Yuri Tikhonov <[EMAIL PROTECTED]> wrote:
> Here we believe in preprocessor: since all PAGE_SIZE, 8, and
> THREAD_SIZE are the constants we expect it will calculate this.
The preprocessor shouldn't be calculating this. I believe it will _only_
calculate expressions for #if. In the situation you're referring to, it
should perform a substitution and nothing more. The preprocessor doesn't
necessarily know how to handle the types involved.
In any case, there's an easy way to find out: you can ask the compiler to give
you the result of running the source through the preprocessor only. For
instance, if you run this:
#define PAGE_SIZE 4096
#define THREAD_SIZE 8192
unsigned long mempages;
unsigned long jump(void)
{
unsigned long max_threads;
max_threads = mempages * PAGE_SIZE / (8 * THREAD_SIZE);
return max_threads;
}
through "gcc -E", you get:
# 1 "calc.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "calc.c"
unsigned long mempages;
unsigned long jump(void)
{
unsigned long max_threads;
max_threads = mempages * 4096 / (8 * 8192);
return max_threads;
}
> In any case, adding braces as follows probably would be better:
>
> + max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
I think you mean brackets, not braces '{}'.
> Right ?
Definitely not.
I added this function to the above:
unsigned long alt(void)
{
unsigned long max_threads;
max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
return max_threads;
}
and ran it through "gcc -S -O2" for x86_64:
jump:
movq mempages(%rip), %rax
salq $12, %rax
shrq $16, %rax
ret
alt:
xorl %eax, %eax
ret
Note the difference? In jump(), x86_64 first multiplies mempages by 4096, and
_then_ divides by 8*8192.
In alt(), it just returns 0 because the compiler realised that you're
multiplying by 0.
If you're going to bracket the expression, it must be:
max_threads = (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);
which should be superfluous.
> E.g. here is the result from this line as produced by cross-gcc
> 4.2.2:
>
> lis r9,0
> rlwinm r29,r29,2,16,29
> stw r29,0(r9)
>
> As you see - only rotate-left, i.e. multiplication to the constant.
Ummm... On powerpc, I believe rotate-left would be a division as it does the
bit-numbering and the bit direction the opposite way to more familiar CPUs
such as x86.
David
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev
