On Tue, Mar 31, 2015 at 6:44 PM, Rob Clark <robdcl...@gmail.com> wrote: > On Tue, Mar 31, 2015 at 9:03 PM, Roland Scheidegger <srol...@vmware.com> > wrote: >> So if this is not enough precision, maybe should state how large the >> error can be? >> > > tbh, if I knew what the error for this approach was, I would have > included it. I'm not the original author, but this is based on > nouveau codegen code (as mentioned in the comment). I guess it is > better than converting to float and dividing and converting back, but > worse than an iterative (ie. looping, ie. divergent flow control) > approach. It is apparently enough to keep piglit happy. > > The original algo in nv50 lowering code is from > 322bc7ed68ed92233c97168c036d0aa50c11a20e (ie. 'nv50/ir: import nv50 > target') which doesn't really give more clue about the origin.. > > if anyone knows, I'm all ears and will add relevant links/info to comment..
FWIW: The DEC Alpha doesn't have an integer divide instruction, and its architecture reference manual [0] suggests in section A.4.2: > For the remaining cases, a table lookup on about a 1000-entry table and > a multiply can give a linear approximation to 1/divisor that is accurate to > 16 bits. > > Using this approximation, a multiply and a back-multiply and a subtract can > generate one 16-bit quotient digit plus a 48-bit new partial dividend. Three > more such steps can generate the full quotient. Having prior knowledge of the > possible sizes of the divisor and dividend, normalizing away leading bytes of > zeros, and performing an early-out test can reduce the average number of > multiplies to about five (compared to a best case of one and a worst case of > nine). I read on IRC that at least some (radeon?) hardware has an instruction like recip_int that might make this a compelling option, since you wouldn't even need the table. I can probably dig up an implementation if this sounds interesting. Otherwise, I think it's a series of subtracts and shifts. [0] http://download.majix.org/dec/alpha_arch_ref.pdf _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
