On Wed, 10 Dec 2008, David Howells wrote:
> Yuri Tikhonov <[EMAIL PROTECTED]> wrote:
> > 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.
The case were the multiplier is 0 (actually smaller than 1, but not integer)
is handled by
#if (8 * THREAD_SIZE) > PAGE_SIZE
max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
#else
...
> If you're going to bracket the expression, it must be:
>
> max_threads = (mempages * PAGE_SIZE) / (8 * THREAD_SIZE);
>
> which should be superfluous.
No, `mempages * PAGE_SIZE' may overflow.
With kind regards,
Geert Uytterhoeven
Software Architect
Sony Techsoft Centre Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium
Phone: +32 (0)2 700 8453
Fax: +32 (0)2 700 8622
E-mail: [EMAIL PROTECTED]
Internet: http://www.sony-europe.com/
A division of Sony Europe (Belgium) N.V.
VAT BE 0413.825.160 · RPR Brussels
Fortis · BIC GEBABEBB · IBAN BE41293037680010
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev
