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.
An even more striking example can be found if you replace the /
with %, where the result of the function is then
unconditionally zero, but every compiler i tried still spat out
multiplication instructions.
Is there a good reason for this, or is it just " * and / aren't
always inverses, so never mind all the cases where they are"?
Now I know that this seems like a unrealistic example, but when
you're in complicated meta-programming situations code like
this can and will appear.
If I'm right, there's a website where I can see assembly
generated by d compiler. And it's not dpaste... any hint?
