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Re: [avr-gcc-list] binutils 2.22 blow up program size with -flto

From: Volker Kuhlmann
Subject: Re: [avr-gcc-list] binutils 2.22 blow up program size with -flto
Date: Sun, 12 Aug 2012 12:20:25 +1200

On Mon 30 Jul 2012 04:07:01 NZST +1200, Weddington, Eric wrote:

> There are known issues with LTO and AVR, though unknown reasons why.

Would you have a link to that known issues list?

> For now I would suggest that you avoid that feature.

That's a shame :-(

> However, please file a bug report at the GCC Bugzilla database. That
> way we can keep track of the various LTO issues and eventually try to
> get them fixed.

When I started on that report I find that I don't have enough
information. My findings are

* binutils 2.22 really increases program size, and the combination with
avr-gcc 4.6.2 and LTO fails. What actually is the reason for this?
And is this a gcc problem?

* Program size from avr-gcc 4.3 to 4.6 decreases steadily, generally
speaking. LTO with 4.6 has a positive effect with binutils 2.19.1
and is non-functional with 2.22.

* 4.7 is, from previous posts on this list, the best optimised and least
buggy gcc version. However its program size gives no advantage over 4.3
in any of the ways I have tried, which is unfortunate. More precisely,
without LTO it produces larger code than 4.3 - no incentive to upgrade.
*With* LTO (and the old binutils) it matches 4.3. Would you like me to
report this as an LTO *problem*? How do you suggest I do that without
looking like an idiot...? ;-)

The 2 problems I see here are:

* binutils 2.22 make programs *bigger*. I wouldn't have expected binutils
to do something like that, what could be the reason?

* gcc 4.7, with all the work Johann has put into it, produces code that
is at least as big as the one from 4.3. Is this expected, or did I build
the tool(s) wrong?

I'll put a report in if it helps, if you can help me be a bit more



Volker Kuhlmann
http://volker.dnsalias.net/     Please do not CC list postings to me.

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