Re: [PATCH]: fix 32bits integer overflow in loops_per_jiffy calculation

Gabriel Paubert
2002-08-22T12:08:28Z

Hi Ben,

> None of the above can happen in the domain of application of this
> function. It's used to scale up/down the loops_per_jiffy value when
> scaling the CPU frequency. Anyway, the above isn't worse than the
> original function. Ideally, we would want 64 bits arithmetics, but we
> decided long ago not to bring the libcc support routines for that in
> the kernel.

Well, first on sane archs which have an easily accessible, fixed
frequency time counter, loops_per_jiffy should never have existed :-)

Second, putting this code there means that one day somebody will
inevitably try to use it outside of its domain of operation (like it
happened for div64 a few months ago when I pointed out that it would not
work for divisors above 65535 or so).

Finally, I agree that we should not import libgcc, but for example on
PPC32 the double lengths shifts (__ashrdi3, __ashldi3, and __lshsldi3)
are implemented somewhere, and the assembly implementation (directly
taken from some appendix in PPC documentation, I just slightly twisted
__ashrdi3 to make it branchless AFAIR) is actually way faster than the
one in libgcc ;-), and less than half the size.

Adding a few subroutines that implement a subset of libgcc's
functionality is necessary for most archs (which functions are needed is
arch, and sometimes compiler's, dependent).

In this case a generic scaling function, while not a standard libgcc/C
library feature has potentially more applications than this simple
cpufreq approximation. But I don't see very much the need for scaling a
long (64 bit on 64 bit archs) value, 32 bit would be sufficient.

Regards,
Gabriel.