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up vote 18 down vote favorite
9

I am using an int type to store a value. By the semantics of the program, the value always varies in a very small range (0 - 36), and int (not a char) is used only because the CPU efficiency.

It seems like that many special arithmetical optimizations can be performed on such a small range of integers. Many function calls on those integers might be optimized into a small set of "magical" operations, and some functions may even be optimized into table look-ups.

So, is it possible to tell the compiler that this int is always in that small range, and is it possible for the compiler to do those optimizations?

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1  
value range optimizations exists in many compilers, eg. llvm but I'm not aware of any language hint to declare it. – Remus Rusanu 4 hours ago
up vote 28 down vote

Yes, it is possible. For example, for gcc you can use __builtin_unreachable to tell the compiler about impossible conditions, like so:

if ( value < 0 || value > 36 ) __builtin_unreachable();

We can wrap condition above in the macro:

#define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)

And use it like so:

assume(x >= 0 && x <= 10);

As you can see, gcc performs optimizations based on this information:

#define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)

int func(int x){
  assume(x >=0 && x <= 10);

    if ( x > 11 ){
      return 2;
    }else{
      return 17;
    }
}

Produces:

func(int):
    mov     eax, 17
    ret

One downside, however, that if your code ever breaks such assumptions, you get undefined behavior.

share|improve this answer
    
Can such optimizations reduce code size? – Xofo 4 hours ago
    
@Xofo probably. I think this can help too when put as a default case in switches. – Asu 4 hours ago
1  
@Xofo, didn't get it, in my example this already happening, as return 2 branch eliminated from the code by the compiler. – deniss 4 hours ago
2  
However, it seems that gcc cannot optimize functions to magical operations or table lookup as OP expected. – jingyu9575 1 hour ago

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