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CPU Features for Nimbus

This document describes the CPU-specific features and compilation flags that significantly improves Nimbus performance.

We focus on x86-64 and ARMv8 (64 bits). Given that the major bottleneck of Nimbus is big integer for cryptography, 64-bit architecture improves elliptic curve cryptography processing by ~2x over 32 bits since we can divide the number of low-level assembly operations by half.

Note: SHA256 isn't improved by 64-bit since it uses 32-bit operations by design

The major bottlenecks that can be improved by CPU specific instructions are:

  • Elliptic curve cryptography for BLS12-381
  • SHA256 hashing

x86

SSSE3 (Supplemental SSE3)

Intel: Core 2, 2006
AMD: Bulldozer, 2011
Flag: -mssse3 Configuration: https://github.com/supranational/blst/blob/v0.3.4/build/assembly.S#L3-L6

SSSE3 improves SHA256 computations. SHA256 is used recursively to hash all consensus objects and to build a Merkle tree. Thanks to caching, SHA256 computation speed is mostly relevant only when receiving new blocks and attestations from the network, but state transitions do not depend on it (unlike a naive spec implementation).

SSSE3 must not be confused with SSE3 from Pentium 3 (2004) and Athlon 64 (2005)

git clone https://github.com/status-im/nim-blscurve
cd nim-blscurve
git submodule update --init
nim c -r -d:danger --passC:"-D__BLST_PORTABLE__" --outdir:build benchmarks/bench_sha256.nim
nim c -r -d:danger --outdir:build benchmarks/bench_sha256.nim

Due to tree hashing, hashing 32 bytes is the most important benchmark.

Without SSSE3

Backend: BLST, mode: 64-bit
==================================================================================

SHA256 - 32B - BLST       4524886.878 ops/s          221 ns/op          660 cycles
SHA256 - 128B - BLST      1776198.934 ops/s          563 ns/op         1689 cycles
SHA256 - 5MB - BLST            70.723 ops/s     14139678 ns/op     42419720 cycles

With SSSE3

Backend: BLST, mode: 64-bit
==================================================================================

SHA256 - 32B - BLST       5376344.086 ops/s          186 ns/op          555 cycles
SHA256 - 128B - BLST      2183406.114 ops/s          458 ns/op         1376 cycles
SHA256 - 5MB - BLST            87.142 ops/s     11475557 ns/op     34427254 cycles

BMI2 & ADX

Intel: Broadwell, 2015
AMD: Ryzen, 2017
Configuration: https://github.com/supranational/blst/blob/v0.3.4/build/assembly.S#L18

The MULX instruction (BMI2), ADCX and ADOX (ADX) significantly improves big integer multiplication and squaring. The speedup is about 20~25% depending on the custom assembly implementation.

All CPUs that support ADX support BMI2.

git clone https://github.com/status-im/nim-blscurve
cd nim-blscurve
git submodule update --init
nim c -r -d:danger --hints:off --warnings:off --verbosity:0 --outdir:build benchmarks/bls_signature.nim
nim c -r -d:danger --passC:"-mbmi2 -madx" --hints:off --warnings:off --verbosity:0 --outdir:build benchmarks/bls_signature.nim

Verification is the bottleneck as it must be done for each block and attestation or aggregate received and verifying a block requires verifying up to 6 signatures (block proposer, RANDAO, aggregate verifification of attestations, proposer slashings, attester slashings, voluntary exits). Signing can become a bottleneck when a node has many validators.

Without BMI2 & ADX

Backend: BLST, mode: 64-bit
=============================================================================================================

BLS signature                                           1960.023 ops/s       510198 ns/op      1530624 cycles
BLS verification                                         743.122 ops/s      1345674 ns/op      4037105 cycles
BLS agg verif of 1 msg by 128 pubkeys                    704.634 ops/s      1419176 ns/op      4257591 cycles
BLS verif of 6 msgs by 6 pubkeys                         120.588 ops/s      8292683 ns/op     24878257 cycles
Serial batch verify 6 msgs by 6 pubkeys (with blinding)  218.027 ops/s      4586595 ns/op     13759932 cycles

With BMI2 & ADX

Backend: BLST, mode: 64-bit
=============================================================================================================

BLS signature                                           2550.084 ops/s       392144 ns/op      1176454 cycles
BLS verification                                         930.081 ops/s      1075175 ns/op      3225589 cycles
BLS agg verif of 1 msg by 128 pubkeys                    878.672 ops/s      1138081 ns/op      3414286 cycles
BLS verif of 6 msgs by 6 pubkeys                         154.833 ops/s      6458588 ns/op     19376076 cycles
Serial batch verify 6 msgs by 6 pubkeys (with blinding)  282.562 ops/s      3539046 ns/op     10617328 cycles

SHA-NI

The hardware SHA instructions has NOT been available in Intel consumer hardware until 2021. AMD has made it available in Zen architecture since 2017.

Intel:

  • Rocket Lake (2021)
  • Ice Lake (low-power laptops 2018)
  • Goldmont (Apollo Lake Pentiums & Celerons 2016, Denverton Atoms 2017)

AMD: Ryzen, 2017
Flag: -msha Configuration: https://github.com/supranational/blst/blob/v0.3.4/src/sha256.h#L11-L12

On Ryzen, hardware SHA is 4X faster than when using SIMD instructions (Table 1, p14).

ARM

32-bit ARM (ARMv6) has a multiplication instruction 32x32 -> 64 called UMULL.

Unfortunately, 64-bit ARM (ARMv8) unlike x86-64 doesn't have a single 64x64 -> 128 multiplication instruction. MUL and UMULH instruction needs to be used for extended precision multiplication.

Concretely, this means that ARMv8 CPUs are impaired compared to x86-64 at equivalent frequency for big integers and cryptography (for example Apple M1).

Cryptographic extensions

Except for Raspberry Pi, ARMv8 processors support the crypto extensions which include hardware implementation of SHA256.

This is detected via

The compilation flag should be either

  • -mfpu=crypto-neon-fp-armv8
  • or -march=armv8-a+crypto

The speedup is expected to be 2x faster than without.
https://patchwork.kernel.org/project/linux-arm-kernel/patch/20150316154835.GA31336@google.com/