Loop Dependency Inter

This Example demonstrates the HLS pragma ‘DEPENDENCE’.Using ‘DEPENDENCE’ pragma, user can provide additional dependency details to the compiler by specifying if the dependency in consecutive loop iterations on buffer is true/false, which allows the compiler to perform unrolling/pipelining to get better performance.

KEY CONCEPTS: Inter Dependence

KEYWORDS: DEPENDENCE, inter, WAR

This example demonstrates how extra information about dependency can be specified for variables in consecutive loop iterations using #pragma HLS DEPENDENCE. It allows the V++ to recognize dependencies and optimize the loop executions.

For example here linebuf has false inter dependency which means each iteration of loop is independent for linebuf access.

#pragma HLS DEPENDENCE allows compiler to ignore dependency of linebuf and generate a pipeline with lower II count. If user does not provide this pragma, by default compiler assumes dependency exists on linebuf between iterations. As a result, compiler schedules unoptimized design with larger II factor which in turn leads to slow performance.

#pragma HLS ARRAY_PARTITION variable = linebuf dim = 1 complete
VConvH:
    for (int col = 0; col < height; ++col) {
    VConvW:
        for (int row = 0; row < width; ++row) {
           #pragma HLS DEPENDENCE variable=linebuf inter false
            int in_val = inStream.read();
            int out_val = 0;
        VConv:
            for (int i = 0; i < K; i++) {
                int vwin_val = i < (K - 1) ? linebuf[i][row] : in_val;
                out_val += vwin_val * vcoeff[i];

                if (i > 0)
                    linebuf[i - 1][row] = vwin_val;
            }
            outStream << out_val;
        }
    }
}

EXCLUDED PLATFORMS:

• Embedded ZC702

• All NoDMA Platforms, i.e u50 nodma etc

DESIGN FILES

Application code is located in the src directory. Accelerator binary files will be compiled to the xclbin directory. The xclbin directory is required by the Makefile and its contents will be filled during compilation. A listing of all the files in this example is shown below

src/host.cpp
src/vconv.cpp
src/vconv.h

Access these files in the github repo by clicking here.

COMMAND LINE ARGUMENTS

Once the environment has been configured, the application can be executed by

./dependence_inter <vconv XCLBIN>