Learn FPGA (from Verilog to HLS)

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        hls, Its full name is high level synthesis. That is to say, the circuit synthesis is completed from a higher-level language . Once upon a time , To write fpga There is only one way , It's all about writing verilog Similar hardware language . But how to use it c、c++ Write integrable logic , This becomes very important , After all c、c++ Engineer ratio fpga There are many more engineers .

1、hls Importance

         In a sense ,hls Will greatly expand the current fpga Application fields of . Comparison mcu、arm soc Come on ,fpga At present, it still focuses on signal sampling 、 Digital signal processing 、 AD conversion these scenarios . But in the field of artificial intelligence ,fpga Not much ink , An example of the opposite is gpu. Once upon a time gpu Just a graphics accelerator , Later, he made continuous efforts in the field of games and artificial , bring gpu There are more and more application fields , The scale of affiliated companies is also growing .

2、c The paradox of language and concurrency

        c Language itself is a serial code , It's not exactly the same as concurrency , In this respect , It can be regarded as a period of 1 Special fpga Code . therefore , In the process of design , The difficulty is not the language itself , But in parallel thinking .c Language itself does not directly become a netlist , It also becomes verilog Language , And then turn it into a net list . To achieve this , Will be in c Make some modifications and restrictions on the language , This is it. hls The purpose of .

3、verilog and waveform Can't throw

        For those with high timing requirements protocol,verilog Can't throw , This one uses hls It's hard to do . For general logic 、 Non standard algorithmic logic 、 Acceleration of scale algorithm , This is a hls The strengths of . Besides ,hls After transformation , How to determine hls Is it the desired effect , In addition to looking at the comprehensive effect , Also need to see the corresponding waveform, This is irreplaceable . Don't expect code to be written well , There is an immediate performance improvement effect .

4、 Write first c Code , To optimize the hls

        For software engineers , First of all c Logic is OK , Optimize step by step . The basic method of optimization is to add various directive, That is to say pragma sign . The basic methods are three ,1、 Collect data while processing ;2、 Concurrent ;3、 Assembly line . It's essentially all kinds of serial restrictions , Reduce the complexity of the algorithm latency.

5、hls Still need a sequence diagram 、 Waveform design

       hls Generally, serial processing is done by default , For example, the following code ,

for(int i = 0; i < 10; i++)
{
    b[i] = a[i] + c + d;
}

          If there is no explanation , That's basically loop body The operation code inside is executed in sequence 10 Time . If explicit acceleration is required , You can turn the loop on 、 use pipeline, In this way, we can basically speed up the processing . Acceleration is not without cost , The basic method is to exchange space for time , There is a trade-off , The algorithm may be fast , But resources may not be enough . A clever way , Is to design graphics first , etc. testbench When , Compare the test graphics with the design graphics , In this way, we can achieve twice the result with half the effort , It is also important not to over optimize .

6、 Pay attention to the interface 、 Memory 、hls Provided function

        hls How and bus Interface communication , How to map the memory in the function , Inside this hls Provide a good way . Besides , For general functions ,hls Corresponding optimizations are also provided , In especial opencv Some of the functions provided ,hls There are corresponding versions .

7、 Study hls Another way to think about it

        It is a good idea to learn concurrent programming with comparative thinking , such as openmp.openmp Itself is actually c、c++ Language , But by #pragma Can achieve the effect of concurrency , This and hls It's kind of similar .hls It is usually suitable for complex algorithm optimization , Not suitable for precise hardware protocols , If you need to put fpga Apply to more occasions , that hls At least for now, it is the only way .

8、hls Video tutorial

https://www.bilibili.com/video/BV1J5411t7uE

ps：

        Many people may find it difficult to understand , since fpga It's so annoying to do algorithms , Why use . I think it's mainly because of the low frequency fpga The performance of the algorithm can be several times that of its own soc As good as , The circuit is relatively simple , cost 、 The supply chain is not so tight . Especially for non-standard products , Especially suitable for .