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Date: Tue, 21 Jun 2011 12:39:00 -0000
From: Georg-Johann Lay <avr@gjlay.de>
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To: Denis Chertykov <chertykov@gmail.com>
CC: gcc-patches@gcc.gnu.org, Anatoly Sokolov <aesok@post.ru>,  "Eric B. Weddington" <eric.weddington@atmel.com>
Subject: Re: [Patch, AVR]: Fix PR33049 (implement extzv)
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Denis Chertykov schrieb:
> 2011/6/21 Georg-Johann Lay:

>> Ok.  d_register_operand or satisfies_constraint_d would do.  But I
>> preferred to introduce a new constraint for 4 and avoid splitting
>> *extzv altogether to keep it simple.  Complexity of splitting is not a
>> real advantage over writing things down directly.
>>
>> As avr is running out of constraint letters, I started 3-letter
>> constraints 'Y**' in the style of bfin.  Would be nice if 'K' was not
>> already occupied or could be deprecated (is'n not really useful in
>> inline asm)...
> 
> Better to choose starting letter different to pointer register names X,Y,Z.
> May be better to use underscore '_' ?
> 
> Denis.

My preference would be to change 'K' to 'K02' and thus deprecate 'K'.

Other letters are 'A', 'B', 'C', D', 'R', 'S', 'T', 'U', 'W', 'Z'.
I do not like '_' as constraint.
'R' might be needed in the future for more register constraints (r24,
r24-r25, r22-25, ...) and 'R' is a nice prefix.
'C' looks appealing for constant.

Johann