Torbjorn Granlund <t...@gmplib.org> writes: > But IIUC, we are thus performing a 32 x 32 -> 64 mul per cycle. > Can one stick addition here without consuming cycles?
As I understand the manual, operations in the main cpu can be done in parallel with the simd instructions. But it also warns about transferring data between core registers and simd registers, with little details. Doing carry propagation with the simd registers seems awkward. One would need some comparisons to get carry conditions. And to make it even worse, it seems the comparison instruction, vcgt, doesn't support 64bit operations. One can do two parallel umlaal using vmull (two 32x32 -> 64), vaddl (two 32 + 32 -> 64), vadd (two 64-bit adds) That avoids carry propagation beyond 64 bits. Is it possible to arrange an addmul_2 (or any other interesting function) with two *independent* umaal-like operations? If we have to accept that we can't do any adds in parallel, addmul_2 would need something like vmull.u32, computing u0*v0 and u1 *v0 vaddl.u32, lo (chain variable) + r0 (result area) vadd.u64, add above to u0*v0 vaddl.u32, hi (chain variable) + high half of above sum vadd.u64, add above sum to u1 * v0 Looks like 6 cycles (which is poor, right?), excluding any data movement. And recurrency latency of four adds, which shouldn't be too bad, I imagine. There's also vmlal (mul and accumulate). One could shave one cycle off the recurrency chain by using vmlal rather than vmull, to add in r0 earlier, and then deleting one of the low adds. And one could possibly add in hi (high part of chain variable) with the same vmlal, but I'm not sure that's very usful. The challenge is that one still has to add the high part of the low product into the low part of the high product, and that's serial, not parallel. But one could potentially reduce the number of instructions vmlal.u32, compute u0*v0 + r0 and u1*v0 + hi vadd.u64, add lo (chain variable) to low product vadd.u64, add high half of above to high product That would be 4 cycles, but one also needs to somehow extend the values we add from 32 bits to 64, which I guess isn't for free. Regards, /Niels -- Niels Möller. PGP-encrypted email is preferred. Keyid C0B98E26. Internet email is subject to wholesale government surveillance. _______________________________________________ gmp-devel mailing list gmp-devel@gmplib.org http://gmplib.org/mailman/listinfo/gmp-devel
