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[avr-gcc-list] Re: Code optimistaion in AVR Tiny13

From: David Brown
Subject: [avr-gcc-list] Re: Code optimistaion in AVR Tiny13
Date: Tue, 17 Feb 2009 01:07:19 +0100
User-agent: Thunderbird (Windows/20081209)

Weddington, Eric wrote:

-----Original Message----- From: address@hidden [mailto:address@hidden org] On Behalf Of David Brown Sent: Monday, February 16, 2009 2:31 PM To: address@hidden Subject: [avr-gcc-list] Re: Code
optimistaion in AVR Tiny13

Another important point in getting code as small as possible is to
use -combine and -fwhole-program - it can significantly improve
code, especially for functions that are called only once but live in different modules.

In my tests on real world programs, it have decreased the code by up
to 25%. And it has also *increased* the code by up to 25%!
Unfortunately I don't know what about the code that causes it to
increase. But clearly it is not always a win and can be a detriment.
So be sure to check your code sizes when you use this.

If these flags *increase* code size, there's a bug somewhere - or at least some badly tuned parameters (such as -finline-limit, or something similar). They give the compiler increased knowledge and more opportunity to combine code and do inter-procedural optimisations across compilation units. I've only used it myself on a couple of programs (boot programs, which are particularly space-concious), and it helped significantly.

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