Date: 15 Dec 2016 22:46:18 -0500 From: "George Spelvin" <linux@...encehorizons.net> To: ak@...ux.intel.com, davem@...emloft.net, David.Laight@...lab.com, ebiggers3@...il.com, hannes@...essinduktion.org, Jason@...c4.com, jeanphilippe.aumasson@...il.com, kernel-hardening@...ts.openwall.com, linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org, linux@...encehorizons.net, luto@...capital.net, netdev@...r.kernel.org, tom@...bertland.com, torvalds@...ux-foundation.org, tytso@....edu, vegard.nossum@...il.com Cc: djb@...yp.to Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF Jean-Philippe Aumasson wrote: > If a halved version of SipHash can bring significant performance boost > (with 32b words instead of 64b words) with an acceptable security level > (64-bit enough?) then we may design such a version. It would be fairly significant, a 2x speed benefit on a lot of 32-bit machines. First is the fact that a 64-bit SipHash round on a generic 32-bit machine requires not twice as many instructions, but more than three. Consider the core SipHash quarter-round operation: a += b; b = rotate_left(b, k) b ^= a The add and xor are equivalent between 32- and 64-bit rounds; twice the instructions do twice the work. (There's a dependency via the carry bit between the two halves of the add, but it ends up not being on the critical path even in a superscalar implementation.) The problem is the rotates. Although some particularly nice code is possible on 32-bit ARM due to its support for shift-and-xor operations, on a generic 32-bit CPU the rotate grows to 6 instructions with a 2-cycle dependency chain (more in practice because barrel shifters are large and even quad-issue CPUs can't do 4 shifts per cycle): temp_lo = b_lo >> (32-k) temp_hi = b_hi >> (32-k) b_lo <<= k b_hi <<= k b_lo ^= temp_hi b_hi ^= temp_lo The resultant instruction counts and (assuming wide issue) latencies are: 64-bit SipHash "Half" SipHash Inst. Latency Inst. Latency 10 3 3 2 Quarter round 40 6 12 4 Full round 80 12 24 8 Two rounds 82 13 26 9 Mix in one word 82 13 52 18 Mix in 64 bits 166 26 61 18 Four round finalization + final XOR 248 39 113 36 Hash 64 bits 330 52 165 54 Hash 128 bits 412 65 217 72 Hash 192 bits While the ideal latencies are actually better for the 64-bit algorithm, that requires an unrealistic 6+-wide superscalar implementation that's more than twice as wide as the 64-bit code requires (which is already optimized for quad-issue). For a 1- or 2-wide processor, the instruction counts dominate, and not only does the 64-bit algorithm take 60% more time to mix in the same number of bytes, but the finalization rounds bring the ratio to 2:1 for small inputs. (And I haven't included the possible savings if the input size is an odd number of 32-bit words, such as networking applications which include the source/dest port numbers.) Notes on particular processors: - x86 can do a 64-bit rotate in 3 instructions and 2 cycles using the SHLD/SHRD instructions instead: movl %b_hi, %temp shldl $k, %b_lo, %b_hi shldl $k, %temp, %b_lo ... but as I mentioned the problem is registers. SipHash needs 8 32-bit words plus at least one temporary, and 32-bit x86 has only 7 available. (And compilers can rarely manage to keep more than 6 of them busy.) - 64-bit SipHash is particularly efficient on 32-bit ARM due to its support for shift-and-op instructions. The 64-bit shift and following xor can be done in 4 instructions. So the only benefit is from the reduced finalization. - Double-width adds cost a little more on CPUs like MIPS and RISC-V without condition codes. - Certain particularly crappy uClinux processors with slow shifts (68000, anyone?) really suffer from extra shifts. One *weakly* requested feature: It might simplify some programming interfaces if we could use the same key for multiple hash tables with a 1-word "tweak" (e.g. pointer to the hash table, so it could be assumed non-zero if that helped) to make distinct functions. That would let us more safely use a global key for multiple small hash tables without the need to add code to generate and store key material for each place that an unkeyed hash is replaced.
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