[U-Boot] [PATCH V2] console: Implement pre-console buffer

[Simon Glass]

Hi Wolfgang,

On Tue, Aug 30, 2011 at 1:57 PM, Wolfgang Denk <wd at denx.de> wrote:
> Dear Simon Glass,
>
> In message <CAPnjgZ1dQXMVeFk47=Gg5y+TCHG7WTdGk0b1WjSqOBhu2d0xnQ at mail.gmail.com> you wrote:
>>
>> In case it is interesting (which may be unlikely) here is the code
>> generated by my compiler (common code stripped) for your example, and
>
> Which architecture / which compiler is this?

It was ARM, gcc version 4.4.3.

>
>> one I added. It seems that % makes it worry about sign.
>
> Indeed!  We should use unsigned numbers here.  MUCH better now:
>

Yes it is much better. Perhaps an (unsigned) cast in the C code is
better than hoping that the board author uses 1024U or similar.

>
> unsigned int ufoo(unsigned int i)
> {
>        return i & (CONFIG_SYS_TMP_CON_BUF_SZ-1);
> }
>
> For ARM, BUF_SZ = 1024
>
>        mov     r0, r0, asl #22
>        mov     r0, r0, lsr #22
>
> For PPC, BUF_SZ = 1024
>
>        rlwinm 3,3,0,22,31
>
> The following two entries make no sense, just for completeness:
>
> For ARM, BUF_SZ = 1021  (Note: 1021 is a prime number)
>
>        and     r0, r0, #1020
>
> For PPC, BUF_SZ = 1021
>
>        rlwinm 3,3,0,22,29
>
>
>
> unsigned int ubar(unsigned int i)
> {
>        return i % CONFIG_SYS_TMP_CON_BUF_SZ;
> }
>
> For ARM, BUF_SZ = 1024
>
>        mov     r0, r0, asl #22
>        mov     r0, r0, lsr #22
>
> For PPC, BUF_SZ = 1024
>
>        rlwinm 3,3,0,22,31
>
> For ARM, BUF_SZ = 1021
>
>        ldr     r2, .L11
>        mov     ip, r0
>        umull   r1, r3, r2, r0
>        rsb     r1, r3, r0
>        add     r3, r3, r1, lsr #1
>        mov     r3, r3, lsr #9
>        mov     r0, r3, asl #10
>        sub     r0, r0, r3, asl #2
>        add     r0, r0, r3
>        rsb     r0, r0, ip
>
> For PPC, BUF_SZ = 1021
>
>        lis 0,0xc0
>        ori 0,0,36973
>        mulhwu 0,3,0
>        subf 9,0,3
>        srwi 9,9,1
>        add 0,0,9
>        srwi 0,0,9
>        mulli 0,0,1021
>        subf 3,0,3
>
>
> So indeed we should use unsigned arithmetics, and try to use
> power-of-two sizes where possible.
>

Yes indeed.

Regards,
Simon

> Best regards,
>
> Wolfgang Denk
>
> --
> DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
> Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
> You can't evaluate a man by logic alone.
>        -- McCoy, "I, Mudd", stardate 4513.3
>