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

[Craig Topper via llvm-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 <
llvm-dev at lists.llvm.org> 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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>
-- 
~Craig
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