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Re: An optimization question


eCos@sunnorth.com.cn wrote:

> Here is an optimization question about gcc compiler, we wonder whether it 
> is a bug or not.
> 
> A simple test program here:
> ===============================================
> int *p;
> 
> int main(void)
> {
>         p++;
>         __asm__ __volatile__ (""::);
>         p++;
> }
> ===============================================
> And it is comiled like this:
> gcc -S -O2 -o xxx.s xxx.c
> 
> Because of the '__asm__ __volatile ...' clause, the 1st 'p++' & the 2nd 
> 'p++' will not be merged together, so
> assembly code is like: 
> 'addl $4, %eax'
> 'addl $4, %eax'
> 
> 
> But if the program is modified as below:
> ===============================================
> int *p;
> 
> int main(void)
> {
>         p++;
>         __asm__ __volatile__ (""::);
>         p+=8;
> }
> ===============================================
> 
> Still, it is compiled like this:
> gcc -S -O2 -o xxx.s xxx.c
> According to the assembly code, we found that gcc merge the 'p++' & 'p+=8' 
> and generate 'addl $36, p'
> 
> We have the intuition that if '__asm__ __volatile ...' is used, the code 
> will be separated into two parts and will not be merged together for 
> optimization.
> Are we right?
> If so, why should the second case happen? Is it a bug or something else?

If you really need a memory barrier, you have to use one:

int main(void)
{
        p++;
        __asm__ __volatile__ ("":::"memory");
        p+=8;
}

Andrew.


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