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buffer: optimize hex parsing. #7602

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5a8b48e
buffer: optimize hex_decode.
chjj Jul 7, 2016
ae21267
buffer: fix hex parsing and add bad hex string test.
chjj Jul 8, 2016
1627283
buffer: lint for hex parsing optimization.
chjj Jul 8, 2016
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27 changes: 27 additions & 0 deletions benchmark/buffers/buffer-hex.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
'use strict';

const common = require('../common.js');

const bench = common.createBenchmark(main, {
len: [0, 1, 64, 1024],
n: [1e7]
});

function main(conf) {
const len = conf.len | 0;
const n = conf.n | 0;
const buf = Buffer.alloc(len);
var i, b;

for (i = 0; i < buf.length; i++)
buf[i] = i & 0xff;

const hex = buf.toString('hex');

bench.start();

for (i = 0; i < n; i += 1)
b = Buffer.from(hex, 'hex');

bench.end(n);
}
37 changes: 24 additions & 13 deletions src/string_bytes.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -143,16 +143,27 @@ const int8_t unbase64_table[256] =
};


template <typename TypeName>
unsigned hex2bin(TypeName c) {
if (c >= '0' && c <= '9')
return c - '0';
if (c >= 'A' && c <= 'F')
return 10 + (c - 'A');
if (c >= 'a' && c <= 'f')
return 10 + (c - 'a');
return static_cast<unsigned>(-1);
}
const int8_t unhex_table[256] =
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@addaleax addaleax Jul 7, 2016

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static const int8_t?

{ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1,
-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1
};

#define unhex(x) \
static_cast<uint8_t>(unhex_table[static_cast<uint8_t>(x)])


template <typename TypeName>
Expand All @@ -162,11 +173,11 @@ size_t hex_decode(char* buf,
const size_t srcLen) {
size_t i;
for (i = 0; i < len && i * 2 + 1 < srcLen; ++i) {
unsigned a = hex2bin(src[i * 2 + 0]);
unsigned b = hex2bin(src[i * 2 + 1]);
unsigned a = unhex(src[i * 2 + 0]);
unsigned b = unhex(src[i * 2 + 1]);
if (!~a || !~b)
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@addaleax addaleax Jul 7, 2016

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Does this still work with unhex()’s return type changed? It seems to me some of the speedup may come from the compiler eliminating this branch completely because it knows the upper bits of a and b will never be set:

$ ./node-master -p 'Buffer(10).write("abcdxx", "hex")'
2
$ ./node -p 'Buffer(10).write("abcdxx", "hex")'
3

(aka we need more tests to cover these cases?)

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@chjj chjj Jul 7, 2016

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@addaleax, good catch. I didn't notice the type difference. It looks like you're right.

Applying this patch:

diff --git a/src/string_bytes.cc b/src/string_bytes.cc
index c216c5d..cb0e78a 100644
--- a/src/string_bytes.cc
+++ b/src/string_bytes.cc
@@ -173,9 +173,9 @@ size_t hex_decode(char* buf,
                   const size_t srcLen) {
   size_t i;
   for (i = 0; i < len && i * 2 + 1 < srcLen; ++i) {
-    unsigned a = unhex(src[i * 2 + 0]);
-    unsigned b = unhex(src[i * 2 + 1]);
-    if (!~a || !~b)
+    uint8_t a = unhex(src[i * 2 + 0]);
+    uint8_t b = unhex(src[i * 2 + 1]);
+    if ((a | b) & 0x80)
       return i;
     buf[i] = (a << 4) | b;
   }

Kills some of the gained perf, but large hex strings are still faster:

$ ./out/Release/node benchmark/buffers/buffer-hex.js
buffers/buffer-hex.js len="0" n="10000000": 5888692.25695
buffers/buffer-hex.js len="1" n="10000000": 1471220.48559
buffers/buffer-hex.js len="64" n="10000000": 1206331.66340
buffers/buffer-hex.js len="1024" n="10000000": 385088.76228

I'll see what else I can do, and probably add a test for bad hex strings.

return i;
buf[i] = a * 16 + b;
buf[i] = (a << 4) | b;
}

return i;
Expand Down