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BlockRng optimizations #40

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@dhardy dhardy commented Dec 23, 2025

Summary

Optimize fill_bytes and next_u64_from_u32

Details

Eliminates an impossible-to-reach panic in fill_bytes's chunks.into_remainder handling.

Reduces next_u64_from_u32 to a single call instruction.

I'm not sure the result is optimal (there are several possible jumps in fill_bytes's chunk-iterator / memcpy code) but it's all I have time for and likely good enough.

Code on godbolt.

Benchmarks in progress...

@dhardy dhardy requested a review from newpavlov December 23, 2025 12:32
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dhardy commented Dec 23, 2025

Benchmarks (using https://github.com/rust-random/rand/pull/1697) 
random_bytes/pcg32      time:   [314.37 ns 314.76 ns 315.27 ns]
                        thrpt:  [3.0250 GiB/s 3.0299 GiB/s 3.0336 GiB/s]
                 change:
                        time:   [+5.1186% +5.3365% +5.5636%] (p = 0.00 < 0.05)
                        thrpt:  [-5.2704% -5.0661% -4.8694%]
                        Performance has regressed.
random_bytes/pcg64      time:   [253.99 ns 254.13 ns 254.29 ns]
                        thrpt:  [3.7503 GiB/s 3.7527 GiB/s 3.7548 GiB/s]
                 change:
                        time:   [+0.2050% +0.2781% +0.3456%] (p = 0.00 < 0.05)
                        thrpt:  [-0.3444% -0.2773% -0.2046%]
                        Change within noise threshold.
Found 24 outliers among 100 measurements (24.00%)
  6 (6.00%) low mild
  6 (6.00%) high mild
  12 (12.00%) high severe
random_bytes/pcg64mcg   time:   [212.15 ns 212.19 ns 212.22 ns]
                        thrpt:  [4.4937 GiB/s 4.4945 GiB/s 4.4953 GiB/s]
                 change:
                        time:   [-3.9152% -3.8398% -3.7600%] (p = 0.00 < 0.05)
                        thrpt:  [+3.9069% +3.9932% +4.0747%]
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  5 (5.00%) high mild
random_bytes/pcg64dxsm  time:   [256.53 ns 256.66 ns 256.82 ns]
                        thrpt:  [3.7134 GiB/s 3.7157 GiB/s 3.7176 GiB/s]
                 change:
                        time:   [+4.4420% +4.5508% +4.6617%] (p = 0.00 < 0.05)
                        thrpt:  [-4.4541% -4.3527% -4.2531%]
                        Performance has regressed.
random_bytes/chacha8    time:   [231.02 ns 231.09 ns 231.19 ns]
                        thrpt:  [4.1251 GiB/s 4.1269 GiB/s 4.1282 GiB/s]
                 change:
                        time:   [-12.047% -11.943% -11.841%] (p = 0.00 < 0.05)
                        thrpt:  [+13.432% +13.563% +13.697%]
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  5 (5.00%) high mild
  6 (6.00%) high severe
random_bytes/chacha12   time:   [301.49 ns 301.58 ns 301.67 ns]
                        thrpt:  [3.1614 GiB/s 3.1623 GiB/s 3.1632 GiB/s]
                 change:
                        time:   [-10.197% -10.080% -9.9426%] (p = 0.00 < 0.05)
                        thrpt:  [+11.040% +11.210% +11.354%]
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  2 (2.00%) low severe
  3 (3.00%) low mild
  1 (1.00%) high mild
random_bytes/chacha20   time:   [439.97 ns 440.10 ns 440.24 ns]
                        thrpt:  [2.1663 GiB/s 2.1670 GiB/s 2.1676 GiB/s]
                 change:
                        time:   [-4.0692% -4.0131% -3.9606%] (p = 0.00 < 0.05)
                        thrpt:  [+4.1239% +4.1808% +4.2418%]
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
  2 (2.00%) high severe
random_bytes/std        time:   [308.83 ns 308.91 ns 308.98 ns]
                        thrpt:  [3.0865 GiB/s 3.0873 GiB/s 3.0880 GiB/s]
                 change:
                        time:   [-3.9856% -3.9010% -3.8220%] (p = 0.00 < 0.05)
                        thrpt:  [+3.9739% +4.0593% +4.1511%]
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild
random_bytes/small      time:   [171.11 ns 171.17 ns 171.23 ns]
                        thrpt:  [5.5696 GiB/s 5.5714 GiB/s 5.5733 GiB/s]
                 change:
                        time:   [-0.2274% -0.1485% -0.0733%] (p = 0.00 < 0.05)
                        thrpt:  [+0.0733% +0.1487% +0.2280%]
                        Change within noise threshold.
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) high mild
random_bytes/os         time:   [1.4060 µs 1.4061 µs 1.4063 µs]
                        thrpt:  [694.44 MiB/s 694.50 MiB/s 694.56 MiB/s]
                 change:
                        time:   [-0.2810% -0.2473% -0.2165%] (p = 0.00 < 0.05)
                        thrpt:  [+0.2170% +0.2480% +0.2818%]
                        Change within noise threshold.
Found 7 outliers among 100 measurements (7.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
  2 (2.00%) high severe
random_bytes/thread     time:   [313.99 ns 314.76 ns 315.48 ns]
                        thrpt:  [3.0229 GiB/s 3.0299 GiB/s 3.0372 GiB/s]
                 change:
                        time:   [-1.0191% -0.8806% -0.7515%] (p = 0.00 < 0.05)
                        thrpt:  [+0.7572% +0.8884% +1.0295%]
                        Change within noise threshold.
Found 14 outliers among 100 measurements (14.00%)
  4 (4.00%) high mild
  10 (10.00%) high severe

