[OT] compiler optimisations

[rumbu via Digitalmars-d]

On Thursday, 23 April 2015 at 08:33:56 UTC, John Colvin wrote:
> Why can no compiler I try optimise this toy example as I would 
> expect?
>
> // uncomment if using a C compiler
> // typedef unsigned int uint;
> uint foo(uint a)
> {
>   if (a < 5)
>     return (a * 3) / 3;
>   else
>     return 0;
> }
>
> So, I would expect the compiler to be able to see that it is 
> equivalent to
>
> uint foo(uint a)
> {
>   return (a < 5) ? a : 0;
> }
>
> But apparently not a single modern compiler I tried can do this 
> optimisation, unless it's hidden in some obscure flag I'm not 
> aware of.
>

I think because of the potential overflow in a * 3 (if we ignore 
the a < 5 condition). To optimize this, a compiler must figure 
out that there is no overflow for any a < 5.

If you change the condition to a > 5 and call foo(uint.max), the 
two expression above are not equivalent.

int foo(uint a)
{
	if (a > 5)
		return (a * 3) / 3;
	else
		return 0;
}

int foo_optimized(uint a)
{
	return (a > 5) ? a : 0;
}

assert(foo(uint.max) == foo_optimized(uint.max)) // -> fail.