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Commit message

encoding/hex: speed up Encode

This CL manually unrolls Encode's loop in order to reduce the number of
bounds checks that the function incurs and speed up its execution. Here
are some benchmark results (no change to allocations):

goos: darwin
goarch: arm64
pkg: encoding/hex
cpu: Apple M4
                │     old      │                 new                 │
                │    sec/op    │   sec/op     vs base                │
Encode/256-10      259.4n ± 0%   156.8n ± 0%  -39.55% (p=0.000 n=20)
Encode/1024-10     997.0n ± 0%   626.0n ± 0%  -37.21% (p=0.000 n=20)
Encode/4096-10     3.948µ ± 0%   2.478µ ± 0%  -37.23% (p=0.000 n=20)
Encode/16384-10   15.750µ ± 0%   9.886µ ± 0%  -37.23% (p=0.000 n=20)
geomean            2.003µ        1.245µ       -37.82%

                │     old      │                  new                  │
                │     B/s      │      B/s       vs base                │
Encode/256-10     941.1Mi ± 0%   1557.5Mi ± 0%  +65.50% (p=0.000 n=20)
Encode/1024-10    979.5Mi ± 0%   1560.1Mi ± 0%  +59.28% (p=0.000 n=20)
Encode/4096-10    989.5Mi ± 0%   1576.2Mi ± 0%  +59.29% (p=0.000 n=20)
Encode/16384-10   992.1Mi ± 0%   1580.6Mi ± 0%  +59.32% (p=0.000 n=20)
geomean           975.3Mi         1.532Gi       +60.82%

Change-Id: I06a6d3ad58e68913aaec614014db08fbf356dfb4

GitHub-Last-Rev: c20c977291d08052e4a9508d5aad9c962672497d

GitHub-Pull-Request: golang/go#81655

Change diff

diff --git a/src/encoding/hex/hex.go b/src/encoding/hex/hex.go
index 4499974..3a2a323 100644
--- a/src/encoding/hex/hex.go
+++ b/src/encoding/hex/hex.go
@@ -43,12 +43,38 @@
 // of bytes written to dst, but this value is always [EncodedLen](len(src)).
 // Encode implements hexadecimal encoding.
 func Encode(dst, src []byte) int {
-	j := 0
-	for _, v := range src {
-		dst[j] = hextable[v>>4]
-		dst[j+1] = hextable[v&0x0f]
-		j += 2
+	if len(src) == 0 {
+		return 0
 	}
+	i := 0
+	// Process the data in predictable chunks (4 src bytes, 8 dst bytes),
+	// to reduce the number of bounds checks.
+	for ; i < len(src)-3; i += 4 {
+		dst := dst[i*2 : i*2+8] // Eliminate bounds checks.
+		v0 := src[i]
+		dst[0] = hextable[v0>>4]
+		dst[1] = hextable[v0&0x0f]
+
+		v1 := src[i+1]
+		dst[2] = hextable[v1>>4]
+		dst[3] = hextable[v1&0x0f]
+
+		v2 := src[i+2]
+		dst[4] = hextable[v2>>4]
+		dst[5] = hextable[v2&0x0f]
+
+		v3 := src[i+3]
+		dst[6] = hextable[v3>>4]
+		dst[7] = hextable[v3&0x0f]
+	}
+
+	// Process the rest.
+	for ; i < len(src); i++ {
+		v := src[i]
+		dst[i*2] = hextable[v>>4]
+		dst[i*2+1] = hextable[v&0x0f]
+	}
+
 	return len(src) * 2
 }

Change information

Files:

M src/encoding/hex/hex.go

Change size: S

Delta: 1 file changed, 31 insertions(+), 5 deletions(-)

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I spotted some possible problems with your PR:

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Gopher Robot . resolved

I spotted some possible problems with your PR:
  1. You usually need to reference a bug number for all but trivial or cosmetic fixes. For this repo, the format is usually 'Fixes #12345' or 'Updates #12345' at the end of the commit message. Should you have a bug reference?
Please address any problems by updating the GitHub PR.
When complete, mark this comment as 'Done' and click the [blue 'Reply' button](https://go.dev/wiki/GerritBot#i-left-a-reply-to-a-comment-in-gerrit-but-no-one-but-me-can-see-it) above. These findings are based on heuristics; if a finding does not apply, briefly reply here saying so.
To update the commit title or commit message body shown here in Gerrit, you must edit the GitHub PR title and PR description (the first comment) in the GitHub web interface using the 'Edit' button or 'Edit' menu entry there. Note: pushing a new commit to the PR will not automatically update the commit message used by Gerrit.
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Ingo Oeser . resolved

Small suggestions.

File src/encoding/hex/hex.go

Line 68, Patchset 4 (Latest): for i := 0; i < len(src); i++ {

Ingo Oeser . unresolved

```suggestion
	for i := range len(arc) {
```

Line 70, Patchset 4 (Latest): dst[i*2] = hextable[v>>4]

Ingo Oeser . unresolved

You could also add a bound check elimination line before this line.

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File src/encoding/hex/hex.go

Line 68, Patchset 4 (Latest): for i := 0; i < len(src); i++ {

Ingo Oeser . unresolved

```suggestion
	for i := range len(arc) {
```

Ingo Oeser

Sorry I meant len(src) here (damn autocorrect).

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File src/encoding/hex/hex.go

Line 68, Patchset 4 (Latest): for i := 0; i < len(src); i++ {

Ingo Oeser . resolved

```suggestion
	for i := range len(arc) {
```

Ingo Oeser

Sorry I meant len(src) here (damn autocorrect).

jub0bs

Done

Line 70, Patchset 4 (Latest): dst[i*2] = hextable[v>>4]

Ingo Oeser . resolved

You could also add a bound check elimination line before this line.

jub0bs

This loops runs for at most three iterations; I'm not sure eliminating one of the two bounds check per iteration is worth it.

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[go] encoding/hex: speed up Encode

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