Re: [avr-gcc-list] [FIX] _clz and friends not found (test builtin-bitops-1)

[Wouter van Gulik]

Dmitry K. schreef:
> On Friday 25 January 2008 22:35, Wouter van Gulik wrote:
> > __clzqi2:
> >     clr     r_count     ; load with 0
> >     com     r_count     ; invert (load with -1) + set carry
> > __clzqi2_loop:
> >     rol     r_arg1L         ; Rotate through carry
> >     inc     r_count         ; Carry not touch by inc
> >     brcc    __clzqi2_loop   ; Branch on no carry
>
> That is splendid!

Thanks, the original idea is from the guy posting in the gcc bug report 
for clz.

> After a superficial view:
> . A short rcall/rjmp is not safe with unknown (big) library
> for intermodule link. Also a conditional branch.

Hmm, this is all ready so for __mulqihi3 and __umulqihi3, they do a rjmp 
to __mulhi3. So I thought it was save.
Also note that avr-libc's libm uses this exclusively. Maybe this is 
fixed by the linker relaxation?

On the conditional branch I totally agree. That is not a smart thing todo.

> . Are this function intended for math functions? If so,
> the strong size optimization is not a best solution (IMHO).
> An addition of few words may speed up in few times for
> some 32/16-bit functions.

Yes they are intended for math I guess. But I think they will never be 
used. CLZ/CTZ is only used when using floats and not linking with 
avr-libc's math library (libm). This has poor result any way, so user 
will probably quickly switch to libm.

I mainly implemented CLZ/popcount to fix the huge use of RAM by these 
functions.

CLZ is already a little optimized for speed. I could optimize further. 
Make the 16 bit check on a high byte being zero and then decide which 
one he should do on byte base. But then I would also need to link in the 
QI implementation, using even more flash.
The qi loop is 4 cycles, the hi loop is 5. Excluding the penalty for 
branching etc. So we could have a negative effect on speed for some values.

There is no bug report from a user that he is missing the other 
functionality, so I thought it is more a matter of having it so gcc 
passes the testsuite, not having it as fast,mean and lean implementation.

Wouter