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Re: [avr-gcc-list] the framepointer

From: Ruud Vlaming
Subject: Re: [avr-gcc-list] the framepointer
Date: Sun, 20 Apr 2008 22:19:58 +0200
User-agent: KMail/1.9.1

On Saturday 19 April 2008 23:26, you wrote:

> The save around call will not use R18 upwards, 
> as these are call clobbered. 
You'r right.

> Without studying source and compiler RTL and optimisation level used  I 
> cannot say why it did not use a lower register.
> It may be the cost assumptions are not quite right for a low number of 
> slots compared to reg push/pops..
Maybe this is a little dumb, but i asked the compiler with -fomit-frame-pointer
explicitly to not use the frame pointers if avoidable. Then it should not
weight any costs, it should simply do push/pop for one variable (or use a 
lower register). Even if it is much more expensive. Only when it is not 
avoidable it should use framepointers, but if you do not explictly alloc 
some space and your functions are not to complex/long this should be rare.
Thus, in my view this is not a matter of optimization.

My knowledge of the gcc internals is near to zero so i have a hard time
finding the right spots to analyze this. Any comments appreciated.

> The reason you don't have RTL prolog/epilog is that it takes forever to 
> get patches approved.  I think I wrote it in 2005 for 4.1 !
Argh! And i thought our goverment was slow ....


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