random_u32/pcg32        time:   [1.0445 ns 1.0446 ns 1.0448 ns]
                        thrpt:  [3.5655 GiB/s 3.5661 GiB/s 3.5667 GiB/s]
                 change:
                        time:   [-0.0583% -0.0350% -0.0122%] (p = 0.00 < 0.05)
                        thrpt:  [+0.0122% +0.0350% +0.0583%]
                        Change within noise threshold.
Found 218 outliers among 1000 measurements (21.80%)
  17 (1.70%) high mild
  201 (20.10%) high severe
random_u32/pcg64        time:   [1.3556 ns 1.3565 ns 1.3573 ns]
                        thrpt:  [2.7447 GiB/s 2.7463 GiB/s 2.7481 GiB/s]
                 change:
                        time:   [-0.6592% -0.5683% -0.4817%] (p = 0.00 < 0.05)
                        thrpt:  [+0.4840% +0.5715% +0.6635%]
                        Change within noise threshold.
Found 206 outliers among 1000 measurements (20.60%)
  28 (2.80%) low severe
  19 (1.90%) high mild
  159 (15.90%) high severe
random_u32/pcg64mcg     time:   [944.56 ps 944.68 ps 944.81 ps]
                        thrpt:  [3.9429 GiB/s 3.9434 GiB/s 3.9439 GiB/s]
                 change:
                        time:   [-0.1245% -0.0994% -0.0739%] (p = 0.00 < 0.05)
                        thrpt:  [+0.0739% +0.0995% +0.1247%]
                        Change within noise threshold.
Found 154 outliers among 1000 measurements (15.40%)
  23 (2.30%) high mild
  131 (13.10%) high severe
random_u32/pcg64dxsm    time:   [1.4340 ns 1.4342 ns 1.4343 ns]
                        thrpt:  [2.5972 GiB/s 2.5975 GiB/s 2.5978 GiB/s]
                 change:
                        time:   [+0.2881% +0.3116% +0.3358%] (p = 0.00 < 0.05)
                        thrpt:  [-0.3347% -0.3107% -0.2873%]
                        Change within noise threshold.
Found 163 outliers among 1000 measurements (16.30%)
  20 (2.00%) low severe
  24 (2.40%) high mild
  119 (11.90%) high severe
random_u32/chacha8      time:   [1.1834 ns 1.1837 ns 1.1839 ns]
                        thrpt:  [3.1465 GiB/s 3.1473 GiB/s 3.1480 GiB/s]
                 change:
                        time:   [-0.6564% -0.6274% -0.5991%] (p = 0.00 < 0.05)
                        thrpt:  [+0.6027% +0.6314% +0.6608%]
                        Change within noise threshold.
Found 139 outliers among 1000 measurements (13.90%)
  58 (5.80%) low mild
  34 (3.40%) high mild
  47 (4.70%) high severe
random_u32/chacha12     time:   [1.2435 ns 1.2439 ns 1.2442 ns]
                        thrpt:  [2.9941 GiB/s 2.9949 GiB/s 2.9957 GiB/s]
                 change:
                        time:   [-1.2178% -1.1768% -1.1374%] (p = 0.00 < 0.05)
