Branch mispredictions are inevitable in modern processors, but their high penalty in power and performance can be alleviated by selectively flushing the pipeline. In this paper, we present a practical technique to opportunistically determine the presence of correct path instructions in the processor pipeline and preserve those instructions instead of throwing them away. In our experiments, selective pipeline flush reduced the fetched instruction count by 7% (up to 26%), while increasing the performance by 2% (up to 15%).

VLIW processors typically deliver high performance on limited budget making them ideal for a variety of communication and signal processing solutions. These processors typically need large multi-ported register files that can have side effects of increased cycle time and high power consumption. The access delay and energy of these register files can also become prohibitive when increasing the register count or the access ports, thus limiting the overall performance of the processor. Most prior art circumvent this problem by using multiple clusters with private register files, to lower the access delay and reduce energy consumption. However, clustering artifacts, like increased inter--cluster communication operations and spill-recovery code, result in a performance penalty. This paper proposes CURE — a novel technique to considerably reduce the negative effects of clustering. CURE augments the ISA to expose the communication registers to the compilers to increase availability of architectural register state to all functional units. The inter--cluster communication operations are integrated into regular ALU and memory operations to improve instruction encoding efficiency. We also propose a new code scheduling heuristic to handle the ISA changes, and to realize the improvements in processor’s performance and energy consumption. Our quantitative analysis estimates that CURE, when compared to the baseline 8--issue uni--cluster processor, boosts average performance by 61% while reducing the average register dynamic energy by 77%.

With the rise of mobile and cloud-based computing, modern processor design has become the task of achieving maximum power efficiency at specific performance targets. This trend, coupled with dwindling improvements in single-threaded performance, has led architects to predominately focus on energy efficiency. In this paper we note that for the majority of benchmarks, a substantial portion of execution time is spent executing simple loops. Capitalizing on the frequency of loops, we design an out-of-order processor architecture that achieves an aggressive level of performance while minimizing the energy consumed during the execution of loops. The Revolver architecture achieves energy efficiency during loop execution by enabling “in-place execution” of loops within the processor's out-of-order backend. Essentially, a few static instances of each loop instruction are dispatched to the out-of-order execution core by the processor frontend. The static instruction instances may each be executed multiple times in order to complete all necessary loop iterations. During loop execution the processor frontend, including instruction fetch, branch prediction, decode, allocation, and dispatch logic, can be completely clock gated. Additionally we propose a mechanism to preexecute future loop iteration load instructions, thereby realizing parallelism beyond the loop iterations currently executing within the processor core. Employing Revolver across three benchmark suites, we eliminate 20, 55, and 84% of all frontend instruction dispatches. Overall, we find Revolver maintains performance, while resulting in 5.3%- 18.3% energy-delay benefit over loop buffers or micro-op cache techniques alone.

The height of the dynamic dependence graph of a program, as executed by a processor, determines the minimum bound on the execution time. This height can be decreased by reducing the effective execution latency of operations that form dependence chains in the graph. In this paper, we propose a technique called REEL to reduce overall latency of chains of dependent floating point (FP) operations by increasing the throughput of computation. REEL comprises of a high-throughput floating point unit (HFP) that allows early issue of an FP Add that is dependent on another FP Add or FP Multiply. This is complemented by instruction scheduler modifications that allow early issue of dependent FP Adds, and a novel checker logic that corrects any precision errors. Unlike conventional static operation fusion, like fused Multiply-Add (FMA), there are no changes to the instruction set to enable utilization of the new hardware, and no recompilation is necessary. Furthermore, unlike ISA-level FMA, our technique produces results that are bit compatible while boosting performance of Add-Add dependence pairs in addition to Multiply-Add pairs. Our evaluation of REEL using CFP2006 benchmarks shows an average performance gain of 7.6% and maximum performance gain of 17% while consuming 1.2% lower energy.

Modern out-of-order processors require a large number of register file access ports. However, adding more ports can drastically increase the delay, power and area of the register file. This relationship imposes constraints on existing superscalar designs while impeding implementation of faster and wider out-of-order processors. In this paper, we present a novel multi-ported register file using concepts from network coding. We split a true multi-ported register file into two interleaved banks, each having half the read and write ports. A third bank, storing the XOR of the write backs to the other two banks, is added to amplify the read and write bandwidth. When compared to a conventional register file, our 8R4W 128-entry coded CRAM register file reduces leakage power by 48%, area by 29% and delay by 9%. In addition, for SPEC2006 benchmarks, our implementation consumes 40% less register file dynamic energy on average with IPC degradation of 3%.