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List:       gcc
Subject:    Using cc (question from avr)
From:       "Paulo J. Matos" <pocmatos () gmail ! com>
Date:       2011-03-09 16:47:52
Message-ID: il8avo$oeo$1 () dough ! gmane ! org
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Hi,

I am having some trouble really understanding the working of cc_status.
In order to understand it better I was looking at the code for avr under 
gcc 4.3.

My assumption is that set_zn, set_* means that an instructions _changes_ 
these flags. So an instruction that set_zn means that Z and N are 
modified but we are not sure to which values. I assume clobber means 
that an instruction changes some flags but we don't know which.

Now, the first thing that surprises me is clobber. Given a processor 
instruction description we know exactly which instructions set what. 
What's the need for clobber?

Check the following example from avr:

(define_insn "*strlenhi"
   [(set (match_operand:HI 0 "register_operand" "=e")
         (unspec:HI [(mem:BLK (match_operand:HI 1 "register_operand" "%0"))
                     (const_int 0)
                     (match_operand:HI 2 "immediate_operand" "i")]
                    UNSPEC_STRLEN))]
   ""
   "ld __tmp_reg__,%a0+ 
 

         tst __tmp_reg__ 
 

         brne .-6"
   [(set_attr "length" "3")
    (set_attr "cc" "clobber")])


 From the instruction manual I have [1] ld changes none, tst changes Z,N 
and brne changes none so I would expect this instruction to have 
(set_attr "cc" "set_zn") instead of the clobber. Why is this?

[1]: http://www.atmel.com/dyn/resources/prod_documents/doc0856.pdf

Cheers,

Paulo Matos

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