                        thrpt:  [+1.1505% +1.1908% +1.2328%]
                        Performance has improved.
Found 101 outliers among 1000 measurements (10.10%)
  96 (9.60%) high mild
  5 (0.50%) high severe
random_u32/chacha20     time:   [1.7942 ns 1.7945 ns 1.7949 ns]
                        thrpt:  [2.0755 GiB/s 2.0759 GiB/s 2.0763 GiB/s]
                 change:
                        time:   [-0.6290% -0.5986% -0.5681%] (p = 0.00 < 0.05)
                        thrpt:  [+0.5714% +0.6022% +0.6329%]
                        Change within noise threshold.
Found 57 outliers among 1000 measurements (5.70%)
  40 (4.00%) high mild
  17 (1.70%) high severe
random_u32/std          time:   [1.1958 ns 1.1961 ns 1.1964 ns]
                        thrpt:  [3.1137 GiB/s 3.1145 GiB/s 3.1153 GiB/s]
                 change:
                        time:   [+0.6913% +0.7184% +0.7474%] (p = 0.00 < 0.05)
                        thrpt:  [-0.7418% -0.7133% -0.6866%]
                        Change within noise threshold.
Found 46 outliers among 1000 measurements (4.60%)
  8 (0.80%) low mild
  29 (2.90%) high mild
  9 (0.90%) high severe
random_u32/small        time:   [636.64 ps 636.77 ps 636.90 ps]
                        thrpt:  [5.8491 GiB/s 5.8503 GiB/s 5.8515 GiB/s]
                 change:
                        time:   [-0.0082% +0.0193% +0.0487%] (p = 0.16 > 0.05)
                        thrpt:  [-0.0486% -0.0193% +0.0082%]
                        No change in performance detected.
Found 218 outliers among 1000 measurements (21.80%)
  21 (2.10%) high mild
  197 (19.70%) high severe
random_u32/os           time:   [14.479 ns 14.482 ns 14.486 ns]
                        thrpt:  [263.34 MiB/s 263.41 MiB/s 263.47 MiB/s]
                 change:
                        time:   [-1.1040% -1.0693% -1.0224%] (p = 0.00 < 0.05)
                        thrpt:  [+1.0330% +1.0809% +1.1164%]
                        Performance has improved.
Found 29 outliers among 1000 measurements (2.90%)
  1 (0.10%) low severe
  24 (2.40%) high mild
  4 (0.40%) high severe
random_u32/thread       time:   [1.2000 ns 1.2003 ns 1.2006 ns]
                        thrpt:  [3.1029 GiB/s 3.1037 GiB/s 3.1045 GiB/s]
                 change:
                        time:   [+0.4258% +0.4542% +0.4839%] (p = 0.00 < 0.05)
                        thrpt:  [-0.4815% -0.4521% -0.4240%]
                        Change within noise threshold.
Found 26 outliers among 1000 measurements (2.60%)
  1 (0.10%) low mild
  24 (2.40%) high mild
  1 (0.10%) high severe

