Re: gEDA-user: interesting links

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Thu, 2007-08-30 at 13:48 -0400, DJ Delorie wrote:
> > I'm not sure what the performance hit might be in removing it, and
> > using a test on the object's type.
> 
> From a gcc developer's point of view...
> 
> A C++ method call off a pointer is two loads and an indirect call.
> Three opcodes.
> 
> The "type" way is a load, compare, perhaps a switch statement (which
> is an indirect jump), and a direct call.

> You take a "big" hit for the compare/switch because it always checks
> for out-of-bounds cases, so you end up with a lot of compares and
> jumps, which are bad for performance.

What if the code "guarantees" type is correct, so we don't need to
compare before indexing the vfunc array?

> Overall, not much difference, but C++ is a bit faster.
> 
> HOWEVER, from *our* point of view, it's looks like this:
> 
> Adding a new object in C++ means editing one file.
> 
> Adding a new object in C means editing *all* the files that have such
> a compare/switch in them.

Surely just updating the one vtable?

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)



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