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Merged
merged 15 commits into from
May 20, 2024
Merged

ext/bcmath: Changed the bcmul calculation method #14213

merged 15 commits into from
May 20, 2024

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SakiTakamachi
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@SakiTakamachi SakiTakamachi commented May 13, 2024 •
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There are still ways to make it faster, but since it would complicate the PR, I will split it into a follow-up PR (#14229).

The difference is quite large, so I will only post rough measurements.

Bench code:

<?php

// 1
$num1 = '1.2345678';
$num2 = '2.1234567';

$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    bcmul($num1, $num2, 20);
}
echo microtime(true) - $start . PHP_EOL;


// 2
$num1 = str_repeat('1234567890', 300);
$num2 = str_repeat('9876543210', 300);

$start = microtime(true);
for ($i = 0; $i < 1000; $i++) {
    bcmul($num1, $num2, 20);
}
echo microtime(true) - $start . PHP_EOL;


// 3
$num1 = '1.2345678901234567890';
$num2 = '2.12345678901234567890';

$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    bcmul($num1, $num2, 20);
}
echo microtime(true) - $start . PHP_EOL;


// 4
$num1 = '-12345678901234567890.12345678901234567890';
$num2 = '-212345678901234567890.12345678901234567890';

$start = microtime(true);
for ($i = 0; $i < 2000000; $i++) {
    bcmul($num1, $num2, 20);
}
echo microtime(true) - $start . PHP_EOL;


// 5
$num1 = '12345678901234567890.00000000000000000000000000000000000000000000000000';
$num2 = '212345678901234567890.00000000000000000000000000000000000000000000000000';

$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    bcmul($num1, $num2, 20);
}
echo microtime(true) - $start . PHP_EOL;

before

1.6423609256744
1.0373518466949
3.7979400157928
2.4885659217834
4.256236076355

after

1.2081251144409
0.096983194351196
1.9056849479675
0.60840702056885
2.0678749084473

@SakiTakamachi
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CI is red

@SakiTakamachi
Changed the bcmul calculation method
91b9343 
Multiplication is performed after converting to unsigned long, resulting in faster calculations.
@SakiTakamachi
Fixed array allocation
ec9f8df
@SakiTakamachi
Fixed array allocation
ac9342d
@SakiTakamachi
Changed unsigned long to BC_UINT_T
899bb39
@SakiTakamachi SakiTakamachi force-pushed the refactor_bcmath_mul branch from 7a944c7 to 899bb39 Compare May 13, 2024 11:04
@SakiTakamachi
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I need to change some of the constant names and comments.

@SakiTakamachi
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done!

@SakiTakamachi
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The red one in CI is openssl.

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There were some corrections that were omitted, so I corrected the variable names and comments. I think everything else is probably fine.

@SakiTakamachi SakiTakamachi mentioned this pull request May 14, 2024
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@SakiTakamachi SakiTakamachi marked this pull request as draft May 14, 2024 13:29
@SakiTakamachi SakiTakamachi marked this pull request as ready for review May 14, 2024 13:30
nielsdos
nielsdos reviewed May 14, 2024
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Nice work. Some comments:

ext/bcmath/libbcmath/src/recmul.c
BC_UINT_T num = 0;
BC_UINT_T base = 1;

for (size_t i = 0; i < len; i++) {
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I once shared a fast way of doing this that I had implemented in a WIP commit: nielsdos@20783c3
Original source: https://kholdstare.github.io/technical/2020/05/26/faster-integer-parsing.html
The article claims it is faster than a regular for loop that multiplies by 10.
(note: I wouldn't use the SSE SIMD approach from the article, just the 4/8 byte fast non-SIMD approach)
However, let's not do that in this PR because this PR is large enough as-is.

ext/bcmath/libbcmath/src/recmul.c Outdated
Let u = u0 + u1*(b^n)
Let v = v0 + v1*(b^n)
Then uv = (B^2n+B^n)*u1*v1 + B^n*(u1-u0)*(v0-v1) + (B^n+1)*u0*v0
BC_UINT_T *buf = emalloc((n1_arr_size + n2_arr_size + prod_arr_size) * sizeof(BC_UINT_T));
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Allocations with multiplications shouldn't be open-coded, because they risk getting overflown.
Use safe_emalloc here please.

