Messages in this thread | | | From | Denys Vlasenko <> | Subject | Re: [PATCH 1/1] vsprintf: optimize decimal conversion (again) | Date | Tue, 27 Mar 2012 01:18:38 +0200 |
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On Monday 26 March 2012 22:18, Geert Uytterhoeven wrote: > On Mon, Mar 26, 2012 at 22:13, Andrew Morton <akpm@linux-foundation.org> wrote: > > On Mon, 26 Mar 2012 21:56:38 +0200 > > Denys Vlasenko <vda.linux@googlemail.com> wrote: > > > >> >> +#if BITS_PER_LONG != 32 || (~(0ULL)>>1) != ((1ULL<<63)-1) > >> > > >> > What's this for? > >> > >> The second check should be just BITS_PER_LONG_LONG != 64, > >> but we don't have BITS_PER_LONG_LONG. > > > > So let's add BITS_PER_LONG_LONG rather than hacking around its absence! > > I don't think Linux runs on anything with BITS_PER_LONG_LONG != 64... > > BTW, what about CPUs with slow 32x32 multiplication and/or slow 64-bit > division?
Without 32x32->64 multiply, the best we can generate is 4 decimal digits: we produce next digit by approximating x/10 with (x * 0xcccd) >> 19, and the first x where it gives wrong result is 81920 if multiply result is truncated to 32 bits. With it, we can generate 9 digits using (x * 0x1999999a) >> 32.
Regrading "slow 64-bit division" - after this patch, 32-bit machines wouldn't use it at all. Only 64-bit machines will perform 64-bit division, one per 9 decimal digits (thus, at most three divisions per one long_long->string conversion).
In fact, with small change to #ifdefs, all machines with long long <= 64 bits can use division-less routine. It might be a good thing to try...
Any people with ARM hardware in hand interesting in running the test program I sent in first email?
-- vda
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