On Friday 19 September 2008 16:25, you wrote:
>
> > Is it true that avr-gcc never produces code that makes
> > explicit use of the T-bit?
>
> No.
> In GCC 4.3.2, in the machine description for the avr
> (gcc/config/avr/avr.md), there is define_insn "abssf2"
> that will use the T bit. It generates a CLT and BLD
> instructions.
Thanks for this information Eric.
> However, I have no idea how often the define_insn
> "abssf2" will trigger. But yes, there is a possibility.
The md file states:
(define_insn "abssf2"
[(set (match_operand:SF 0 "register_operand" "=d,r")
(abs:SF (match_operand:SF 1 "register_operand" "0,0")))]
""
"@
andi %D0,0x7f
clt\;bld %D0,7"
[(set_attr "length" "1,2")
(set_attr "cc" "set_n,clobber")])
I don't know how to read it, but loosely interpreted i
understand the following:
abssf2 seems to be involved in the calculation of the
absolute value of a single precesion float. There are
two ways to clean the msb of register %DO:
- one is and immideate with 0x7f and the
- second is clear the T bit and load that zero on msb
Now since the first only works on registers r16 to r31
sometimes the compiler chooses the second option?
Ruud
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