policies that match program execution behavior to machine design. Intuitively, the slack of a dynamic instruction i is the number of cycles i can be delayed with no effect on execution time. This property makes slack a natural candidate for hiding non-uniform latencies. We make three contributions in our exploration of slack. First, we formally define slack, distinguish three variants (local, global and apportioned), and perform a limit study to show that slack is prevalent in our SPEC2000 workload. Second, we show how to predict slack in hardware. Third, we illustrate how to create a control policy based on slack for steering instructions among fast (high power) and slow (lower power) pipelines.