Messages in this thread | | | Date | Wed, 12 Mar 2003 02:48:36 +0200 | From | Matti Aarnio <> | Subject | Re: Runaway cron task on 2.5.63/4 bk? |
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On Tue, Mar 11, 2003 at 03:02:31PM -0800, Linus Torvalds wrote: > On Tue, 11 Mar 2003, Andrew Morton wrote: > > gcc will generate 64bit * 64bit multiplies without resorting to > > any library code > > However, gcc is unable to do-the-right-thing and generate 32x32->64 > multiplies, or 32x64->64 multiplies, even though those are both a _lot_ > faster than the full 64x64->64 case. > > And in quite a _lot_ of cases, that's actually what you want. It might > actually make sense to add a "do_mul()" thing to allow architectures to do > these cases right, since gcc doesn't.
Some architectures have a bit stricter limitations -- S390 limits divisor to 2^31-1, for example.
A number of systems simply flaunt the task, and instead implement mere 32/32 division in do_div(). (arm, cris, m68knommu, sh (?), sparc(32), v850)
The original pure C code to do 64/32 division to 64/32 results is very much gone in favour of architecture specific assembly codes (where system isn't 64 bit one already..) Ah, include/asm-parisc/div64.h still has it in 2.5.64 sources..
> > and you can probably do the division with do_div().
If you need arbitrary divisions at all. Filesystems for example can (in most cases) do with power-of-two divisions, e.g.: LL >> count You may, perhaps, need to pre-calculate a number of those shift-counts when mounting a filesystem.
> Yes. This is the same issue - gcc will always promote a 64-bit divide to > be _fully_ 64-bit, even if the mixed-size 64/32 -> [64,32] case is much > faster and simpler. Which is why do_div() exists in the first place.
Originally it was lib/vsprintf.c's internal (and very portable) divide numerator by small base, produce changed numerator, and remainder... The innermost element in arbitrary base number printing.
In 2.5 there is some odd: #define sector_div(a, b) do_div(a, b) (only with CONFIG_LDB), and usage in jiffie-to-clock conversion... ... and all over the code in various odd nooks, XFS filesystem included...
> Linus
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