Hello David,
On Wednesday, December 10, 2008 you wrote:
> 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 '{}'.
Yes, it was a typo.
>> 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.
I think Geert has already commented this: you've compiled your alt()
functions having 4K PAGE_SIZE and 8K THREAD_SIZE - this case is
handled by the old code in fork_init.
> 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.
On powerpc shifting left is multiplication by 2, as this has the most
significant bit first.
Regards, Yuri
--
Yuri Tikhonov, Senior Software Engineer
Emcraft Systems, www.emcraft.com
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