Repository: go
Issue: cmd/compile: For some sizes, zeroing memory through OpZero could be faster (AMD64)
Labels: NeedsDecision,compiler/runtime

I'm working on a patch for inlining known-size memclrNoHeapPointers calls and replacing them with an OpZero. 

The inlining significantly speeds up clearing, but for some clear size, AMD64's OpZero is actually slower than calling memcrlNoHeapPointers,

see:

Intel Alder Lake 12600k:

name                      old time/op    new time/op     delta
MemclrKnownSize1-16         0.59ns ± 3%     0.38ns ± 6%   -36.00%  (p=0.000 n=19+18)
MemclrKnownSize2-16         0.57ns ± 1%     0.19ns ± 5%   -66.27%  (p=0.000 n=19+19)
MemclrKnownSize4-16         0.66ns ± 2%     0.36ns ±21%   -45.12%  (p=0.000 n=19+20)
MemclrKnownSize8-16         0.74ns ± 1%     0.30ns ±26%   -59.81%  (p=0.000 n=18+20)
MemclrKnownSize16-16        1.00ns ± 7%     0.21ns ± 8%   -79.51%  (p=0.000 n=20+19)
MemclrKnownSize32-16        0.95ns ± 1%     0.40ns ± 1%   -57.61%  (p=0.000 n=20+18)
MemclrKnownSize64-16        1.20ns ± 2%     0.41ns ± 0%   -65.82%  (p=0.000 n=20+18)
MemclrKnownSize112-16       1.27ns ± 2%     1.03ns ± 0%   -19.35%  (p=0.000 n=20+18)
MemclrKnownSize128-16       1.34ns ± 2%     1.03ns ± 0%   -23.02%  (p=0.000 n=20+20)
MemclrKnownSize192-16       1.92ns ± 2%     1.44ns ± 0%   -24.89%  (p=0.000 n=20+16)
MemclrKnownSize248-16       2.77ns ± 1%     3.29ns ± 0%   +18.81%  (p=0.000 n=20+16)
MemclrKnownSize256-16       1.92ns ± 1%     1.86ns ± 0%    -3.49%  (p=0.000 n=19+15)
MemclrKnownSize512-16       2.81ns ± 2%     3.49ns ± 0%   +24.15%  (p=0.000 n=20+17)
MemclrKnownSize1024-16      4.02ns ± 1%     6.78ns ± 0%   +68.44%  (p=0.000 n=20+18)
MemclrKnownSize4096-16      17.2ns ± 2%     14.4ns ± 0%   -16.73%  (p=0.000 n=20+17)
MemclrKnownSize512KiB-16    6.71µs ± 1%     6.52µs ± 0%    -2.85%  (p=0.000 n=20+18)
[Geo mean]                  2.60ns          1.71ns        -34.06%

name                      old speed      new speed       delta
MemclrKnownSize1-16       1.71GB/s ± 3%   2.67GB/s ± 6%   +56.39%  (p=0.000 n=19+18)
MemclrKnownSize2-16       3.52GB/s ± 2%  10.43GB/s ± 6%  +196.04%  (p=0.000 n=20+20)
MemclrKnownSize4-16       6.06GB/s ± 1%  10.83GB/s ±11%   +78.63%  (p=0.000 n=19+18)
MemclrKnownSize8-16       10.7GB/s ± 1%   27.0GB/s ±21%  +151.49%  (p=0.000 n=18+20)
MemclrKnownSize16-16      16.0GB/s ± 8%   78.1GB/s ± 7%  +387.24%  (p=0.000 n=20+19)
MemclrKnownSize32-16      33.6GB/s ± 1%   79.4GB/s ± 1%  +135.89%  (p=0.000 n=20+18)
MemclrKnownSize64-16      53.3GB/s ± 2%  155.9GB/s ± 0%  +192.58%  (p=0.000 n=20+18)
MemclrKnownSize112-16     88.0GB/s ± 2%  109.1GB/s ± 0%   +23.97%  (p=0.000 n=20+18)
MemclrKnownSize128-16     95.3GB/s ± 2%  123.8GB/s ± 0%   +29.88%  (p=0.000 n=20+20)
MemclrKnownSize192-16      100GB/s ± 2%    133GB/s ± 0%   +33.12%  (p=0.000 n=20+17)
MemclrKnownSize248-16     89.7GB/s ± 1%   75.5GB/s ± 0%   -15.84%  (p=0.000 n=20+19)
MemclrKnownSize256-16      133GB/s ± 1%    138GB/s ± 0%    +3.61%  (p=0.000 n=19+14)
MemclrKnownSize512-16      182GB/s ± 2%    147GB/s ± 0%   -19.46%  (p=0.000 n=20+17)
MemclrKnownSize1024-16     254GB/s ± 1%    151GB/s ± 0%   -40.64%  (p=0.000 n=20+18)
MemclrKnownSize4096-16     237GB/s ± 2%    285GB/s ± 0%   +20.09%  (p=0.000 n=20+17)
MemclrKnownSize512KiB-16  78.2GB/s ± 1%   80.4GB/s ± 0%    +2.93%  (p=0.000 n=20+18)
[Geo mean]                42.1GB/s        63.8GB/s        +51.53%