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Fixed it

ext/bcmath/libbcmath/src/recmul.c
}
*pvptr-- = sum % BASE;
sum = sum / BASE;
while (pend >= pptr) {
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I also had a faster way than this of writing the BCD representation back. See nielsdos@20783c3#diff-5550a7de511402b266a0fc932bd11298d2e02ef720879507b89651add38f380dR177
But we should keep that as a follow-up imo.

ext/bcmath/libbcmath/src/recmul.c Outdated
_bc_rec_mul(u1, u1->n_len, v1, v1->n_len, &m1);
/* Convert n2 to uint[] */
i = 0;
while (n2len > 0) {
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Hmmm, we're doing the same for n1, would be great if this was factored out.

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fixed it

ext/bcmath/libbcmath/src/recmul.c
char *pend = pptr + prodlen - 1;
i = 0;
while (i < prod_arr_size - 1) {
for (size_t j = 0; j < BC_MUL_UINT_DIGITS; j++) {
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Also an opportunity for the faster BCD write method for follow-up.

ext/bcmath/libbcmath/src/recmul.c

/* Do the multiply */
_bc_rec_mul(n1, len1, n2, len2, &pval);
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One thing we're going to lose here now is the divide-and-conquer approach for very large numbers, and I think we should probably keep that. WDYT?

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Good point. Sorry, I should have explained this in the PR description.

The existing implementation uses divide-and-conquer when the number of digits is 80 or more.

This is compared in test case 2, and clearly this PR is faster, so it's not worth keeping the original implementation as is.

However, combining this PR approach with a divide and conquer approach has shown, in my local testing, to probably be nearly twice as effective.

Therefore, we should use the divide and conquer method (commonly called the Karatsuba algorithm in Japanese), but probably separate PRs.

Girgias
Girgias reviewed May 15, 2024
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nielsdos
nielsdos reviewed May 15, 2024
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Well, it's a good thing I allocated time for this, took me a while to think about this.
It looks right except for some small nits.

ext/bcmath/libbcmath/src/recmul.c Outdated
*/
static void _bc_rec_mul(bc_num u, size_t ulen, bc_num v, size_t vlen, bc_num *prod)
/*
* If the n_values ​​of n1 and n2 are both 4 (32-bit) or 8 (64-bit) digits or less,
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I can see in my IDE there's zero-width-spaces on this line.

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It seems that Google Translate mixed it up. Fixed.

ext/bcmath/libbcmath/src/recmul.c Outdated
* e.g. 12345678901234567890 => {34567890, 56789012, 1234}
*
* Multiply and add these groups of numbers to perform multiplication fast.
* How much to shift the digits when adding values ​​can be calculated from the index of the array.
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Also zero-width-spaces on this line.

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Fixed

ext/bcmath/libbcmath/src/recmul.c

bc_num d1 = bc_sub(u1, u0, 0);
bc_num d2 = bc_sub(v0, v1, 0);
BC_UINT_T *buf = safe_emalloc(n1_arr_size + n2_arr_size + prod_arr_size, sizeof(BC_UINT_T), 0);
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I think this deserves a comment to explain why the addition can't overflow in practice.
I.e. let's say that N is the max of n1len and n2len (and a multiple of BC_MUL_UINT_DIGITS for simplicity), then this sum is <= N/BC_MUL_UINT_DIGITS + N/BC_MUL_UINT_DIGITS + N/BC_MUL_UINT_DIGITS + N/BC_MUL_UINT_DIGITS - 1 which is equal to N - 1 if BC_MUL_UINT_DIGITS is 4, and N/2 - 1 if BC_MUL_UINT_DIGITS is 8.

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Added, thx!

nielsdos
nielsdos approved these changes May 16, 2024
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Seems right. Maybe @Girgias wants to have a final look?

Girgias
Girgias approved these changes May 20, 2024
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Looks fine to me

@SakiTakamachi SakiTakamachi merged commit 1d38656 into php:master May 20, 2024
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@SakiTakamachi SakiTakamachi deleted the refactor_bcmath_mul branch May 20, 2024 11:29
@nielsdos nielsdos mentioned this pull request May 20, 2024
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This broken 32-bit builds, see https://github.com/php/php-src/actions/runs/9183895160/job/25255358600. @SakiTakamachi Please have a look.

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Okay, thanks!

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@nielsdos nielsdos nielsdos approved these changes

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