Improve performance of decoding #62

taiki-e · 2021-07-18T09:13:52Z

This change makes decoding about 10x faster than the current implementation, on my machine.

Before:

hex_decode              time:   [128.28 us 129.41 us 130.70 us]

After:

hex_decode              time:   [12.206 us 12.242 us 12.284 us]

Benchmarks per commit

Current main branch (aa8f300)

hex_decode              time:   [128.28 us 129.41 us 130.70 us]

First commit of this PR: Use lookup table for decoding (870590e)

hex_decode              time:   [63.538 us 63.936 us 64.419 us]
                        change: [-50.833% -50.462% -50.088%] (p = 0.00 < 0.05)
                        Performance has improved.

Second commit of this PR: Inline val function (3b75b32)

hex_decode              time:   [32.263 us 32.382 us 32.521 us]
                        change: [-49.441% -49.098% -48.682%] (p = 0.00 < 0.05)
                        Performance has improved.

Third commit of this PR: Use chunks_exact instead of chunks (ff1d115)

hex_decode              time:   [24.701 us 24.814 us 24.936 us]
                        change: [-24.459% -23.911% -23.395%] (p = 0.00 < 0.05)
                        Performance has improved.

Fourth commit of this PR: Use decode_to_slice in Vec::from_hex (4fd0ed8)

hex_decode              time:   [16.060 us 16.297 us 16.608 us]
                        change: [-35.137% -34.468% -33.556%] (p = 0.00 < 0.05)
                        Performance has improved.

Fifth commit of this PR: Inline decode_to_slice function (3e2aa25)

hex_decode              time:   [12.206 us 12.242 us 12.284 us]                        
                        change: [-21.954% -20.254% -18.150%] (p = 0.00 < 0.05)
                        Performance has improved.

codecov-commenter · 2021-07-18T09:16:53Z

Codecov Report

Merging #62 (f3251bd) into main (aa8f300) will decrease coverage by 0.44%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   94.55%   94.11%   -0.45%     
==========================================
  Files           4        4              
  Lines         147      153       +6     
==========================================
+ Hits          139      144       +5     
- Misses          8        9       +1

Impacted Files	Coverage Δ
tests/version-number.rs	`100.00% <ø> (ø)`
src/lib.rs	`96.99% <100.00%> (-0.65%)`	⬇️

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Luro02

Looks good :)

Luro02 · 2021-08-27T07:32:49Z

src/lib.rs

+
+// Lookup table for ascii to hex decoding.
+#[rustfmt::skip]
+static DECODE_TABLE: [u8; 256] = [


Why is this static and not const?

Suggested change

static DECODE_TABLE: [u8; 256] = [

const DECODE_TABLE: [u8; 256] = [

If this table were const, one could make the val function const: playground

The lookup table is relatively large, so using const may bloat the binary size by inlining.

IIRC, the standard library uses static for that use-case. For example: https://github.com/rust-lang/rust/blob/1.54.0/library/core/src/unicode/unicode_data.rs

See also https://stackoverflow.com/questions/52751597/what-is-the-difference-between-a-constant-and-a-static-variable-and-which-should

By the way, benchmarking (on my machine) shows that changing this to const does not change the performance.

Luro02 · 2021-08-27T07:37:46Z

src/lib.rs

+            index: idx + 1,
+        });
+    }
+    Ok((upper << 4) | lower)


What does this bitshift + or do?

Suggested change

Ok((upper << 4) | lower)

// upper and lower are only 4 bits large, so of the 8 bits only the first 4 are used.

// this merges the two 4 bit numbers into one 8 bit number:

//

// upper: 0 0 0 0 U U U U

// lower: 0 0 0 0 L L L L

// result: U U U U L L L L

Ok((upper << 4) | lower)

(does not hurt to explain?)

src/lib.rs

Luro02 · 2021-08-27T07:46:03Z

src/lib.rs

+];
+
+#[inline]
+fn val(bytes: &[u8], idx: usize) -> Result<u8, FromHexError> {


Suggested change

fn val(bytes: &[u8], idx: usize) -> Result<u8, FromHexError> {

const fn val(bytes: &[u8], idx: usize) -> Result<u8, FromHexError> {

The new implementation of val function cannot be const

If the table were const and not static, this function could be constant: playground

Luro02 · 2021-08-27T07:49:42Z

src/lib.rs


-use core::iter;
+use core::{iter, u8};


uhm I do not think that you should import the u8 module, where u8::MAX is deprecated.

The u8 type itself has an associated constant u8::MAX.

Suggested change

use core::{iter, u8};

use core::iter;

I used u8 module because the u8::MAX associated constant requires a relatively new compiler.

See also: #55

You have a point. Personally, I do not really care about MSRV (because I want to use the new features and updating the compiler is just a simple rustup update) so I did not think much about it.

#58 (comment)

taiki-e added 5 commits July 18, 2021 17:12

Use lookup table for decoding

870590e

Inline val function

3b75b32

Use chunks_exact instead of chunks

ff1d115

Use decode_to_slice in Vec::from_hex

4fd0ed8

Inline decode_to_slice function

3e2aa25

Fix build errors

f3251bd

taiki-e force-pushed the pref branch from 5ac1916 to f3251bd Compare July 18, 2021 09:45

Luro02 suggested changes Aug 27, 2021

View reviewed changes

taiki-e mentioned this pull request Aug 28, 2021

hex::encode performance #18

Open

taiki-e mentioned this pull request Dec 20, 2022

Draft: Add BWoS-queue backend to tokio tokio-rs/tokio#5283

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Improve performance of decoding #62

Improve performance of decoding #62

taiki-e commented Jul 18, 2021

codecov-commenter commented Jul 18, 2021 •

edited

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Luro02 Aug 27, 2021

taiki-e Aug 28, 2021 •

edited

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taiki-e Aug 28, 2021

Luro02 Aug 27, 2021

Luro02 Aug 27, 2021

taiki-e Aug 28, 2021

Luro02 Aug 28, 2021

Luro02 Aug 27, 2021

taiki-e Aug 28, 2021

Luro02 Aug 28, 2021

	static DECODE_TABLE: [u8; 256] = [
	const DECODE_TABLE: [u8; 256] = [

-    Ok((upper << 4) | lower)
+    // upper and lower are only 4 bits large, so of the 8 bits only the first 4 are used.
+    // this merges the two 4 bit numbers into one 8 bit number:
+    //
+    // upper:  0 0 0 0 U U U U
+    // lower:  0 0 0 0 L L L L
+    // result: U U U U L L L L
+    Ok((upper << 4) | lower)

	fn val(bytes: &[u8], idx: usize) -> Result<u8, FromHexError> {
	const fn val(bytes: &[u8], idx: usize) -> Result<u8, FromHexError> {

Improve performance of decoding #62

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Improve performance of decoding #62

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taiki-e commented Jul 18, 2021

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codecov-commenter commented Jul 18, 2021 •

edited

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taiki-e Aug 28, 2021 •

edited

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