Compiler optimizations

[Walter Bright]

Robert Jacques wrote:
> I haven't seen I compiler yet to do that optimization and DMD 1.038 
> doesn't:
> foreach(ref val; array)  val /= x;
> is ten times slower than
> foreach(ref val; array)  val *= invx;
> on a 100,000 element array.

The reason the optimizer doesn't replace floating divides with floating 
multiplies of the reciprocal is that one gets slightly different answers 
(due to roundoff).

> My general rule of thumb is than the optimizer is pretty good within a 
> single code block and less so across blocks. And there's really no 
> substitute for profiling and testing the user-optimization to see if 
> it's actually worthwhile.

It's also straightforward to run obj2asm on the output and see what it's 
doing.