use uint64 in reduce when possible

[maddyblue]

name          old time/op  new time/op  delta
GDA/reduce-4   215µs ±30%   124µs ±10%  -42.48%  (p=0.008 n=5+5)

Idea from: https://github.com/cockroachdb/cockroach/blob/0ba52ff84a3e67efe3532b9119058aab8231478a/pkg/util/encoding/decimal.go#L773

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This change is 

[RaduBerinde]

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Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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decimal.go, line 480 at r1 (raw file):

		i := d.Coeff.Uint64()
		e := d.Exponent
		for i%10 == 0 {

maybe add a for i >= 10000 && i % 10000 == 0 { i /= 10000; e += 4 }

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Comments from Reviewable

[maddyblue]

Review status: 0 of 1 files reviewed at latest revision, 1 unresolved discussion.

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decimal.go, line 480 at r1 (raw file):

Previously, RaduBerinde wrote…

maybe add a for i >= 10000 && i % 10000 == 0 { i /= 10000; e += 4 }

Wow, this yielded another very large speed up in the benchmark. Thanks.

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Comments from Reviewable

[RaduBerinde]

decimal.go, line 480 at r1 (raw file):

Previously, mjibson (Matt Jibson) wrote…

Wow, this yielded another very large speed up in the benchmark. Thanks.

👍

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Comments from Reviewable

[RaduBerinde]

decimal.go, line 480 at r1 (raw file):

Previously, RaduBerinde wrote…

👍

The same idea can be useful in the big loop below as well. We could:

• divide by 10^10 until the remainder is non-zero
• convert the remainder to uint64
• use uint64 code to count the zeros on the remainder
• do a final Quo with the exact power of 10 (as determined in the previous step). We can precalculate all powers of ten from 1 to 10.

Probably beyond the intention of this PR but maybe file an issue if you think this would help.

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Comments from Reviewable