Re: Fwd: about udelay in mips

[Sergei Shtylyov <sshtylyov@mvista.com>]

Hello.

On 27-01-2011 12:20, loody wrote:

> hi all:
> I guess there seems be some differences about
> "unsigned long long" and "ull" so I forward the letter to gcc-help.
> If my guess is correct, what are the differences between them and why
> "unsigned long long" cannot let compiler compile the 64-bits
> operations as I want?

    Probably because in 2.6.30 you only cast the result of 32-bit multiplies 
to 64 bits. In the 2.6.33 kernel, the mutliplies are 64-bit as the 
0x000010c7ull constant is 64-bit...

> thanks a lot,
> miloody

> I found my kernel will compile udelay(xx) as zero no matter what xx I filled in.
> below are the dis-assemblies:
> (as you can see the v0 = v1 = zero.)
> My version is 2.6.30.9:
> void __udelay(unsigned long us)
> {
>         unsigned int lpj = current_cpu_data.udelay_val;
>
>         __delay(((unsigned long long)(us * 0x000010c7 * HZ * lpj)) >> 32);
> 80306ed0:       00001821        move    v1,zero
> 80306ed4:       00601021        move    v0,v1
> #include<asm/compiler.h>
> #include<asm/war.h>
>
> inline void __delay(unsigned int loops)
> {
>         __asm__ __volatile__ (
> 80306ed8:       1440ffff        bnez    v0,80306ed8 <__udelay+0x8>
> 80306edc:       2442ffff        addiu   v0,v0,-1
> void __udelay(unsigned long us)

> I have double checked the __delay source code of 2.6.33.4
> and the dis-assemblies:

> void __udelay(unsigned long us)
> {
>         unsigned int lpj = current_cpu_data.udelay_val;
>
>         __delay((us * 0x000010c7ull * HZ * lpj) >> 32);
> 802f7310:       3c02804f        lui     v0,0x804f
> 802f7314:       8c429360        lw      v0,-27808(v0)
> 802f7318:       3c050010        lui     a1,0x10
> 802f731c:       34a56256        ori     a1,a1,0x6256
> 802f7320:       00450019        multu   v0,a1
> 802f7324:       00002821        move    a1,zero
> 802f7328:       00001012        mflo    v0
> 802f732c:       00001810        mfhi    v1
> 802f7330:       00a20018        mult    a1,v0
> 802f7334:       70640000        madd    v1,a0
> 802f7338:       00003012        mflo    a2
> 802f733c:       00440019        multu   v0,a0
> 802f7340:       00001810        mfhi    v1
> 802f7344:       00c31021        addu    v0,a2,v1
> #include<asm/compiler.h>
> #include<asm/war.h>
>
> inline void __delay(unsigned int loops)
> {
>         __asm__ __volatile__ (
> 802f7348:       1440ffff        bnez    v0,802f7348 <__udelay+0x38>
> 802f734c:       2442ffff        addiu   v0,v0,-1
> void __udelay(unsigned long us)
> {
>         unsigned int lpj = current_cpu_data.udelay_val;
>
>         __delay((us * 0x000010c7ull * HZ * lpj)>>  32);
> }
> 802f7350:       03e00008        jr      ra

WBR, Sergei