[boost] Contracts and optimizing (was Circular log buffer)

From: Darren Cook (darren_at_[hidden])
Date: 2004-10-29 23:21:45

>>I'm hoping Thorsten Ottosen's contract programming will make it into
>>the standard so the compiler will understand asserts and be able to
>>optimize away the redundant ones.
> 
> Compilers are already allowed to do that, and I predict no proposal
> that requires it to be done will ever be accepted into the standard.

"be able" falls somewhere between "allowed to" and "required to". An 
optimizer will have a hard job analyzing all the code in a function so see 
if any can be skipped. But if it can see a precondition is always satisfied 
then it can jump directly to the body of the function and skip the precondition.

E.g.
   void f(int x){
   precondition{ x>=0 && x<=99; }	//1
   ... //2
   }

   f(atoi(argv[2]));	//Jumps to f, at 1
   f(34);	//Jumps to f, at 2

   void g(int z){
   precondition{ z>=10 && z<50; }
   f(z*2);	//Can also jump to f at 2, as range is 20..98
   }

I don't think Thorsten's proposal requires optimizations like this. But I 
don't know enough about compiler writing to know if the above is ever likely 
to be implemented.

Darren