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math/big: fix BitLen performance regression

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CL 450055 replaced BitLen with a slower constant-time implementation,
which caused a performance regression in some ecosystem benchmarks.

https://perf.golang.org/search?q=upload%3A20221130.13+pkg%3Agithub.com%2Fericlagergren%2Fdecimal%2Fbenchmarks

Current tip vs this CL

name                                   old time/op  new time/op  delta
Pi/foo=ericlagergren_(Go)/prec=100-4    151µs ± 0%   129µs ± 0%  -14.89%  (p=0.000 n=10+9)
Pi/foo=ericlagergren_(GDA)/prec=100-4   305µs ± 0%   269µs ± 1%  -11.88%  (p=0.000 n=9+10)
Pi/foo=cockroachdb/apd/prec=100-4      5.74ms ± 0%  5.33ms ± 0%   -7.02%  (p=0.000 n=9+10)
Pi/foo=shopspring/prec=100-4            265µs ±16%   268µs ±11%     ~     (p=0.796 n=10+10)
Pi/foo=apmckinlay/prec=100-4           3.10µs ± 0%  3.08µs ± 0%   -0.60%  (p=0.000 n=8+10)
Pi/foo=go-inf/prec=100-4                132µs ± 9%   137µs ± 9%     ~     (p=0.182 n=10+9)
Pi/foo=float64/prec=100-4              4.97µs ± 0%  4.98µs ± 0%     ~     (p=0.196 n=10+10)

CL 450055's parent vs this CL

name                                   old time/op  new time/op  delta
Pi/foo=ericlagergren_(Go)/prec=100-4    129µs ± 1%   129µs ± 0%    ~     (p=0.182 n=10+9)
Pi/foo=ericlagergren_(GDA)/prec=100-4   267µs ± 1%   269µs ± 1%  +0.93%  (p=0.001 n=9+10)
Pi/foo=shopspring/prec=100-4            252µs ± 9%   268µs ±11%    ~     (p=0.052 n=10+10)
Pi/foo=apmckinlay/prec=100-4           3.10µs ± 1%  3.08µs ± 0%  -0.66%  (p=0.000 n=9+10)
Pi/foo=go-inf/prec=100-4                135µs ± 6%   137µs ± 9%    ~     (p=0.605 n=9+9)
Pi/foo=float64/prec=100-4              4.97µs ± 0%  4.98µs ± 0%  +0.23%  (p=0.005 n=8+10)

goos: linux
goarch: amd64
pkg: github.com/ericlagergren/decimal_benchmarks
cpu: Intel(R) Core(TM) i5-7400 CPU @ 3.00GHz

Fixes #57014

Change-Id: I08478bea122212320a592ad2652e33077807de09
Reviewed-on: https://go-review.googlesource.com/c/go/+/454617
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Filippo Valsorda 2022-12-02 17:58:00 +01:00 committed by Gopher Robot
parent 6ec6492f55
commit eb12245645
1 changed files with 11 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
src/math/big/nat.go
View File

@ -664,15 +664,18 @@ func (x nat) bitLen() int {
// This function is used in cryptographic operations. It must not leak
// anything but the Int's sign and bit size through side-channels. Any
// changes must be reviewed by a security expert.
//
// In particular, bits.Len and bits.LeadingZeros use a lookup table for the
// low-order bits on some architectures.
if i := len(x) - 1; i >= 0 {
l := i * _W
for top := x[i]; top != 0; top >>= 1 {
l++
}
return l
// bits.Len uses a lookup table for the low-order bits on some
// architectures. Neutralize any input-dependent behavior by setting all
// bits after the first one bit.
top := uint(x[i])
top |= top >> 1
top |= top >> 2
top |= top >> 4
top |= top >> 8
top |= top >> 16
top |= top >> 16 >> 16 // ">> 32" doesn't compile on 32-bit architectures
return i*_W + bits.Len(top)
}
return 0
}