On Fri, Dec 21, 2018 at 12:29:46PM -0800, Matthew Wilcox wrote: > > > > > > That's inefficient. > > > > > > pattern = (unsigned char)c; > > > pattern |= pattern << 8; > > > pattern |= pattern << 16; > > > pattern |= pattern << 32; > > > > Won't > > > > pattern = 0x0101010101010101 * c; > > > > do the same but faster? > > Depends on your CPU. Some yes, some no. > > (Also you need to cast 'c' to unsigned char to avoid someone passing in > 0x1234 and getting 0x4646464646464634 instead of 0x3434343434343434)
Cast to unsigned char is needed in any case. And as far as I remember we've been using this multiplication trick for a really long time in x86 land. I'm out of sources right now but it should be somewhere in assembly libs.
