architecture specifications Figure 3. PICO's hierarchical design flow.

Chip multiprocessors with multiple simpler cores are gaining popularity because they have the potential to drive future performance gains without exacerbating the problems of power dissipation and complexity. Current chip multiprocessors increase throughput by utilizing multiple cores to perform computation in parallel. These designs provide real benefits for server-class applications that are explicitly multi-threaded. However, for desktop and other systems where single-thread applications dominate, multicore systems have yet to offer much benefit. Chip multiprocessors are most efficient at executing coarse-grain threads that have little communication. However, general-purpose applications do not provide many opportunities for identifying such threads, due to frequent use of pointers, recursive data structures, if-then-else branches, small function bodies, and loops with small trip counts. To attack this mismatch, this paper proposes a multicore architecture, referred to as Voltron, that extends traditional multicore systems in two ways. First, it provides a dual-mode scalar operand network to enable efficient inter-core communication and lightweight synchronization. Second, Voltron can organize the cores for execution in either coupled or decoupled mode. In coupled mode, the cores execute multiple instruction streams in lock-step to collectively function as a wide-issue VLIW. In decoupled mode, the cores execute a set of fine-grain communicating threads extracted by the compiler. This paper describes the Voltron architecture and associated compiler support for orchestrating bi-modal execution. 1

In this article, we present a novel linear time algorithm for data remapping, that is, (i) lightweight; (ii) fully automated; and (iii) applicable in the context of pointer-centric programming languages with dynamic memory allocation support. All previous work in this area lacks one or more of these features. We proceed to demonstrate a novel application of this algorithm as a key step in optimizing the design of an embedded memory system. Specifically, we show that by virtue of locality enhancements via data remapping, we may reduce the memory subsystem needs of an application by 50%, and hence concomitantly reduce the associated costs in terms of size, power, and dollar-investment (61%). Such a reduction overcomes key hurdles in designing highperformance embedded computing solutions. Namely, memory subsystems are very desirable from a performance standpoint, but their costs have often limited their use in embedded systems. Thus, our innovative approach offers the intriguing possibility of compilers playing a significant role in exploring and optimizing the design space of a memory subsystem for an embedded design. To this end and in order to properly leverage the improvements afforded by a compiler optimization, we identify a range of measures for quantifying the cost-impact of popular notions of locality, prefetching, regularity of memory access and others. The proposed methodology will

One of the new extensions in EPIC architectures are cache hints. On each memory instruction, two kinds of hints can be attached: a source cache hint and a target cache hint. The source hint indicates the true latency of the instruction, which is used by the compiler to improve the instruction schedule. The target hint indicates at which cache levels it is profitable to retain data, allowing to improve cache replacement decisions at run time. A compile-time method is presented which calculates appropriate cache hints. Both kind of hints are based on the locality of the instruction, measured by the reuse distance metric. Two

Modern instruction sets extend their load/store-instructions with cache hints, as an additional means to bridge the processor-memory speed gap. Cache hints are used to specify the cache level at which the data is likely to be found, as well as the cache level where the data is stored after accessing it. In order to improve a program's cache behavior, the cache hint is selected based on the data locality of the instruction. We

retargetable compilers, table-driven compilers, machine description, processor description, instruction-level parallelism, EPIC processors, VLIW processors,

retargetable compilers, table-driven compilers, machine description, processor description, instruction-level parallelism, EPIC processors, VLIW processors, EPIC compilers, VLIW compilers, code generation, scheduling, register allocation In the past, due to the restricted gate count available on an inexpensive chip, embedded DSPs have had limited parallelism, few registers and irregular, incomplete interconnectivity. More recently, with increasing levels of integration, embedded VLIW processors have started to appear. Such processors typically have higher levels of instruction-level parallelism, more registers, and a relatively regular interconnect between the registers and the functional units. The central challenges faced by a code generator for an EPIC (Explicitly Parallel Instruction Computing) or VLIW processor are quite different from those for the earlier DSPs and, consequently, so is the structure of a code generator that is designed to be easily retargetable. In this report, we explain the nature of the challenges faced by an EPIC or VLIW compiler and present a strategy for performing code generation in an incremental fashion that is best suited to generating high-quality code efficiently. We also describe the Operation Binding Lattice, a formal model for incrementally binding the opcodes and register assignments in an EPIC code generator. As we show, this reflects the phase structure of the EPIC code generator. It also defines the structure of the machine-description database, which is queried by the code generator for the information that it needs about the target processor. Lastly, we discuss general features of our implementation of these ideas and techniques in Elcor, our EPIC compiler research infrastructure.

This paper introduces a compiler-orchestrated prefetching system as a unified framework geared toward ameliorating the gap between processing speeds and memory access latencies. We focus the scope of the optimization on specific subsets of the program dependence graph that succinctly characterize the memory access pattern of both regular array-based applications and irregular pointer-intensive programs. We illustrate how program embedded precomputation via speculative execution can accurately predict and effectively prefetch future memory references with negligible overhead. The proposed techniques reduce the total running time of seven SPEC benchmarks and two OLDEN benchmarks by 27% on an Itanium 2 processor. The improvements are in addition to several state-of-the-art optimizations including software pipelining and data prefetching. In addition, we use cycle-accurate simulations to identify important and lightweight architectural innovations that further mitigate the memory system bottleneck. In particular, we focus on the notoriously challenging class of pointerchasing applications, and demonstrate how they may benefit from a novel scheme of sentineled prefetching. Our results for twelve SPEC benchmarks demonstrate that 45% of the processor stalls that are caused by the memory system are avoidable. The techniques in this paper can effectively mask long memory latencies with little instruction overhead, and can readily contribute to the performance of processors today.

Media- and telecommunications-focused processors, increasingly designed as deeply pipelined, staticallyscheduled VLIWs, rely on loop buffers for low-overhead execution of simple loops. Key loops containing control flow pose a substantial problem---full predication has a high encoding overhead, and partial predication techniques do not support if-conversion, the transformation of general acyclic control flow into predicated blocks. Using a set of significant media processing benchmarks, drawn from MediaBench and contemporary telecommunications standards, we explore a compromise approach. We demonstrate a compiler using if-conversion and specialized loop transformations to arrange for 70-99% of fetched operations to come from a simple, statically managed 256-instruction loop buffer, saving instruction fetch power and eliminating branch penalties. To complement this we introduce a "niche" form of predication specialized to permit general if-conversion with only a single bit in the encoding of each operation and to eliminate much of the hardware overhead of a predicate register-based approach.

this report, we elaborate on these claims and provide, as an example, an overview of PICO, the architecture synthesis system that the authors and their colleagues have been developing over the past five years