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

From: Ned Konz
Subject: Re: [avr-gcc-list] More on C macros vs. C++ templates
Date: Fri, 28 Apr 2006 09:27:34 -0700

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

00800060 00000042 B myQ
0000008e 00000046 T main

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

Attachment: templateDemo.tar.gz
Description: GNU Zip compressed data

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