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

We describe the design and implementation of Dynamo, a software dynamic optimization system that is capable of transparently improving the performance of a native instruction stream as it executes on the processor. The input native instruction stream to Dynamo can be dynamically generated (by a JIT for example), or it can come from the execution of a statically compiled native binary. This paper evaluates the Dynamo system in the latter, more challenging situation, in order to emphasize the limits, rather than the potential, of the system. Our experiments demonstrate that even statically optimized native binaries can be accelerated Dynamo, and often by a significant degree. For example, the average performance of --O optimized SpecInt95 benchmark binaries created by the HP product C compiler is improved to a level comparable to their --O4 optimized version running without Dynamo. Dynamo achieves this by focusing its efforts on optimization opportunities that tend to manifest only at runtime, and hence opportunities that might be difficult for a static compiler to exploit. Dynamo's operation is transparent in the sense that it does not depend on any user annotations or binary instrumentation, and does not require multiple runs, or any special compiler, operating system or hardware support. The Dynamo prototype presented here is a realistic implementation running on an HP PA-8000 workstation under the HPUX 10.20 operating system.

This paper examines the effect of technology scaling and microarchitectural trends on the rate of soft errors in CMOS memory and logic circuits. We describe and validate an end-to-end model that enables us to compute the soft error rates (SER) for existing and future microprocessor-style designs. The model captures the effects of two important masking phenomena, electrical masking and latchingwindow masking, which inhibit soft errors in combinational logic. We quantify the SER in combinational logic and latches for feature sizes from 600nm to 50nm and clock rates from 16 to 6 fan-out-of-4 delays. Our model predicts that the SER per chip of logic circuits will increase eight orders of magnitude by the year 2011 and at that point will be comparable to the SER per chip of unprotected memory elements. Our result emphasizes the need for computer system designers to address the risks of SER in logic circuits in future designs. 1

Data dependence speculation is used in instruction-level parallel (ILP) processors to allow early execution of an instruction before a logically preceding instruction on which it may be data dependent. If the instruction is independent, data dependence speculation succeeds; if not, it fails, and the two instructions must be synchronized. The modern dynamically scheduled processors that use data dependence speculation do so blindly (i.e., every load instruction with unresolved dependences is speculated). In this paper, we demonstrate that as dynamic instruction windows get larger, significant performance benefits can result when intelligent decisions about data dependence speculation are made. We propose dynamic data dependence speculation techniques: (i) to predict if the execution of an instruction is likely to result in a data dependence mis-speculation, and (ii) to provide the synchronization needed to avoid a mis-speculation. Experimental results evaluating the effectiveness of the...

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The delay of pipeline structures in superscalar processors are studied to determine their potential for limiting clock cycle times in future designs. 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, and 0:18 m.

This paper studies techniques to improve the performance of memory consistency models for shared-memory multiprocessors with ILP processors. The first part of this paper extends earlier work by studying the impact of current hardware optimizations to memory consistency implementations, hardware-controlled non-binding prefetching and speculative load execution, on the performance of the processor consistency (PC) memory model. We find that the optimized implementation of PC performs significantly better than the best implementation of sequential consistency (SC) in some cases because PC relaxes the store-to-load ordering constraint of SC. Nevertheless, release consistency (RC) provides significant benefits over PC in some cases, because PC suffers from the negative effects of premature store prefetches and insufficient memory queue sizes. The second part of the paper proposes and evaluates a new technique, speculative retirement, to improve the performance of SC. Speculative retirement ...

Selective Dual Path Execution (SDPE) reduces branch misprediction penalties by selectively forking a second path and executing instructions from both paths following a conditional branch instruction. SDPE restricts the number of simultaneously executed paths to two, and uses a branch prediction confidence mechanism to fork selectively only for branches that are more likely to be mispredicted. A branch forking policy defines the behavior of SDPE when a low confidence branch prediction is encountered while two paths are already being executed. Trace driven simulation is used to evaluate the effectiveness of SDPE with three different forking policies. SDPE can reduce the cycles lost to branch mispredictions by 34 to 50%, resulting in an approximate 10% reduction in overall execution time. However, it is shown that both branch mispredictions and low confidence predictions tend to occur in clusters, limiting the effectiveness of SDPE. A number of design parameters are studied via simulation. These include prediction and confidence table sizes. Finally, a number of implementation issues are discussed, with emphasis on instruction fetching mechanisms and register renaming. 1.0

dynamic optimization, compiler, trace selection, binary translation © Copyright Hewlett-Packard Company 1999 Dynamic optimization refers to the runtime optimization of a native program binary. This report describes the design and implementation of Dynamo, a prototype dynamic optimizer that is capable of optimizing a native program binary at runtime. Dynamo is a realistic implementation, not a simulation, that is written entirely in user-level software, and runs on a PA-RISC machine under the HPUX operating system. Dynamo does not depend on any special programming language,

Modern out-of-order processors tolerate long latency memory operations by supporting a large number of inflight instructions. This is particularly useful in numerical applications where branch speculation is normally not a problem and where the cache hierarchy is not capable of delivering the data soon enough. In order to support more in-flight instructions, several resources have to be up-sized, such as the Reorder Buffer (ROB), the general purpose instructions queues, the Load/Store queue and the number of physical registers in the processor. However, scaling-up the number of entries in these resources is impractical because of area, cycle time, and power consumption constraints. In this paper we propose to increase the capacity of future processors by augmenting the number of in-flight instructions. Instead of simply up-sizing resources, we push for new and novel microarchitectural structures that achieve the same performance benefits but with a much lower need for resources. Our main contribution is a new checkpointing mechanism that is capable of keeping thousands of in-flight instructions at a practically constant cost. We also propose a queuing mechanism that takes advantage of the differences in waiting time of the instructions in the flow. Using these two mechanisms our processor has a performance degradation of only 10 % for SPEC2000fp over a conventional processor requiring more than an order of magnitude additional entries in the ROB and instruction queues, and about a 200 % improvement over a current processor with a similar number of entries. 1.