Subj : Re: OO compilers and efficiency
To   : comp.programming
From : Gerry Quinn
Date : Wed Jul 20 2005 12:47 am

In article <dbj5f7$rdv$1@gondor.sdsu.edu>, brian@rohan.sdsu.edu says...
> Is there a good approachable book that covers OO compilers?  Something
> like a Red Dragon for the ages? 
> 
> OO seems like it must be very wasteful.  Extending a class usually
> means taking the super class' existing method and tacking on some 
> new secret sauce stuff.

If the original method works, nothing needs to be added.  If it 
doesn't, you'd have to change it anyway.

> So is this wrapping one stack frame on top of another?  First the
> subclasses method gets called then super's? Are compilers generally 
> smart enough to inline the two calls into one?

They might or they might not.

> Getter and setter functions are another simple example of what seems
> to be waste.  The method call works like this.  The existing register
> states are pushed onto the stack.  The call is made.  The getter
> method returns a simple value on the stack.  The call is returned.  
> The registers are popped.  That must be about 20 instructions for
> what might only take one, a simple memory read, or better yet, a
> register read. 

That is a more likely candidate for auto-inlining, I would guess.

> Anyway, those are some of the things that kind of nag at me.
> I think it's a safe statement to say C can beat any OO compiled
> program pound for pound given the same programmer skill and
> adherence to language goals.  
> 
> Does everyone drop into C for critical code?

You can use getter and setter functions in C too if you want.  Or not 
use them in C++.  I don't see their use as a specific indicator of OO.

If your derived class is doing heavy calculation with certain members, 
they are normally its own.  So you can write it like C if you want.

> Or am I the only one who worries about this stuff?  Is it a 
> concern or should I kind of push it back into the recesses
> of my mind and say the wastefulness is part of the tradeoff
> for programming OO code?

There's a certain cost in terms of extra aliasing etc. going from C to 
C++.  If it looks likely to hurt, you can write pretty much in C style 
where it counts.  But it's unlikely that more than about 1% of the code 
- even of an action game - is so time critical that it makes a 
difference.  

- Gerry Quinn

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