By Adam Taylor

So far on this journey, we have significantly increased performance by accelerating matrix multiplication using the Zynq SoC’s PL (programmable logic). However if you look at the code within the mmult() function, you will notice that not only is the function implemented in C++ but it also contains a number of pragmas. A pragma is a directive that tells the compiler—or SDSoC in this case—how it should process its input. Exploring these pragmas and their role in increasing performance provides a great starting point for looking at HLS (high-level synthesis) and how it is used within SDSoC and Vivado HLS.

We use pragmas to control optimizations and synthesis behavior within SDSoC in the same way we use them when developing designs using standard SoC/FPGA coding methods.

The code above contains four pragmas. However, what is their role and what do they achieve?

Here’s what each of the four above pragmas accomplish:

#pragma HLS INLINE self – The INLINE command removes any hierarchy within a function, which allows for optimizations across boundaries. This is similar to setting the flatten hierarchy option in HDL synthesis.

#pragma HLS PIPELINE II=1 – Pipelining is the way FPGA engineers have traditionally increased the operating frequency of a circuit, while also increasing the latency. SDSoC pipelining behaves in a similar manner, allowing us to pipeline functions or loops. Within the example code above, the pipeline pragma is located before the final loop to ensure that HLS will unroll the innermost loop and perform all the multiplications at once. Within this pragma, the immediately following “II” refers to “initiation interval.” This parameter defines a function’s or loop’s target number of clock cycles for processing a new input. Setting this parameter to 1 means the function or loop will process a new input every clock cycle. In this example, the result is parallel multiplications. HLS will attempt to achieve this specified target. If it cannot, it will create a design with the lowest possible initiation interval and issue a warning, allowing us to analyze and optimise the design to try and achieve the required initiation interval.

#pragma HLS array_partition – This pragma partitions an array into small elements (using two pragmas for in_a and in_b in the above example). A partitioned array allows for multiple simultaneous reads and writes, which potentially increases throughput. Array partitioning is important in the example application above because we have unrolled the last “for” loop and are performing all multiplications in the Zynq SoC’s PL hardware at the same time. The partitioning “factor” of 16 specifies that 16 arrays are to be created while the “dim” parameter defines the dimension of the array to be partitioned. The specified value of 2 creates a two-dimensional array.

Use of these pragmas demonstrates an optimal solution for this example. Over the next few blogs, we will begin to look at Vivado HLS and how we can develop our application to ensure that we obtain the best results when SDSoC calls the high-level synthesis tool as part of the build process.

Note: In some documentation you may see AP used in place of HLS. The two are equivalent. However the term AP is obsolete and should not be used going forward.

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Please see the previous entries in this MicroZed Chronicles series by Adam Taylor:

Adam Taylor’s MicroZed Chronicles Part 88: SDSoC Part 4—a look under the hood

Adam Taylor’s MicroZed Chronicles Part 87: Getting SDSoC up and running Part 3

Adam Taylor’s MicroZed Chronicles Part 86: Getting SDSoC up and running

Adam Taylor’s MicroZed Chronicles Part 85: SDSoC—the first instalment

Adam Taylor’s MicroZed(ish) Chronicles Part 84: Simple Communication Interfaces Part 4

Adam Taylor’s MicroZed(ish) Chronicles Part 83: Simple Communication Interfaces Part 3

Adam Taylor’s MicroZed(ish) Chronicles Part 82: Simple Communication Interfaces Part 2

Adam Taylor’s MicroZed(ish) Chronicles Part 81: Simple Communication Interfaces

Adam Taylor’s MicroZed Chronicles Part 80: LWIP Stack Configuration

Adam Taylor’s MicroZed Chronicles Chronicles Part 79: Zynq SoC Ethernet Part III

Adam Taylor’s MicroZed Chronicles Chronicles Part 78: Zynq SoC Ethernet Part II

Adam Taylor’s MicroZed Chronicles Microzed Chronicles Part 77 – Introducing the Zynq SoC’s Ethernet

