Re: [boost] proposal - Statistically robust function timing for performance decisions.

Subject: Re: [boost] proposal - Statistically robust function timing for performance decisions.
From: Edward Grace (ej.grace_at_[hidden])
Date: 2009-07-18 05:14:25

On 18 Jul 2009, at 07:40, joel wrote:

> Add me to the bandwagon ;)
>
> How do you handle the fact that sometimes, compiler may otpimize out
> benchamrk code because it has no side-effects ?

Easy.  I don't!

When I started this I was concerned that might happen and was quite  
surprised to see that the compiler (g++ 4.0.1)  didn't optimise out  
the function.

More generally, I don't think a function such as:

void f();

can be considered free of side-effects unless you can guarantee that  
it does not access global variables (or otherwise sneakily modify  
state).  From the prototype of a compiled function I don't see how  
there's any way to know this.  With fully templated code it may be  
far more subtle since the compiler can potentially 'see' everything.

That said, at the moment I want to concentrate on making the timings  
reliable on the assumption that problems such as you describe can be  
circumvented later on by someone who knows more about compilers than me!

-ed