cpp-codegen

Cpp-codegen is a library that generates, compiles and runs C++ code at runtime.

• Performs code specialization at runtime (jit compilation)
• Control over individual compilation parameters
• Can produce more optimized code

The library is in experimental/development stage.

How it works

In a nutshell, at run time, the compile shell command and then the dynamic linking loader are called through the C interfaces, system() and dlsym() respectively.

Instructions

Below there are instructions to build and use the library, as well as an example demonstrating potential performance benefits.

Requirements

gtest, cmake, libopenblas-dev

Build

mkdir build && cd build
cmake -DCMAKE_INSTALL_PREFIX=<your/installation/path> .. 
make
make install

Run simple example

export CXX=g++ //or your favorite C++ compiler
cd <your/installation/path>/bin
helloworld

Run Convolution example

To demonstrate the potential benefits to performance, two optimized functions for the convolution operation are generated

• 1. Substitute the loop boundary variables with their runtime values
• 2. Completely eliminate the 2 innermost loops traversing the kernel

Performance comparison against Normal Convolution and Im2col-Gemm Convolution

export CXX=g++ //or your favorite C++ compiler
cd <your/installation/path>/bin
./time_conv <input channel> <input rows> <input cols> <output channel> \
		<filter rows> <filter cols> <stride> <pad> <threads>

Specifically with Alexnet's conv2 layer:

./time_conv 96 27 27 256 5 5 1 1 1
./time_conv 96 27 27 256 5 5 1 1 2

	1 thread	2 threads
Normal	0.378811s	0.223168s
Im2col	0.018317s	0.020192s
Cgen-loop	0.180567s	0.091323s
Cgen-unroll	0.088904s	0.045188s

In this case code specialization enables performance comparable to the widely used im2col-gemm implementations without their memory overheads

Create your own program

Step 1. Create the C++ file:

This can be achieved by using plain strings or making a template:

Case 1. String

// src/mymain.cpp
...
  StringExec se ;
  std::string str;
  str.append("extern \"C\" int func()\n");
  str.append("{\n");
  str.append("return 5+5;\n");
  str.append("}\n");

  se.append(str);
  se.save();
  se.compile();
...

Case 2. Template

• Create a template

// templates/mytemplate.txt
extern "C" int func()
{
   return $hook;
}

• On the main file append code from template and replace hooks if any

// src/mymain.cpp
...
   StringExec se ;
   se.append("../templates/test.txt");
   se.replace("$hook","5+5");
   se.save();
   se.compile();
...

Step 2. Compile and run the function:

// src/mymain.cpp
...
  se.openlib();
  se.loadlib("func");
  se.func();
  se.closelib();
...

TODO

• Fix possible name conflict of same named generated libraries used by different programs:

  • Use unique directory name suffix per programm or function call

• More fine grained compilation parameters:

  • Define compilation parameters at object init as strings