Combining the Three Paradigms - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

Functions and loops are the main focus of most optimizations in the user's program. Today's optimization tools typically operate at the function/procedure level. Each function can be converted into a specific hardware component. Each such hardware component is like a class definition and many objects (or instances) of this component can be created and instantiated in the eventual hardware design. Each hardware component will in turn be composed of many smaller predefined components that typically implement basic functions such as add, sub, and multiply. Functions may call other functions although recursion is not supported. Functions that are small and called less often can be also inlined into their callers just like how software functions can be inlined. In this case, the resources needed to implement the function are subsumed into the caller function's component which can potentially allow for better sharing of common resources. Constructing your design as a set of communicating functions lends to inferring parallelism when executing these functions.

Loops are one of the most important constructs in your program. Since the body of a loop is iterated over a number of times, this property can be easily exploited to achieve better parallelism. There are several transformations (such as pipelining and unrolling) that can be made to loops and loop nests in order to achieve efficient parallel execution. These transformations enable both memory-system optimizations as well as mapping to multi-core and SIMD execution resources. Many programs in science and engineering applications are expressed as operations over large data structures. These may be simple element-wise operations on arrays or matrices or more complex loop nests with loop-carried dependencies - i.e. data dependencies across the iterations of the loop. Such data dependencies impact the parallelism achievable in the loop. In many such cases, the code must be restructured such that loop iterations can be executed efficiently and in parallel on modern parallel platforms.

The following diagrams illustrate different overlapping executions for a simple example of 4 consecutive tasks (i.e., C/C++ functions) A, B, C, and D, where A produces data for B and C, in two different arrays, and D consumes data from two different arrays produced by B and C. Let us assume that this “diamond” communication pattern is to be run twice (two invocations) and that these two runs are independent.

void diamond(data_t vecIn[N], data_t vecOut[N])
   data_t c1[N], c2[N], c3[N], c4[N];
   #pragma HLS dataflow
   A(vecIn, c1, c2);
   B(c1, c3);
   C(c2, c4);
   D(c3, c4, vecOut);

The code example above shows the C/C++ source snippet for how these functions are invoked. Note that tasks B and C have no mutual data dependencies. A fully-sequential execution corresponds to the figure below where the black circles represent some form of synchronization used to implement the serialization.

Figure 1. Sequential Execution - Two Runs

In the diamond example, B and C are fully-independent. They do not communicate nor do they access any shared memory resource, and so if no sharing of computation resource is required, they can be executed in parallel. This leads to the diagram in the figure below, with a form of fork-join parallelism within a run. B and C are executed in parallel after A ends while D waits for both B and C, but the next run is still executed in series.

Figure 2. Task Parallelism within a Run

Such an execution can be summarized as (A; (B || C); D); (A; (B || C); D) where “;” represents serialization and “||” represents full parallelism. This form of nested fork-join parallelism corresponds to a subclass of dependent tasks, namely series-parallel task graphs. More generally, any DAG (directed acyclic graph) of dependent tasks can be implemented with separate fork-and-join-type synchronization. Also, it is important to note that this is exactly like how a multithreaded program would run on a CPU with multiple threads and using shared memory.

On FPGAs, you can explore what other forms of parallelism are available. The previous execution pattern exploited task-level parallelism within an invocation. What about overlapping successive runs? If they are truly independent, but if each function (i.e., A, B, C, or D) reuses the same computation hardware as for its previous run, we may still want to execute, for example, the second invocation of A in parallel with the first invocations of B and C. This is a form of task-level pipelining across invocations, leading to a diagram as depicted in the following figure. The throughput is now improved because it is limited by the maximum latency among all tasks, rather than by the sum of their latencies. The latency of each run is unchanged but the overall latency for multiple runs is reduced.

Figure 3. Task Parallelism with Pipelining

Now, however, when the first run of B reads from the memory where A placed its first result, the second run of A is possibly already writing in the same memory. To avoid overwriting the data before it is consumed, you can rely on a form of memory expansion, namely double buffering or PIPOs to allow for this interleaving. This is represented by the black circles between the tasks.

An efficient technique to improve throughput and reuse computational resources is to pipeline operators, loops, and/or functions. If each task can now overlap with itself, you can achieve simultaneously task parallelism within a run and task pipelining across runs, both of which are examples of macro-level parallelism. Pipelining within the tasks is an example of micro-level parallelism. The overall throughput of a run is further improved because it now depends on the minimum throughput among the tasks, rather than their maximum latency. Finally, depending on how the communicated data are synchronized, only after all are produced (PIPOs) or in a more element-wise manner (FIFOs), some additional overlapping within a run can be expected. For example, in the following figure, both B and C start earlier and are executed in a pipelined fashion with respect to A, while D is assumed to still have to wait for the completion of B and C. This last type of overlap within a run can be achieved if A communicates to B and C through FIFO streaming accesses (represented as lines without circles). Similarily, D can also be overlapped with B and C, if the channels are FIFOs instead of PIPOs. However, unlike all previous execution patterns, using FIFOs can lead to deadlocks and so these streaming FIFOs need to be sized correctly.

Figure 4. Task Parallelism and Pipelining within a Run, Pipelining of Runs, and Pipelining within a Task

In summary, the three paradigms presented in the earlier section show how parallelism can be achieved in your design without needing the complexities of multi-threading and/or parallel programming languages. The producer-consumer paradigm coupled with streaming channels allows for the composition of small to large scale systems easily. As mentioned before, streaming interfaces allow for easy coupling of parallel tasks or even hierarchical dataflow networks. This is in part due to the flexibility in the programming language (C/C++) to support such specifications and the tools to implement them on the heterogeneous computing platform available on today's FPGA devices.