On Thursday, 23 April 2015 at 10:04:47 UTC, Rikki Cattermole
wrote:
On 23/04/2015 10:02 p.m., Andrea Fontana wrote:
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?
asm.dlang.org
and d.godbolt.org
This isn't a D-specific question though, so gcc.godbolt.org would
allow you to test a wider range of backends.
