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Re: [avr-gcc-list] how to safely use register locked variables with avr-

From: Erik Christiansen
Subject: Re: [avr-gcc-list] how to safely use register locked variables with avr-libc?
Date: Sun, 31 May 2015 19:13:03 +1000
User-agent: Mutt/1.5.21 (2010-09-15)

On 26.05.15 15:55, Britton Kerin wrote:
> What I don't know is what I must do to ensure that avr-libc isn't using the
> registers in an unsafe way.  It does use the registers:
> $ avr-objdump -d /usr/lib/avr/lib/libc.a > /tmp/libc
> $ egrep '\<r2\>' /tmp/libc | more
>  1ac:   28 2e           mov     r2, r24
>  1f4:   20 2e           mov     r2, r16
>  204:   82 2d           mov     r24, r2
> ...
> and apparently since there are no push-pop pairs, it doesn't seem guaranteed
> that its saving/restoring them.  But I'm told that it might not need to,
> because a compiler optimization causes only clobbered registers to be
> preserved.  I'm told that what I need to check is whether avr-gcc "pushes
> the clobber list"?

What do you see when you "avr-objdump -d" the compiled object of a small
program which calls one avr-libc function? Does the compiler provide
push-pop pairs for registers used both by the calling function and the
avr-libc function?

It would be less efficient to push everything, whether needed or not.
Avr-gcc only needs to protect registers which are in use. What do you
find when you look?

My recollection of the AVR ABI is that r2-r17 are preserved across
function calls. As avr-libc lacks obvious signs of doing that
intrinsically, you can either hope that avr-gcc does it, or quickly and
easily verify the behaviour by observation.


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