Memory dependence prediction | EPFL Graph Search

Memory dependence prediction is a technique, employed by high-performance out-of-order execution microprocessors that execute memory access operations (loads and stores) out of program order, to predict true dependencies between loads and stores at instruction execution time. With the predicted dependence information, the processor can then decide to speculatively execute certain loads and stores out of order, while preventing other loads and stores from executing out-of-order (keeping them in-order). Later in the pipeline, memory disambiguation techniques are used to determine if the loads and stores were correctly executed and, if not, to recover. By using the memory dependence predictor to keep most dependent loads and stores in order, the processor gains the benefits of aggressive out-of-order load/store execution but avoids many of the memory dependence violations that occur when loads and stores were incorrectly executed. This increases performance because it reduces the number

High-Level Synthesis of Dynamically Scheduled Circuits

Lana Josipovic

High-Level Synthesis (HLS) tools generate hardware designs from high-level programming languages. These tools almost universally build datapaths that are controlled using a centralized controller which relies on a static, compile-time schedule to determine the cycle when each operation executes. Such an approach results in high-throughput pipelines in cases where memory accesses are provably independent and critical control decisions are determinable during code compilation. Unfortunately, when this is not the case, the tools must make pessimistic assumptions, yielding inferior schedules and lower performance. An alternative HLS approach is to create dataflow circuits out of high-level code. Dataflow circuits are built out of units which communicate using point-to-point pairs of handshake control signals; data is propagated from unit to unit as soon as memory and control dependences allow it and stalled by the handshaking mechanism otherwise. This distributed control mechanism effectively implements a dynamic schedule, where scheduling decisions are made locally in the circuit as it runs, hence achieving behaviors which are beyond the capabilities of statically scheduled circuits. Although translating high-level code into dataflow circuits seems relatively straightforward, a naive translation is not sufficient to achieve functional correctness, high performance, and area efficiency. Firstly, without appropriate buffer placement and sizing, dataflow circuits exhibit only limited pipelining capabilities. Secondly, in the absence of a static schedule, resource sharing opportunities are difficult to identify; in addition, sharing may cause deadlock and compromise the functionality of the circuit. Thirdly, memory accesses in a dataflow circuit may execute in an order different than the one specified in the original program---a naive memory interface is not always sufficient to guarantee that all memory dependences are honored. Finally, standard dataflow circuits do not support speculation, i.e., the ability to execute some operations before it is certain whether they are correct or required, which prevents pipelining when a memory or a control dependence takes a long time to resolve. The contribution of this thesis is to develop techniques that make dataflow circuits truly competitive in the HLS context. We first present a complete set of rules and transformations to create dataflow circuits out of high-level specifications (i.e., C/C++ programs). We detail a methodology to systematically place and size buffers in dataflow circuits to achieve high-throughput pipelines. We show how to automatically identify performance-acceptable resource sharing opportunities and describe a sharing mechanism which achieves functionally correct and deadlock-free dataflow designs. We detail the construction of a memory interface (i.e., a load-store queue) for dataflow circuits that can correctly handle memory accesses arriving out of order and show how to automatically customize this interface to a particular application. Further, we present a generic framework for handling speculation in dataflow circuits. Finally, we show that these techniques can reap significant area/performance benefits in appropriate situations. All these features enable dataflow circuits to achieve dynamic behaviors similar to those of modern superscalar processors; we believe that these behaviors are key for HLS to be successful in new contexts and broader application domains.

EPFL2021

From C/C plus plus Code to High-Performance Dataflow Circuits

Andrea Guerrieri, Paolo Ienne, Lana Josipovic

High-level synthesis (HLS) tools typically generate statically scheduled datapaths. Static scheduling implies that the resulting circuits have a hard time exploiting parallelism in code with potential memory dependences, with control dependences, or where performance is limited by long latency control decisions. In this work, we describe an HLS approach which generates dynamically scheduled, dataflow circuits out of imperative code. We detail a complete set of rules to transform a standard compiler intermediate representation into a high-performance dataflow circuit that is able to dynamically resolve memory dependences and adapt its behavior on the fly to particular control flow decisions and operation latencies. Compared to a traditional HLS tool, the result is a different tradeoff between performance and circuit complexity: statically scheduled circuits display the best performance per cost in regular applications, but general-purpose, irregular, and control-dominated computing tasks require the runtime flexibility of dynamic scheduling. Therefore, enabling dynamic behavior in HLS is key to dealing with the increasing computational demands of new contexts and broader application domains.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

High-Level Synthesis of Dynamically Scheduled Circuits

Lana Josipovic

EPFL2021