How to compile PTX code

Question

I need to modify the PTX code and compile it directly. The reason is that I want to have some specific instructions right after each other and it is difficult to write a cuda code that results my target PTX code, So I need to modify ptx code directly. The problem is that I can compile it to (fatbin and cubin) but I dont know how to compile those (.fatbin and .cubin) to "X.o" file.

Answer 1

There may be a way to do this with an orderly sequence of nvcc commands, but I'm not aware of it and haven't discovered it.

One possible approach however, albeit messy, is to interrupt and restart the cuda compilation sequence, and edit the ptx file in the interim (before the restart). This is based on information provided in the nvcc manual, and I would not consider this a standard methodology, so your mileage may vary. There may be any number of scenarios that I haven't considered where this doesn't work or isn't feasible.

In order to explain this I shall present an example code:

#include <stdio.h>

__global__ void mykernel(int *data){

  (*data)++;
}

int main(){

  int *d_data, h_data = 0;
  cudaMalloc((void **)&d_data, sizeof(int));
  cudaMemcpy(d_data, &h_data, sizeof(int), cudaMemcpyHostToDevice);
  mykernel<<<1,1>>>(d_data);
  cudaMemcpy(&h_data, d_data, sizeof(int), cudaMemcpyDeviceToHost);
  printf("data = %d\n", h_data);
  return 0;
}

For this purpose, I am dispensing with cuda error checking and other niceties, in favor of brevity.

Ordinarily we might compile the above code as follows:

nvcc -arch=sm_20 -o t266 t266.cu 

(assuming the source file is named t266.cu)

Instead, based on the reference manual, we'll compile as follows:

nvcc -arch=sm_20 -keep -o t266 t266.cu

This will build the executable, but will keep all intermediate files, including t266.ptx (which contains the ptx code for mykernel)

If we simply ran the executable at this point, we'd get output like this:

$ ./t266
data = 1
$

The next step will be to edit the ptx file to make whatever changes we want. In this case, we'll have the kernel add 2 to the data variable instead of adding 1. The relevant line is:

    add.s32         %r2, %r1, 2;
                              ^
                              |
                          change the 1 to a 2 here

Now comes the messy part. The next step is to capture all the intermediate compile commands, so we can rerun some of them:

nvcc -dryrun -arch=sm_20 -o t266 t266.cu --keep 2>dryrun.out

(Using linux redirection of stderr here). We then want to edit that dryrun.out file so that:

1. we retain all the commands after the creation of the ptx file, up to the end of the file. The line that creates the ptx file will be evident as the one which specifies -o "t266.ptx"
2. we strip out the leading #$ that each line begins with, so in effect we are creating a script.

When I perform the above 2 steps, I end up with a script like this:

ptxas  -arch=sm_20 -m64  "t266.ptx"  -o "t266.sm_20.cubin"
fatbinary --create="t266.fatbin" -64 --key="xxxxxxxxxx" --ident="t266.cu" "--image=profile=sm_20,file=t266.sm_20.cubin" "--image=profile=compute_20,file=t266.ptx" --embedded-fatbin="t266.fatbin.c" --cuda
gcc -D__CUDA_ARCH__=200 -E -x c++   -DCUDA_DOUBLE_MATH_FUNCTIONS   -D__CUDA_PREC_DIV -D__CUDA_PREC_SQRT "-I/usr/local/cuda/bin/..//include"   -m64 -o "t266.cu.cpp.ii" "t266.cudafe1.cpp"
gcc -c -x c++ "-I/usr/local/cuda/bin/..//include"   -fpreprocessed -m64 -o "t266.o" "t266.cu.cpp.ii"
nvlink --arch=sm_20 --register-link-binaries="t266_dlink.reg.c" -m64   "-L/usr/local/cuda/bin/..//lib64" "t266.o"  -o "t266_dlink.sm_20.cubin"
fatbinary --create="t266_dlink.fatbin" -64 --key="t266_dlink" --ident="t266.cu " -link "--image=profile=sm_20,file=t266_dlink.sm_20.cubin" --embedded-fatbin="t266_dlink.fatbin.c"
gcc -c -x c++ -DFATBINFILE="\"t266_dlink.fatbin.c\"" -DREGISTERLINKBINARYFILE="\"t266_dlink.reg.c\"" -I. "-I/usr/local/cuda/bin/..//include"   -m64 -o "t266_dlink.o" "/usr/local/cuda/bin/crt/link.stub"
g++ -m64 -o "t266" -Wl,--start-group "t266_dlink.o" "t266.o"   "-L/usr/local/cuda/bin/..//lib64" -lcudart_static  -lrt -lpthread -ldl  -Wl,--end-group

Finally, execute the above script. (in linux you can make this script file executable using chmod +x dryrun.out or similar.) If you haven't made any mistakes while editing the .ptx file, the commands should all complete successfully, and create a new t266 executable file.

When we run that file, we observe:

$ ./t266
data = 2
$

Indicating that our changes were successful.

Answer 2

Usually, when handling with cubin or ptx-files one uses the CUDA Driver API and not the Runtime API; doing so, you load the ptx or cubin file manually at runtime with cuModuleLoadDataEx. If you want to stick with the Runtime API you need to mimic manually what NVCC does, but this is not (entirely) documented. I only found this Nvidia forum entry on how to do this.

Answer 3

You can load cubin or fatbin at runtime using cuModuleLoad* functions in CUDA: Here's the API

You can use it to include PTX into your build, though the method is somewhat convoluted. For instance, suricata compiles its .cu files into PTX files for different architectures and then converts them into an .h file that contains PTX code as a 'C' array, and then just includes it from one of the files during the build.

Answer 4

I am rather late but GPU Lynx does exactly that: take a CUDA fat binary, parse the PTX, and modify it before emitting the result to the driver for execution on a GPU. You can optionally print out the modified PTX as well.

Answer 5

This sequence of nvcc commands seems to do the trick. Please see here for more details.

Create your ptx files to modify

nvcc file1.cu file2.cu file3.cu -rdc=true --ptx

Link ptx files into an object file

nvcc file1.ptx file2.ptx file3.ptx -dlink

I did this on Windows so it popped out a_dlink.obj. As the documentation points out host code has been discarded by this point. Run

nvcc file1.cu file2.cu file3.cu -rdc=true --compile

to create object files. They will be .obj for Windows or .o for Linux. Then create a library output file

nvcc file1.obj file2.obj file3.obj a_dlink.obj --lib -o myprogram.lib

Then run

nvcc myprogram.lib

which will pop out an exectuable a.exe on Windows or a.out on Linux. This procedure works for cubin and fatbin files too. Just substitute those names in place of ptx.