This problem doesn't appear on arm64 (M1 Pro):

name                      old time/op     new time/op     delta
MemclrKnownSize1-10          2.03ns ± 0%     0.31ns ± 0%    -84.69%  (p=0.000 n=18+19)
MemclrKnownSize2-10          1.97ns ± 0%     0.31ns ± 0%    -84.19%  (p=0.000 n=12+19)
MemclrKnownSize4-10          2.02ns ± 0%     0.31ns ± 0%    -84.56%  (p=0.000 n=12+20)
MemclrKnownSize8-10          2.02ns ± 0%     0.31ns ± 0%    -84.59%  (p=0.000 n=14+19)
MemclrKnownSize16-10         2.15ns ± 0%     0.31ns ± 0%    -85.50%  (p=0.000 n=18+19)
MemclrKnownSize32-10         2.48ns ± 0%     0.31ns ± 0%    -87.48%  (p=0.000 n=20+19)
MemclrKnownSize64-10         1.93ns ± 0%     0.62ns ± 0%    -67.88%  (p=0.000 n=20+19)
MemclrKnownSize112-10        2.48ns ± 0%     1.80ns ± 0%    -27.74%  (p=0.000 n=19+20)
MemclrKnownSize128-10       10.0ns ±112%      2.0ns ± 0%    -79.76%  (p=0.000 n=18+17)
MemclrKnownSize192-10       27.4ns ±103%      2.6ns ± 0%    -90.38%  (p=0.000 n=16+19)
MemclrKnownSize248-10        9.67ns ±43%     3.26ns ± 0%    -66.29%  (p=0.000 n=19+19)
MemclrKnownSize256-10       85.4ns ±148%      3.3ns ± 0%    -96.18%  (p=0.000 n=20+20)
MemclrKnownSize512-10         223ns ±54%        6ns ± 0%    -97.42%  (p=0.000 n=18+20)
MemclrKnownSize1024-10        216ns ±26%       11ns ± 0%    -95.00%  (p=0.000 n=18+15)
MemclrKnownSize4096-10        265ns ± 2%       88ns ± 0%    -66.84%  (p=0.000 n=19+17)
MemclrKnownSize512KiB-10     9.91µs ± 1%    10.23µs ± 2%     +3.14%  (p=0.000 n=19+19)
[Geo mean]                   15.6ns           2.5ns         -83.62%

name                      old speed       new speed       delta
MemclrKnownSize1-10         493MB/s ± 0%   3216MB/s ± 0%   +553.04%  (p=0.000 n=18+19)
MemclrKnownSize2-10        1.02GB/s ± 0%   6.43GB/s ± 0%   +532.33%  (p=0.000 n=16+19)
MemclrKnownSize4-10        1.99GB/s ± 0%  12.86GB/s ± 0%   +547.67%  (p=0.000 n=18+20)
MemclrKnownSize8-10        3.96GB/s ± 0%  25.72GB/s ± 0%   +548.81%  (p=0.000 n=19+19)
MemclrKnownSize16-10       7.46GB/s ± 0%  51.43GB/s ± 0%   +589.72%  (p=0.000 n=20+19)
MemclrKnownSize32-10       12.9GB/s ± 0%  102.9GB/s ± 0%   +698.60%  (p=0.000 n=20+18)
MemclrKnownSize64-10       33.1GB/s ± 0%  103.0GB/s ± 0%   +211.34%  (p=0.000 n=19+19)
MemclrKnownSize112-10      45.1GB/s ± 0%   62.4GB/s ± 0%    +38.38%  (p=0.000 n=19+20)
MemclrKnownSize128-10     13.3GB/s ±107%   63.5GB/s ± 0%   +378.03%  (p=0.000 n=19+18)
MemclrKnownSize192-10     6.97GB/s ±139%  72.72GB/s ± 0%   +943.44%  (p=0.000 n=19+19)
MemclrKnownSize248-10      25.9GB/s ±46%   76.1GB/s ± 0%   +194.16%  (p=0.000 n=20+17)
MemclrKnownSize256-10     8.64GB/s ±196%  78.51GB/s ± 0%   +808.19%  (p=0.000 n=20+20)
MemclrKnownSize512-10      2.33GB/s ±86%  89.13GB/s ± 0%  +3719.50%  (p=0.000 n=17+20)
MemclrKnownSize1024-10     4.85GB/s ±32%  94.93GB/s ± 0%  +1856.74%  (p=0.000 n=18+19)
MemclrKnownSize4096-10     15.4GB/s ± 2%   46.6GB/s ± 0%   +201.55%  (p=0.000 n=19+18)
MemclrKnownSize512KiB-10   52.9GB/s ± 1%   51.3GB/s ± 2%     -3.04%  (p=0.000 n=19+19)
[Geo mean]                 7.54GB/s       42.86GB/s        +468.76%

It seems that either: on AMD64 OpZero could be improved or for some clears we could use memcrlNoHeapPointers instead.

I'll send the patch soon. Creating this issue so that it can be linked from the change.