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Date: Tue, 02 Jul 2024 18:20:35 +0200
From: "Joel Jacobson" <joel@compiler.org>
To: "Dean Rasheed" <dean.a.rasheed@gmail.com>
Cc: =?UTF-8?Q?Dagfinn_Ilmari_Manns=C3=A5ker?= <ilmari@ilmari.org>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Optimize numeric multiplication for one and two base-NBASE digit
 multiplicands.
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 boundary=1f74c0b692524120a03b9507a65425f2
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--1f74c0b692524120a03b9507a65425f2
Content-Type: text/plain

On Tue, Jul 2, 2024, at 13:44, Dean Rasheed wrote:
>> Can you think of an example that should trigger the bug?
>
> 9999.0001 * 5000.9999_9999 with rscale = 0 triggers it (returned
> 50004999 instead of 50005000).

Thanks, helpful.

Attached patch adds the var1ndigits=3 case.

Benchmark:

/*
 * Apple M3 Max
 */

SELECT SUM(var1*var2) FROM bench_mul_var_var1ndigits_3; -- HEAD
Time: 3809.005 ms (00:03.809)
Time: 3438.260 ms (00:03.438)
Time: 3453.920 ms (00:03.454)

SELECT SUM(var1*var2) FROM bench_mul_var_var1ndigits_3; -- v3-optimize-numeric-mul_var-small-var1-arbitrary-var2.patch
Time: 3230.017 ms (00:03.230)
Time: 3244.094 ms (00:03.244)
Time: 3246.370 ms (00:03.246)

/*
 * Intel Core i9-14900K
 */

SELECT SUM(var1*var2) FROM bench_mul_var_var1ndigits_3; -- HEAD
Time: 4082.759 ms (00:04.083)
Time: 4077.372 ms (00:04.077)
Time: 4080.830 ms (00:04.081)

SELECT SUM(var1*var2) FROM bench_mul_var_var1ndigits_3; -- v3-optimize-numeric-mul_var-small-var1-arbitrary-var2.patch
Time: 3989.021 ms (00:03.989)
Time: 3978.263 ms (00:03.978)
Time: 3955.331 ms (00:03.955)

/*
 * AMD Ryzen 9 7950X3D
 */

SELECT SUM(var1*var2) FROM bench_mul_var_var1ndigits_3; -- HEAD
Time: 3791.271 ms (00:03.791)
Time: 3760.138 ms (00:03.760)
Time: 3758.773 ms (00:03.759)

SELECT SUM(var1*var2) FROM bench_mul_var_var1ndigits_3; -- v3-optimize-numeric-mul_var-small-var1-arbitrary-var2.patch
Time: 3400.824 ms (00:03.401)
Time: 3373.714 ms (00:03.374)
Time: 3345.505 ms (00:03.346)

Regards,
Joel
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