On Sunday, 26 May 2013 at 20:23:45 UTC, Martin Nowak wrote:
On 05/26/2013 06:12 AM, Timothee Cour wrote:
> Which means that the extra check to make sure the output
reference
> doesn't escape a local incurs a 20% cost in my example.
>
> Is there a better implementation (i have a single pointer
comparison
> though so I'm not sure how that would be optimized) ?
>
> If not, it will mean user will have to choose between
enabling runtime
> safety checks (20% in my simple example), or throwing away
ref safety by
> disabling checkboundsref flag.
>
If you look at the asm output and lower the iteration count,
you'll see that you bench llvm's inliner rather than the cost
of the runtime check.
Also the version that doesn't perform the bounds check doesn't
use the ptr parameter, so the variable can be optimized away.
In a real implementation computing the frame pointer doesn't
incur much cost, it is already available in a register or it is
a constant offset from the stack pointer.
The branch is easily predictable, as the condition is always
the same.
I think the solution proposed in the DIP is worth checking, not
solely due to performance reasons, but mainly for better safety.
A runtime check cannot check all possible code paths when
testing, and an undetected bug can surprise you when you don't
expect it.
