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Re: [avr-gcc-list] using exceptions

From: David Brown
Subject: Re: [avr-gcc-list] using exceptions
Date: Wed, 02 May 2012 12:49:07 +0200
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20120420 Thunderbird/12.0

On 01/05/2012 15:04, Georg-Johann Lay wrote:
Gabriel Dos Reis schrieb:
Georg-Johann Lay wrote:
Kövesdi György schrieb:

I am using avr-g++ for building my projects for avr and i am
happy with it. :-)
But i would need exception support, and could not find how to
get it work. I found some mails in the archive (dated many
years ago) about possibilityof building libstdc++ or libsupc++
with exception support, but could not find how to do it.
Reportedly it uses 10...20 kbytes of text, which would be acceptable.

Currently i have gcc 4.6.2 sources, and have tried the options
--with-exceptions, --with-rtti, --enable-libstdc++, etc... no one

libstdc++ is turned off in configury, for example configure.ac reads

# Disable libstdc++-v3 for some systems.
case "${target}" in
noconfigdirs="$noconfigdirs target-libstdc++-v3"

This is current trunk. There were some changes not very long ago.

Would it be reasonable to expect at least libsupc++?
That would be a useful start...

IMHO extending what avr-gcc/avr-g++ can do is reasonable and

However, avr notoriously is short of developers, and if some
problems pop up, there is nobody who would fix it.

For example, PR50925 could block a build, or it might be the
case that unwinding does not work properly.

I am not familiar with C++, it's a dreaded language I always
avoided. If I need to operate on a "big" system, I use Java
or Python, and I am in the happy situation not to have to
program big GUIs that base on C++.

I would not go so far as to say that C++ is a "dreaded" language - it all depends on how you use it. If you want to use exceptions, multiple inheritance, virtual functions, RTTI, STL, and modern "design patterns" for factory functions, singletons, etc., then (assuming someone writes the support libraries) your AVR would crawl like a drunken snail.

However, there is a lot of C++ that works without extra overhead. Basic class usage without multiple inheritance and without virtual functions is pretty much just syntactic sugar for code you could write in C. The same applies to many other C++ features such as overloading, default values, namespaces, templates (which will often replace C macros), etc. C++ was designed with the aim that you don't pay for features you don't use. And the use of C++ structuring can lead to more modular programs with less overhead than trying to do the equivalent abstraction in C - as well as having better typechecking and static error checking than macros.

The kind of C++ that needs the libraries, however, is to deal with exceptions, RTTI, and dynamic memory handling - and all of these are of questionable value on a small device. Personally, I think exceptions are often of questionable value in /any/ program - it makes it hard to see the control flow, as exceptions are a "hidden goto". They also constrict the optimiser - as the compiler has to assume that exceptions can occur at lots of different places in the code, causing jumps, stack-unwinds, etc., it has far less freedom to re-organise the code for better efficiency.



Presumably, exception/unwind support could be made much more
efficient with avr-specific unwind info, but obviously
noone ever tried to get it to work.


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