Questions about CUDA macro __CUDA_ARCH__

Question

I have a simple cuda code in ttt.cu

#include <iostream>
__global__ void example(){
   printf("__CUDA_ARCH__: %d \n", __CUDA_ARCH__);
}
int main(){
example<<<1,1>>>();
}

with CMakeLists.txt:

cmake_minimum_required(VERSION 3.18)
project(Hello)
find_package(CUDA REQUIRED)

cuda_add_executable(sss ttt.cu)

Then I got the error: identifier "__CUDA_ARCH__" is undefined. I would like to know why does this happen and what should I do for making the __CUDA_ARCH__ valid? And can we use valid __CUDA_ARCH__ in host code within a header .h file?

Update:

I intended to use the following cmake for generating a 750 cuda arch, however, this always results in a __CUDA_ARCH__ = 300 (2080 ti with cuda 10.1). I tried both set_property and target_compile_options, which all failed.

cmake_minimum_required(VERSION 3.18)
project(Hello)
find_package(CUDA REQUIRED)
cuda_add_executable(oounne ttt.cu)
set_property(TARGET oounne PROPERTY CUDA_ARCHITECTURES 75)
#target_compile_options(oounne PRIVATE  $<$<COMPILE_LANGUAGE:CUDA>:-gencode 
arch=compute_75,code=sm_75>)

Answer 1

__CUDA_ARCH__ is a compiler macro.

can we use valid __CUDA_ARCH__ in host code

No, it is intended to be used in device code only:

The host code (the non-GPU code) must not depend on it.

You cannot print a compiler macro the way you are imagining. It is not an ordinary numerical variable defined in C++. You could do something like this but that would print at compile-time, not at run-time.

To print at run-time, you could do something like this:

$ cat t2.cu
#include <cstdio>
#define STR_HELPER(x) #x
#define STR(x) STR_HELPER(x)

__device__ void print_arch(){
  const char my_compile_time_arch[] = STR(__CUDA_ARCH__);
  printf("__CUDA_ARCH__: %s\n", my_compile_time_arch);
}
__global__ void example()
{
   print_arch();
}

int main(){

example<<<1,1>>>();
cudaDeviceSynchronize();
}
$ nvcc -o t2 t2.cu

$ ./t2
__CUDA_ARCH__: 520
$

Note that there are quite a few questions here on the cuda tag discussing __CUDA_ARCH__, you may wish to review some of them.