Volatile function

Question

Summary: What does the keyword volatile do when applied to a function declaration in C and in C++?

Details:

I see that it's possible to compile a function that is marked as volatile. However, I'm not sure what compiler optimization (if any) this prevents. For instance I created the following test case:

volatile int foo() {
  return 1;
}

int main() {
  int total = 0;
  int i = 0;
  for(i = 0; i < 100; i++) {
    total += foo();
  }

  return total;
}

When I compile with clang -emit-llvm -S -O3 test.c (gcc would also work but the llvm IR is more readable in my opinion) I get:

target triple = "x86_64-unknown-linux-gnu"

define i32 @foo() #0 {
  ret i32 1
}

define i32 @main() #0 {
  ret i32 100
}

So obviously the compiler was able to optimize away the calls to function foo() so that main() returns a constant, even though foo() is marked as volatile. So my question is whether volatile does anything at all when applied to a function declaration in terms of limiting compiler optimizations.

(Note my interest in this question is mostly curiosity to understand what volatile does rather than to solve any specific problem.)

(Also I have marked this question as both C and C++ not because I think they are the same language, but because I am interested to know if there are differences in what volatile does in this case in these two languages).

Answer 1

In your code, the volatile keyword does not apply to the function, but to the return type, it is the equivalent of:

typedef volatile int Type;
Type foo();

Now, in C++ you can make a member function volatile, in the same way that the const qualifier, and the behavior is the same:

struct test {
   void vfunction() volatile;
};

Basically you cannot call a non-volatile (alterantively non-const) function on a volatile (const respectively) instance of the type:

struct test {
   void vfunction() volatile;
   void function();
};
volatile test t;
t.vfunction();      // ok
t.function();       // error

Answer 2

foo() is not volatile.

It's a function that returns a volatile int.

Which is legal. But strange for a returned int.

Member functions, on the other hand, can be volatile for the same reason they can be const -- both describe the object this is pointing to.