random_u64/pcg32        time:   [2.0870 ns 2.0877 ns 2.0882 ns]
                        thrpt:  [3.5679 GiB/s 3.5688 GiB/s 3.5699 GiB/s]
                 change:
                        time:   [-0.1059% -0.0577% -0.0153%] (p = 0.01 < 0.05)
                        thrpt:  [+0.0153% +0.0577% +0.1060%]
                        Change within noise threshold.
Found 167 outliers among 1000 measurements (16.70%)
  8 (0.80%) low severe
  28 (2.80%) high mild
  131 (13.10%) high severe
random_u64/pcg64        time:   [1.3354 ns 1.3391 ns 1.3426 ns]
                        thrpt:  [5.5495 GiB/s 5.5641 GiB/s 5.5792 GiB/s]
                 change:
                        time:   [-0.0613% +0.1906% +0.4601%] (p = 0.16 > 0.05)
                        thrpt:  [-0.4580% -0.1902% +0.0613%]
                        No change in performance detected.
Found 286 outliers among 1000 measurements (28.60%)
  141 (14.10%) low severe
  17 (1.70%) high mild
  128 (12.80%) high severe
random_u64/pcg64mcg     time:   [948.41 ps 948.86 ps 949.30 ps]
                        thrpt:  [7.8485 GiB/s 7.8522 GiB/s 7.8559 GiB/s]
                 change:
                        time:   [-0.2421% -0.1750% -0.1138%] (p = 0.00 < 0.05)
                        thrpt:  [+0.1139% +0.1753% +0.2427%]
                        Change within noise threshold.
Found 39 outliers among 1000 measurements (3.90%)
  36 (3.60%) high mild
  3 (0.30%) high severe
random_u64/pcg64dxsm    time:   [1.2211 ns 1.2214 ns 1.2218 ns]
                        thrpt:  [6.0982 GiB/s 6.1000 GiB/s 6.1016 GiB/s]
                 change:
                        time:   [-14.876% -14.855% -14.833%] (p = 0.00 < 0.05)
                        thrpt:  [+17.416% +17.447% +17.476%]
                        Performance has improved.
Found 43 outliers among 1000 measurements (4.30%)
  41 (4.10%) high mild
  2 (0.20%) high severe
random_u64/chacha8      time:   [1.6276 ns 1.6279 ns 1.6282 ns]
                        thrpt:  [4.5759 GiB/s 4.5768 GiB/s 4.5777 GiB/s]
                 change:
                        time:   [+3.2994% +3.3265% +3.3525%] (p = 0.00 < 0.05)
                        thrpt:  [-3.2438% -3.2194% -3.1940%]
                        Performance has regressed.
Found 93 outliers among 1000 measurements (9.30%)
  4 (0.40%) low mild
  26 (2.60%) high mild
  63 (6.30%) high severe
random_u64/chacha12     time:   [2.1290 ns 2.1293 ns 2.1296 ns]
                        thrpt:  [3.4986 GiB/s 3.4991 GiB/s 3.4996 GiB/s]
                 change:
                        time:   [-0.0708% -0.0436% -0.0160%] (p = 0.00 < 0.05)
                        thrpt:  [+0.0160% +0.0436% +0.0708%]
                        Change within noise threshold.
Found 92 outliers among 1000 measurements (9.20%)
  2 (0.20%) low severe
  4 (0.40%) low mild
  22 (2.20%) high mild
  64 (6.40%) high severe
random_u64/chacha20     time:   [3.2094 ns 3.2099 ns 3.2104 ns]
                        thrpt:  [2.3207 GiB/s 2.3211 GiB/s 2.3215 GiB/s]
                 change:
                        time:   [-0.2781% -0.2449% -0.2122%] (p = 0.00 < 0.05)
                        thrpt:  [+0.2126% +0.2455% +0.2789%]
                        Change within noise threshold.
Found 245 outliers among 1000 measurements (24.50%)
  126 (12.60%) low severe
  14 (1.40%) low mild
  18 (1.80%) high mild
  87 (8.70%) high severe
random_u64/std          time:   [2.0725 ns 2.0767 ns 2.0808 ns]
                        thrpt:  [3.5807 GiB/s 3.5878 GiB/s 3.5949 GiB/s]
                 change:
                        time:   [+3.5284% +3.6897% +3.8350%] (p = 0.00 < 0.05)
                        thrpt:  [-3.6933% -3.5584% -3.4081%]
                        Performance has regressed.
random_u64/small        time:   [650.12 ps 650.26 ps 650.40 ps]
                        thrpt:  [11.455 GiB/s 11.458 GiB/s 11.460 GiB/s]
                 change:
                        time:   [-0.8041% -0.7763% -0.7482%] (p = 0.00 < 0.05)
                        thrpt:  [+0.7539% +0.7824% +0.8106%]
                        Change within noise threshold.
