`On Mon, 26 Mar 2012 20:51:24 +0200Denys Vlasenko <vda.linux@googlemail.com> wrote:> commit 01a2904d31d2373886f489429ec662c9be64a6ab> Author: Denys Vlasenko <vda.linux@googlemail.com>> Date:   Mon Mar 26 20:40:53 2012 +0200> >     vsprintf: optimize decimal conversion (again)> >     Previous code was using optimizations which were developed>     to work well even on narrow-word CPUs (by today's standards).>     But Linux runs only on 32-bit and wider CPUs. We can use that.> >     First: using 32x32->64 multiply and trivial 32-bit shift,>     we can correctly divide by 10 much larger numbers, and thus>     we can print groups of 9 digits instead of groups of 5 digits.> >     Next: there are two algorithms to print larger numbers.>     One is generic: divide by 1000000000 and repeatedly print>     groups of (up to) 9 digits. It's conceptually simple,>     but requires an (unsigned long long) / 1000000000 division.> >     Second algorithm splits 64-bit unsigned long long into 16-bit chunks,>     manipulates them cleverly and generates groups of 4 decimal digits.>     It so happens that it does NOT require long long division.> >     If long is > 32 bits, division of 64-bit values is relatively easy,>     and we will use the first algorithm.>     If long long is > 64 bits (strange architecture with VERY large long long),>     second algorithm can't be used, and we again use the first one.> >     Else (if long is 32 bits and long long is 64 bits) we use second one.> >     And third: there is a simple optimization which takes fast path>     not only for zero as was done before, but for all one-digit numbers.> >     In all tested cases new code is faster than old one, in many cases by 30%,>     in few cases by more than 50% (for example, on x86-32, conversion of 12345678).>     Code growth is ~0 in 32-bit case and ~130 bytes in 64-bit case.> This patch is so nutty that I like it.> +#if BITS_PER_LONG != 32 || (~(0ULL)>>1) != ((1ULL<<63)-1)What's this for?`