Re: [avr-gcc-list] More on C macros vs. C++ templates

[Ned Konz]

On Apr 28, 2006, at 7:06 AM, Dave Hylands wrote (in response to a  
message from "dimax un"):

> > WOW. I will definitely study your message. It will take time to  
> > all of us.
> > But my scepticism was not about templates but about writing  
> > embedded code in
> > C++ instead of C/Assembler/InlineAssembler. Interesting what would  
> > you get
> > if you would write one more example in plain good C.
>
> I pasted some circular buffer code that I did. In C & C++, I used
> macros in C, and in C++ I used a template.
>
> I did a bunch of analysis, and the C++ is very close in size to the C
> one. The biggest size differences are all related to how unit8_t's get
> promoted to ints.
>
> The C++ version is also missing a bit of functionality but it would be
> easy to add. I think at the time I didn't fully appreciate what was
> affecting the size and could probably get the C++ one to be as small
> as the C one now that I understand whats going on.

OK, here's a modified version of yours that generates the same size  
object file using GCC 4.1.0 (actually, the C++ version is 2 bytes  
larger because of a bad register allocation choice, but who's  
counting?).

If you uncomment the commented-out constructor in the C++ version, it  
becomes 36 bytes larger (108 vs. 72 bytes) because of the global  
constructor/destructor mechanism. In this case, of course, the  
constructor is not necessary for a static CBUF object, as the indices  
need to start at 0 anyway.

Here's how it looks when you uncomment the constructor and run
avr-nm -C --size-sort --print-size testc*.elf

testc.elf:
00800060 00000042 B myQ
0000008e 00000046 T main

testcpp.elf:
000000ba 0000000e t global constructors keyed to myQ
000000a6 00000014 t __static_initialization_and_destruction_0(int, int)
00800060 00000042 B myQ
000000c8 00000048 T main

So you see that main() is only 2 bytes larger in the C++ version.

I also made the C interface parameterizable for IndexType and  
EntryType, added the missing functions to the C++ version (though I'm  
not sure that they should be public at all), and added some comments.

--
Ned Konz
MetaMagix embedded consulting
address@hidden

templateDemo.tar.gz
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