Re: [boost] [Function][Bind] Re: "Small bufferoptimization"forboost::function?

From: Doug Gregor (dgregor_at_[hidden])
Date: 2006-01-09 12:51:28

On Jan 9, 2006, at 11:55 AM, Peter Dimov wrote:
> Yes. It would be interesting to measure the performance for larger 
> buffer
> sizes, though. The break-even point should occur somewhere around 32 
> or 48,
> maybe even 64 if the allocator is bad enough.

Yeah, that's possible. I don't think it's just a matter of finding the 
break-even point for performance, though. function<> is supposed to 
replace function pointers, closures, etc. If it's significantly larger 
than those entities, it becomes harder to justify the use of 
function<>. We have two things to optimize here :(

>> If we go to 16 bytes, then bind(&X::f, &x, _1, true) will fit but
>> bind(&X::f, &x,
>> _1, true, true) won't.
>
> It probably will unless 'true' is 4 bytes.

Some ABIs are actually that strange :)

> In my experience, the closure case is indeed very common in code 
> written by
> people who don't take advantage of the full expressive power of 
> boost::bind,
> probably because they have a Borland/delegate background.
> [snip]
> and synthesize show/hide with boost::bind. My code tends towards the 
> latter
> variety, so I won't be seeing much of the SBO with a &X::f+&x cutoff.

Me too :)

> BTW, this talk about 12 byte buffer is assuming g++. A member pointer 
> is
> 4-16 on MSVC, 8 on g++, 12 on Borland, 4 on Digital Mars (!). There's 
> a nice
> table in
>
> http://www.codeproject.com/cpp/FastDelegate.asp

Yeah, I know. The actual code for function<> has a union containing a 
struct with an unknown member pointer and a void pointer in it. I guess 
we could pad that with an integer or two if we want to expand the 
buffer...

> Anyway, I committed a storage optimization to the CVS. <crosses 
> fingers>

Very cool. Works like a charm on GCC, at least. Once I get a chance to 
write up documentation for the changes to Function, I'll commit 
everything to CVS and we'll see who screams :)

        Doug