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Re: [avr-gcc-list] Align attribute in gcc requiring alignment of 2 for X


From: Thomas, George
Subject: Re: [avr-gcc-list] Align attribute in gcc requiring alignment of 2 for Xmegas with USB.
Date: Mon, 19 Nov 2012 19:10:14 +0000

I made a mistake with the patch. 

I had a version of 3.4.1 release of toolchain in path which was of version 
4.6.2 on which I was trying the changes. 
The 4.6.2 version was giving a warning saying 
"requested alignment for 'a' is greater than implemented alignment of 1" 
and the alignment was not happening properly. 

I tried changing the BIGGEST_ALIGNMENT to 16 but applied the patch to the trunk 
version which already had the fix. 

Sorry for the mistake.

-George-

PS :- Changing the BIGGEST_ALIGNMENT to 16 removed the warning when tried in 
the 4.6.2 toolchain but it did not change the alignment in the output elf when 
the disassembly was checked.


________________________________________
From: Georg-Johann Lay address@hidden
Sent: Monday, November 19, 2012 11:34 PM
To: Weddington, Eric
Cc: Thomas, George; address@hidden
Subject: Re: [avr-gcc-list] Align attribute in gcc requiring alignment of 2 for 
Xmegas with USB.

Weddington, Eric wrote:
>
>>
>>> Section 20.13.7 of the Xmega manual speaks about the requirement of 16
>>> bit alignment.
>>>
>>> http://www.atmel.com/images/doc8331.pdf
>>>
>>> As the MAXIMUM_ALIGNMENT in avr.h is set to 1,  it throws an error when
>>> trying to compile.
>>>
>>> I have attached a patch which sets it to 2 by default in the compiler. I
>>> have also attached a case which fails.
>>>
>>> Would this be an acceptable change ?
>> Hi George,
>>
>> No, this would not be an acceptable change, because it is changing the
>> data alignment for the whole compiler, when there is only a restriction on
>> a data structure for a single peripheral for a small subset of the AVR
>> processors.
>>
>> This is why the data attribute "aligned" exists. Just do this for your
>> data structure: __attribute__ ((aligned (2)))
>
> Excuse me, perhaps I have this wrong... (It's still early for me).
>
> Does changing MAXIMUM_ALIGNMENT allow the aligned attribute to work? If we
> don't change MAXIMUM_ALIGNMENT, are you saying that the aligned attribute
> fails?

See PR53448.  In general, that is no issue in < 4.7 because only xmega needs
align(2) and these versions don't support xmega.

Besides that, 4.7 had quite some clean-up of w.r.t. ELF, e.g. avr/elf.h.

Even if it works with changing MAXIMUM_ALIGNMENT, that's not the purpose of
MAXIMUM_ALIGNMENT.

Johann




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