Abstract
Designed for real-time systems, the Patmos instruction-set architecture’s features ensure a high degree of predictability. One such feature is its dual-issue pipeline, which can issue and execute bundles of up to two instructions at a time. Executing instructions in the second issue slot is a predictable way to increase the throughput of a processor, but without dedicated support from the compiler, this benefit cannot be unlocked. A compiler generates highly predictable programs by generating single-path code. This technique produces code that always follows the same trace of instructions. While Patmos’ compiler can already produce single-path code, it does not assign any instructions to the second issue-slot. This limitation is unfortunate, as single-path code inherently possesses a high degree of instruction-level parallelism. In this paper, we present a single-path code generation technique with support for dual-issue pipelines. It can also support different bundling algorithms, which allows changing algorithms without having to edit other parts of the compiler. We present a simple bundling algorithm plugged into the single-path code generator. It looks for branches and bundles the basic blocks on each path of the branch. While this specific bundling algorithm is too simple to provide a real-world benefit, it highlights the potential that further work on bundling algorithms can unlock.
Highlights
Single-path code allows time-predictable execution of real-time applications [1]
We describe the overall implementation highlighting what modifications we made to the original implementation
We implemented a simple and naive bundling algorithm to prove the single-path transformation’s effectiveness in supporting any bundling algorithm. To show that it works and can increase performance, we measure the number of cycles used when executing various benchmark programs with and without bundling
Summary
Single-path code allows time-predictable execution of real-time applications [1]. It allows for the most precise argumentation about code timing. Patmos [2], as part of the T-CREST project [3], has been specially designed to support the execution of single-path code: (1) each instruction can be predicated and (2) a dual-issue pipeline can execute code from both branches of an if-converted if/else decision. This paper presents an instruction scheduler for single-path code that uses the two issue-slots of the Patmos pipeline. Previous work produced single-path code that uses only the first issue slot [4]; this paper presents a technique for transforming the code such that the second slot is used as well. The contributions of this paper are: (1) a description of a generator for dual-issue single-path code that can support any instruction bundling algorithm, (2) an implementation of an automatic dual-issue single-path code generator in Patmos’ compiler that uses a simple proof-of-concept bundling algorithm and (3) an evaluation of the performance gain achieved over single-issue single-path code.
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