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Re: Fwd: about udelay in mips

To: loody <miloody@gmail.com>
Subject: Re: Fwd: about udelay in mips
From: Sergei Shtylyov <sshtylyov@mvista.com>
Date: Thu, 27 Jan 2011 14:28:15 +0300
Cc: gcc-help <gcc-help@gcc.gnu.org>, Linux MIPS Mailing List <linux-mips@linux-mips.org>
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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

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