Found 48 outliers among 1000 measurements (4.80%)
  48 (4.80%) high mild
random_u64/os           time:   [20.870 ns 20.875 ns 20.881 ns]
                        thrpt:  [365.38 MiB/s 365.47 MiB/s 365.56 MiB/s]
                 change:
                        time:   [-1.4342% -1.3997% -1.3659%] (p = 0.00 < 0.05)
                        thrpt:  [+1.3848% +1.4195% +1.4551%]
                        Performance has improved.
Found 217 outliers among 1000 measurements (21.70%)
  22 (2.20%) low severe
  11 (1.10%) low mild
  16 (1.60%) high mild
  168 (16.80%) high severe
random_u64/thread       time:   [2.0048 ns 2.0052 ns 2.0056 ns]
                        thrpt:  [3.7148 GiB/s 3.7156 GiB/s 3.7164 GiB/s]
                 change:
                        time:   [+0.2088% +0.2418% +0.2755%] (p = 0.00 < 0.05)
                        thrpt:  [-0.2747% -0.2412% -0.2084%]
                        Change within noise threshold.
Found 44 outliers among 1000 measurements (4.40%)
  43 (4.30%) high mild
  1 (0.10%) high severe
reseeding_bytes/chacha20_4k
                        time:   [381.28 µs 381.36 µs 381.45 µs]
                        thrpt:  [2.5602 GiB/s 2.5607 GiB/s 2.5613 GiB/s]
                 change:
                        time:   [-1.2454% -1.1847% -1.1231%] (p = 0.00 < 0.05)
                        thrpt:  [+1.1359% +1.1989% +1.2612%]
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe
reseeding_bytes/chacha20_16k
                        time:   [366.15 µs 366.23 µs 366.32 µs]
                        thrpt:  [2.6659 GiB/s 2.6665 GiB/s 2.6671 GiB/s]
                 change:
                        time:   [-0.6439% -0.6060% -0.5690%] (p = 0.00 < 0.05)
                        thrpt:  [+0.5723% +0.6096% +0.6480%]
                        Change within noise threshold.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe
reseeding_bytes/chacha20_32k
                        time:   [363.90 µs 364.02 µs 364.13 µs]
                        thrpt:  [2.6819 GiB/s 2.6828 GiB/s 2.6836 GiB/s]
                 change:
                        time:   [-0.3650% -0.3314% -0.2986%] (p = 0.00 < 0.05)
                        thrpt:  [+0.2995% +0.3325% +0.3663%]
                        Change within noise threshold.
Found 6 outliers among 100 measurements (6.00%)
  5 (5.00%) high mild
  1 (1.00%) high severe
reseeding_bytes/chacha20_64k
                        time:   [362.82 µs 363.09 µs 363.42 µs]
                        thrpt:  [2.6871 GiB/s 2.6896 GiB/s 2.6916 GiB/s]
                 change:
                        time:   [-0.5035% -0.4485% -0.3832%] (p = 0.00 < 0.05)
                        thrpt:  [+0.3846% +0.4505% +0.5060%]
                        Change within noise threshold.
Found 7 outliers among 100 measurements (7.00%)
  3 (3.00%) high mild
  4 (4.00%) high severe
reseeding_bytes/chacha20_256k
                        time:   [361.53 µs 361.67 µs 361.83 µs]
                        thrpt:  [2.6990 GiB/s 2.7002 GiB/s 2.7012 GiB/s]
                 change:
                        time:   [-0.3746% -0.3252% -0.2711%] (p = 0.00 < 0.05)
                        thrpt:  [+0.2719% +0.3263% +0.3760%]
                        Change within noise threshold.
Found 9 outliers among 100 measurements (9.00%)
  3 (3.00%) high mild
  6 (6.00%) high severe
reseeding_bytes/chacha20_1024k
                        time:   [361.01 µs 361.06 µs 361.12 µs]
                        thrpt:  [2.7042 GiB/s 2.7047 GiB/s 2.7051 GiB/s]
                 change:
                        time:   [-1.0130% -0.9190% -0.8275%] (p = 0.00 < 0.05)
                        thrpt:  [+0.8344% +0.9275% +1.0234%]
                        Change within noise threshold.
Found 5 outliers among 100 measurements (5.00%)
  2 (2.00%) low mild
  3 (3.00%) high mild

random_bytes output for ChaCha* looks a bit better (4-13.5% more throughput); otherwise little has changed (most results under 1% deviation, though there are a few spurious results, most notably random_u64/pcg64dxsm is ~15% faster).

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@dhardy