commit 4c227a091e9200ed9757dedd8efbc6e254750c2c
author Josh Bleecher Snyder <josharian@gmail.com> Sun Mar 03 12:54:01 2019 -0800
committer Josh Bleecher Snyder <josharian@gmail.com> Sat Mar 09 20:33:13 2019 +0000
tree af4dc96bb929c4c1cf4a98dc1adffedfc1a75413
parent 788e038e5d5fcdc1cc44ec9af1885db55a19977c

math/big: remove bounds checks in pure Go implementations

These routines are quite sensitive to BCE.

This change eliminates bounds checks from loops.
It does so at the cost of a bit of safety:
malformed input will now return incorrect answers
instead of panicking.

This isn't as bad as it sounds: math/big has very good
test coverage, and the alternative implementations are in
assembly, which could do much worse things with malformed input.

If the compiler's BCE improves, so could these routines.

Notable BCE improvements for these routines would be:

* Allowing and propagating more cross-slice length hints.
  Then hints like _ = y[:len(z)] would eliminate bounds checks for y[i].

* Propagating enough information so that we could do
  n := len(x)
  if len(z) < n {
    n = len(z)
  }
  and then have i < n eliminate the same bounds checks as
  i < len(x) && i < len(z) currently does.

* Providing some way to do BCE for unrolled loops.
  Now that we have math/bits implementations,
  it is possible to write things like ADC chains in
  pure Go, if you can reasonably unroll loops.

Benchmarks below are for amd64, using -tags=math_big_pure_go.

name            old time/op    new time/op    delta
AddVV/1-8         5.15ns ± 3%    4.65ns ± 4%   -9.81%  (p=0.000 n=93+86)
AddVV/2-8         6.40ns ± 2%    5.58ns ± 4%  -12.78%  (p=0.000 n=90+95)
AddVV/3-8         7.07ns ± 2%    6.66ns ± 2%   -5.88%  (p=0.000 n=87+83)
AddVV/4-8         7.94ns ± 5%    7.41ns ± 4%   -6.65%  (p=0.000 n=94+98)
AddVV/5-8         8.55ns ± 1%    8.80ns ± 0%   +2.92%  (p=0.000 n=87+92)
AddVV/10-8        12.7ns ± 1%    12.3ns ± 1%   -3.12%  (p=0.000 n=83+71)
AddVV/100-8        119ns ± 5%     117ns ± 4%   -1.60%  (p=0.000 n=93+90)
AddVV/1000-8      1.14µs ± 4%    1.14µs ± 5%     ~     (p=0.812 n=95+91)
AddVV/10000-8     11.4µs ± 5%    11.3µs ± 5%     ~     (p=0.503 n=97+96)
AddVV/100000-8     114µs ± 4%     113µs ± 5%   -0.98%  (p=0.002 n=97+90)

name            old time/op    new time/op    delta
SubVV/1-8         5.23ns ± 5%    4.65ns ± 3%  -11.18%  (p=0.000 n=89+91)
SubVV/2-8         6.49ns ± 5%    5.58ns ± 3%  -14.04%  (p=0.000 n=92+94)
SubVV/3-8         7.10ns ± 3%    6.65ns ± 2%   -6.28%  (p=0.000 n=87+80)
SubVV/4-8         8.04ns ± 1%    7.44ns ± 5%   -7.49%  (p=0.000 n=83+98)
SubVV/5-8         8.55ns ± 2%    8.32ns ± 1%   -2.75%  (p=0.000 n=84+92)
SubVV/10-8        12.7ns ± 1%    12.3ns ± 1%   -3.09%  (p=0.000 n=80+75)
SubVV/100-8        119ns ± 0%     116ns ± 3%   -1.83%  (p=0.000 n=87+98)
SubVV/1000-8      1.13µs ± 5%    1.13µs ± 3%     ~     (p=0.082 n=96+98)
SubVV/10000-8     11.2µs ± 1%    11.3µs ± 3%   +0.76%  (p=0.000 n=87+97)
SubVV/100000-8     112µs ± 2%     113µs ± 3%   +0.55%  (p=0.000 n=76+88)

