Richard Henderson schrieb:
> On 06/15/2011 02:47 AM, Georg-Johann Lay wrote:
>> +#if defined (L_loop_ffsqi2)
>> +;; Helper for ffshi2, ffssi2
>> +;; r25:r24 = r26 + zero_extend16 (ffs8(r24))
>> +;; r24 must be != 0
>> +;; clobbers: r26
>> +DEFUN __loop_ffsqi2
>
> Why does this function have "loop" in its name? The actual
> implementation is surely irrelevant.
hmmm. I needed some global name that can be referenced from __ffshi2
resp. __ffssi2. The function in itself is not very helpful as stand
alone. You prefer some other naming for such global helpers?
>> +DEFUN __ffshi2
>> + clr r26
>> + cpse r24, __zero_reg__
>> +1: XJMP __loop_ffsqi2
>> + ldi r26, 8
>> + or r24, r25
>
> It probably doesn't matter to execution speed, but why the
> OR here, when you know that r24 is 0? Wouldn't the logic
> be clearer spelling this with MOV?
The following instruction is BRNE, a conditional branch.
MOV does not modify condition code. So OR is used. Alternative would
be EOR. Or MOV+TST (note that TST Rx is sugar for AND Rx,Rx).
>> +#if defined (L_ctzsi2)
>> +;; count trailing zeros
>> +;; r25:r24 = ctz32 (r25:r22)
>> +;; ctz(0) = 32
>
> Note that GCC does not define ctz(0). It's explicitly undefined.
> Why are you forcing a particular value here?
Yes, you are right. Following patchlet ok?
Johann
* config/avr/libgcc.S (__ctzsi2, __ctzhi2):
Map zero to 255.
Index: config/avr/libgcc.S
===================================================================
--- config/avr/libgcc.S (Revision 175104)
+++ config/avr/libgcc.S (Arbeitskopie)
@@ -977,12 +977,10 @@ ENDF __loop_ffsqi2
#if defined (L_ctzsi2)
;; count trailing zeros
;; r25:r24 = ctz32 (r25:r22)
-;; ctz(0) = 32
+;; ctz(0) = 255
DEFUN __ctzsi2
XCALL __ffssi2
dec r24
- sbrc r24, 7
- ldi r24, 32
ret
ENDF __ctzsi2
#endif /* defined (L_ctzsi2) */
@@ -990,12 +988,10 @@ ENDF __ctzsi2
#if defined (L_ctzhi2)
;; count trailing zeros
;; r25:r24 = ctz16 (r25:r24)
-;; ctz(0) = 16
+;; ctz(0) = 255
DEFUN __ctzhi2
XCALL __ffshi2
dec r24
- sbrc r24, 7
- ldi r24, 16
ret
ENDF __ctzhi2
#endif /* defined (L_ctzhi2) */
> r~
