Make Cosim Great Again

Vivado HLS is a handy(?) high-level synthesis (HLS) tool that supports C++11 features. However, its C-RTL co-simulation (cosim) is broken in the sense that it doesn’t fully support C++11. The cosim compiler seems to be stuck at g++ 4.6.3 forever. This makes it impossible to make fancy stuff like Google Test work as part of the testbench with the cosim compiler.

Let’s recap the situation:

We are talking about the Linux toolchains.
The cosim framework is designed to be started via a TCL command and it is hard to invoke it manually.
The cosim framework requires that the testbench contains a main function so you cannot make the cosim executable a library and link it with other programs.
The cosim compiler is a customized version of g++ 4.6.3 and cannot be upgraded or replaced. Note that although clang++ is provided as an alternative, it never worked for me.
Although some ldflags work, I never got the cosim compiler correctly link a library against the source code.

山重水复疑无路，柳暗花明又一村。————陆游

When there’s a will, there’s a way — the ABI is still compatible, isn’t it? You can compile whatever program with whichever compiler, as long as it can be loaded as a shared object by the cosim executable. So the idea is clear:

Compile the fancy part of the code into a shared object with whichever compiler you want (just keep the ABI compatible). To create a shared object with g++ or clang++, add -fPIC when compiling and add -shared when linking.

Make the cosim executable load the shared object at runtime. This can be done with something like this

#include <dlfcn.h>
// This function pointer will be used to invoke the function in the shared object.
void (*entry_function)();
void* handle = dlopen("foo.so", RTLD_LAZY);
if (handle == nullptr) {
  clog &lt;&lt; dlerror() &lt;&lt; endl;
  exit(EXIT_FAILURE);
}
entry_function = reinterpret_cast<void(*)()>(dlsym(handle, "EntryFunction"));
char *error;
if ((error = dlerror()) != nullptr)  {
  clog &lt;&lt; dlerror() &lt;&lt; endl;
  exit(EXIT_FAILURE);
}
(*entry_function)();
// Don't dlclose(handle) or cosim won't work.</void(*)()></dlfcn.h>

If the function invoked in the shared object needs to invoke the hardware module, put the top-level function in a wrapper instead of invoking it directly. This is because cosim is actually doing a source-to-source transformation to the testbench and invoking a top-level module directly will only run C simulation. Note that you need to export the symbols in the testbench by adding -Wl,--export-dynamic to the ldflags of the cosim_design TCL command to invoke functions in the testbench from the shared object.
Compile the antique part of the code with the cosim compiler. Add -ldl to the ldflags of the cosim_design TCL command to enable dynamic library loading.

I managed to get cosim working with Google Test!

References

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