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