Adam Taylor’s MicroZed Chronicles Part 76: Constraints for Relatively Placed Macros

Adam Taylor’s MicroZed Chronicles, Part 75: Placement Constraints – Pblocks

Adam Taylor’s MicroZed Chronicles, Part 73: Physical Constraints

Adam Taylor’s MicroZed Chronicles, Part 73: Working with other Zynq-Based Boards

Adam Taylor’s MicroZed Chronicles, Part 72: Multi-cycle Constraints

Adam Taylor’s MicroZed Chronicles, Part 70: Constraints—Clock Relationships and Avoiding Metastability

Adam Taylor’s MicroZed Chronicles, Part 70: Constraints—Introduction to timing and defining a clock

Adam Taylor’s MicroZed Chronicles Part 69: Zynq SoC Constraints Overview

Adam Taylor’s MicroZed Chronicles Part 68: AXI DMA Part 3, the Software

Adam Taylor’s MicroZed Chronicles Part 67: AXI DMA II

Adam Taylor’s MicroZed Chronicles Part 66: AXI DMA

Adam Taylor’s MicroZed Chronicles Part 65: Profiling Zynq Applications II

Adam Taylor’s MicroZed Chronicles Part 64: Profiling Zynq Applications

Adam Taylor’s MicroZed Chronicles Part 63: Debugging Zynq Applications

Adam Taylor’s MicroZed Chronicles Part 62: Answers to a question on the Zynq XADC

Adam Taylor’s MicroZed Chronicles Part 61: PicoBlaze Part Six

Adam Taylor’s MicroZed Chronicles Part 60: The Zynq and the PicoBlaze Part 5—controlling a CCD

Adam Taylor’s MicroZed Chronicles Part 59: The Zynq and the PicoBlaze Part 4

Adam Taylor’s MicroZed Chronicles Part 58: The Zynq and the PicoBlaze Part 3

Adam Taylor’s MicroZed Chronicles Part 57: The Zynq and the PicoBlaze Part Two

Adam Taylor’s MicroZed Chronicles Part 56: The Zynq and the PicoBlaze

Adam Taylor’s MicroZed Chronicles Part 55: Linux on the Zynq SoC

Adam Taylor’s MicroZed Chronicles Part 54: Peta Linux SDK for the Zynq SoC

Adam Taylor’s MicroZed Chronicles Part 53: Linux and SMP

Adam Taylor’s MicroZed Chronicles Part 52: One year and 151,000 views later. Big, Big Bonus PDF!

Adam Taylor’s MicroZed Chronicles Part 51: Interrupts and AMP

Adam Taylor’s MicroZed Chronicles Part 50: AMP and the Zynq SoC’s OCM (On-Chip Memory)

Adam Taylor’s MicroZed Chronicles Part 49: Using the Zynq SoC’s On-Chip Memory for AMP Communications

Adam Taylor’s MicroZed Chronicles Part 48: Bare-Metal AMP (Asymmetric Multiprocessing)

Adam Taylor’s MicroZed Chronicles Part 47: AMP—Asymmetric Multiprocessing on the Zynq SoC

Adam Taylor’s MicroZed Chronicles Part 46: Using both of the Zynq SoC’s ARM Cortex-A9 Cores

Adam Taylor’s MicroZed Chronicles Part 44: MicroZed Operating Systems—FreeRTOS

Adam Taylor’s MicroZed Chronicles Part 43: XADC Alarms and Interrupts

Adam Taylor’s MicroZed Chronicles MicroZed Part 42: MicroZed Operating Systems Part 4

Adam Taylor’s MicroZed Chronicles MicroZed Part 41: MicroZed Operating Systems Part 3

Adam Taylor’s MicroZed Chronicles MicroZed Part 40: MicroZed Operating Systems Part Two

Adam Taylor’s MicroZed Chronicles MicroZed Part 39: MicroZed Operating Systems Part One

Adam Taylor’s MicroZed Chronicles MicroZed Part 38 – Answering a question on Interrupts

