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RFC: optimize `CheckBlock` input checks (duplicate detection & nulls)

Placeholder l0rinc requested to merge github/fork/l0rinc/l0rinc/optimize-CheckBlock-input-duplicate-check into master

This PR is in draft mode to gather comments since it contains consensus code refactorings.

CheckBlock's latency is critical for efficiently validating correct inputs during transaction validation, including mempool acceptance and new block creation.

This PR improves performance and maintainability by introducing the following changes:

  • Simplified checks for the most common cases (1 or 2 inputs - 70-90% of transactions have a single input).
  • Optimized the general case by replacing std::set with sorted std::vector for improved locality.
  • Simplified Null prevout checks from linear to constant time.

The goal of this change is to make block validation faster via a low-risk alternative.

C++ compiler .......................... AppleClang 16.0.0.16000026

Before:

ns/block block/s err% total benchmark
372,743.63 2,682.81 1.1% 10.99 CheckBlockBench
ns/op op/s err% total benchmark
3,304,694.54 302.60 0.5% 11.05 DuplicateInputs
9,585.10 104,328.55 0.1% 11.03 ProcessTransactionBench

After:

ns/block block/s err% total benchmark
179,971.00 5,556.45 0.3% 11.02 CheckBlockBench
ns/op op/s err% total benchmark
963,177.98 1,038.23 1.7% 10.92 DuplicateInputs
9,410.90 106,259.75 0.3% 11.01 ProcessTransactionBench
  • 2.07x faster CheckBlockBench
  • 3.4x faster DuplicateInputs
  • 1.8% faster ProcessTransactionBench
C++ compiler .......................... GNU 13.3.0

Before:

ns/block block/s err% ins/block cyc/block IPC bra/block miss% total benchmark
1,096,261.84 912.19 0.1% 7,963,390.88 3,487,375.26 2.283 1,266,941.00 1.8% 11.03 CheckBlockBench
ns/op op/s err% ins/op cyc/op IPC bra/op miss% total benchmark
8,366,309.48 119.53 0.0% 23,865,177.67 26,620,160.23 0.897 5,972,887.41 4.0% 10.78 DuplicateInputs
56,199.57 17,793.73 0.1% 229,263.01 178,766.31 1.282 15,509.97 0.5% 10.91 ProcessTransactionBench

After:

ns/block block/s err% ins/block cyc/block IPC bra/block miss% total benchmark
834,855.94 1,197.81 0.0% 6,518,548.86 2,656,039.78 2.454 919,160.84 1.5% 10.78 CheckBlockBench
ns/op op/s err% ins/op cyc/op IPC bra/op miss% total benchmark
4,261,492.75 234.66 0.0% 17,379,823.40 13,559,793.33 1.282 4,265,714.28 3.4% 11.00 DuplicateInputs
55,819.53 17,914.88 0.1% 227,828.15 177,520.09 1.283 15,184.36 0.4% 10.91 ProcessTransactionBench
  • 31% faster CheckBlockBench
  • 2x faster DuplicateInputs
  • 0.5% faster ProcessTransactionBench

While the point of the change isn't necessarily to speed up IBD, but you can see the measurements at https://github.com/bitcoin/bitcoin/pull/31682#issuecomment-2606765689

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