Re: [llvm-dev] Splitting C/C++ code into pure and side-effecting code

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Re: [llvm-dev] Splitting C/C++ code into pure and side-effecting code

Tom Stellard via cfe-dev
This probably a better question for the clang developer list cfe-dev.

On Sun, Apr 9, 2017 at 12:01 PM Suman Thakur via llvm-dev <[hidden email]> wrote:
Hello folks,

I'm not a compiler expert or subscribed to this mailing list, but I have a unique problem. I need to split a large piece of C/C++ code into two separate libraries: one library that only has pure code (i.e., code that doesn't require operating system interactions) and other library that can have both pure code and side-effecting code. 

I was was wondering if it's possible to achieve this by adding something like __attribute__((annotate("pure"))) and __attribute__((annotate("call_impure"))) and how much effort will it be to add such a functionality. Basically, I want "pure" attribute to be sticky, in the sense that every function that's pure, can only call pure code unless explicitly marked inside the function to make impure call. For example, the following code should create one library for pure code, and a driver which has all the impure code.


__attribute__((annotate("pure"))
int increment(int x){
    return x + 1;
}

__attribute__((annotate("pure")))
int add(uint32_t a, uint32_t b)  {
  int c = a;

  /* addition via pieno arithmetic */
  while(b != 0){
    c = increment(c);  // Okay to call pure code directly.
    b--;
  }

  printf("%d + %d = %d\n", a, b, c) __attribute__((annotate("call_impure")));
  // calling impure code requires explicit annotation
  // all the arguments to the function are copied 
  // and don't share the same stack as pure code
  return c;
}


int main(int argc, char* argvp[]){
   a = 1;
   b = 2;.
   c = add(a,b)
   printf("%d + %d = %d", a, b); // okay to call impure from impure
}


The call to printf in pure code should create a stub function call printf_impure_call(), which in the impure library just calls printf. 

I know very little about compilers (mathematician by training), but I will really appreciate if someone can comment about the feasibility of this.

Thanks
Suman

PS: I'm not subscribed to LLVM mailing list so please reply-all.
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