Adam Taylor’s MicroZed Chronicles Part 37: Driving Adafruit RGB NeoPixel LED arrays with MicroZed Part 8

Adam Taylor’s MicroZed Chronicles Part 36: Driving Adafruit RGB NeoPixel LED arrays with MicroZed Part 7

Adam Taylor’s MicroZed Chronicles Part 35: Driving Adafruit RGB NeoPixel LED arrays with MicroZed Part 6

Adam Taylor’s MicroZed Chronicles Part 34: Driving Adafruit RGB NeoPixel LED arrays with MicroZed Part 5

Adam Taylor’s MicroZed Chronicles Part 33: Driving Adafruit RGB NeoPixel LED arrays with the Zynq SoC

Adam Taylor’s MicroZed Chronicles Part 32: Driving Adafruit RGB NeoPixel LED arrays

Adam Taylor’s MicroZed Chronicles Part 31: Systems of Modules, Driving RGB NeoPixel LED arrays

Adam Taylor’s MicroZed Chronicles Part 30: The MicroZed I/O Carrier Card

Zynq DMA Part Two – Adam Taylor’s MicroZed Chronicles Part 29

The Zynq PS/PL, Part Eight: Zynq DMA – Adam Taylor’s MicroZed Chronicles Part 28

The Zynq PS/PL, Part Seven: Adam Taylor’s MicroZed Chronicles Part 27

The Zynq PS/PL, Part Six: Adam Taylor’s MicroZed Chronicles Part 26

The Zynq PS/PL, Part Five: Adam Taylor’s MicroZed Chronicles Part 25

The Zynq PS/PL, Part Four: Adam Taylor’s MicroZed Chronicles Part 24

The Zynq PS/PL, Part Three: Adam Taylor’s MicroZed Chronicles Part 23

The Zynq PS/PL, Part Two: Adam Taylor’s MicroZed Chronicles Part 22

The Zynq PS/PL, Part One: Adam Taylor’s MicroZed Chronicles Part 21

Introduction to the Zynq Triple Timer Counter Part Four: Adam Taylor’s MicroZed Chronicles Part 20

Introduction to the Zynq Triple Timer Counter Part Three: Adam Taylor’s MicroZed Chronicles Part 19

Introduction to the Zynq Triple Timer Counter Part Two: Adam Taylor’s MicroZed Chronicles Part 18

Introduction to the Zynq Triple Timer Counter Part One: Adam Taylor’s MicroZed Chronicles Part 17

The Zynq SoC’s Private Watchdog: Adam Taylor’s MicroZed Chronicles Part 16

Implementing the Zynq SoC’s Private Timer: Adam Taylor’s MicroZed Chronicles Part 15

MicroZed Timers, Clocks and Watchdogs: Adam Taylor’s MicroZed Chronicles Part 14

More About MicroZed Interrupts: Adam Taylor’s MicroZed Chronicles Part 13

MicroZed Interrupts: Adam Taylor’s MicroZed Chronicles Part 12

Using the MicroZed Button for Input: Adam Taylor’s MicroZed Chronicles Part 11

Driving the Zynq SoC's GPIO: Adam Taylor’s MicroZed Chronicles Part 10

Meet the Zynq MIO: Adam Taylor’s MicroZed Chronicles Part 9

MicroZed XADC Software: Adam Taylor’s MicroZed Chronicles Part 8

Getting the XADC Running on the MicroZed: Adam Taylor’s MicroZed Chronicles Part 7

A Boot Loader for MicroZed. Adam Taylor’s MicroZed Chronicles, Part 6

Figuring out the MicroZed Boot Loader – Adam Taylor’s MicroZed Chronicles, Part 5

Running your programs on the MicroZed – Adam Taylor’s MicroZed Chronicles, Part 4

Zynq and MicroZed say “Hello World”-- Adam Taylor’s MicroZed Chronicles, Part 3

Adam Taylor’s MicroZed Chronicles: Setting the SW Scene

Bringing up the Avnet MicroZed with Vivado