name            old time/op    new time/op    delta
AddVW/1-8         4.30ns ± 4%    3.96ns ± 6%  -8.02%  (p=0.000 n=89+97)
AddVW/2-8         5.15ns ± 2%    4.91ns ± 1%  -4.56%  (p=0.000 n=87+80)
AddVW/3-8         5.59ns ± 3%    5.75ns ± 2%  +2.91%  (p=0.000 n=91+88)
AddVW/4-8         6.20ns ± 1%    6.03ns ± 1%  -2.71%  (p=0.000 n=75+90)
AddVW/5-8         6.93ns ± 3%    6.49ns ± 2%  -6.35%  (p=0.000 n=100+82)
AddVW/10-8        10.0ns ± 7%     9.6ns ± 0%  -4.02%  (p=0.000 n=98+74)
AddVW/100-8       91.1ns ± 1%    90.6ns ± 1%  -0.55%  (p=0.000 n=84+80)
AddVW/1000-8       866ns ± 1%     856ns ± 4%  -1.06%  (p=0.000 n=69+96)
AddVW/10000-8     8.64µs ± 1%    8.53µs ± 4%  -1.25%  (p=0.000 n=67+99)
AddVW/100000-8    84.3µs ± 2%    85.4µs ± 4%  +1.22%  (p=0.000 n=89+99)

name            old time/op    new time/op    delta
SubVW/1-8         4.28ns ± 2%    3.82ns ± 3%  -10.63%  (p=0.000 n=91+89)
SubVW/2-8         4.61ns ± 1%    4.48ns ± 3%   -2.67%  (p=0.000 n=94+96)
SubVW/3-8         5.54ns ± 1%    5.81ns ± 4%   +4.87%  (p=0.000 n=92+97)
SubVW/4-8         6.20ns ± 1%    6.08ns ± 2%   -1.99%  (p=0.000 n=71+88)
SubVW/5-8         6.91ns ± 3%    6.64ns ± 1%   -3.90%  (p=0.000 n=97+70)
SubVW/10-8        9.85ns ± 2%    9.62ns ± 0%   -2.31%  (p=0.000 n=82+62)
SubVW/100-8       91.1ns ± 1%    90.9ns ± 3%   -0.14%  (p=0.010 n=71+93)
SubVW/1000-8       859ns ± 3%     867ns ± 1%   +0.98%  (p=0.000 n=99+78)
SubVW/10000-8     8.54µs ± 5%    8.57µs ± 2%   +0.38%  (p=0.007 n=98+92)
SubVW/100000-8    84.5µs ± 3%    84.6µs ± 3%     ~     (p=0.334 n=95+94)

name                old time/op    new time/op    delta
AddMulVVW/1-8         5.43ns ± 3%    4.36ns ± 2%  -19.67%  (p=0.000 n=95+94)
AddMulVVW/2-8         6.56ns ± 4%    6.11ns ± 1%   -6.90%  (p=0.000 n=91+91)
AddMulVVW/3-8         8.00ns ± 1%    7.80ns ± 4%   -2.52%  (p=0.000 n=83+95)
AddMulVVW/4-8         9.81ns ± 2%    9.53ns ± 1%   -2.86%  (p=0.000 n=77+64)
AddMulVVW/5-8         11.4ns ± 3%    11.3ns ± 5%   -0.89%  (p=0.000 n=95+97)
AddMulVVW/10-8        18.9ns ± 5%    19.1ns ± 5%   +0.89%  (p=0.000 n=91+94)
AddMulVVW/100-8        165ns ± 5%     165ns ± 4%     ~     (p=0.427 n=97+98)
AddMulVVW/1000-8      1.56µs ± 3%    1.56µs ± 4%     ~     (p=0.167 n=98+96)
AddMulVVW/10000-8     15.7µs ± 5%    15.6µs ± 5%   -0.31%  (p=0.044 n=95+97)
AddMulVVW/100000-8     156µs ± 3%     157µs ± 8%     ~     (p=0.373 n=72+99)

Change-Id: Ibc720785d5b95f6a797103b1363843205f4d56bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/164966
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>