twmb commented on a change in pull request #11721:
URL: https://github.com/apache/kafka/pull/11721#discussion_r801157664
##########
File path: clients/src/main/java/org/apache/kafka/common/utils/ByteUtils.java
##########
@@ -386,18 +386,39 @@ public static void writeDouble(double value, ByteBuffer
buffer) {
buffer.putDouble(value);
}
+ final static int[] LEADING_ZEROS_TO_U_VARINT_SIZE = new int[] {
+ // 32 bits, and each 7-bits adds one byte to the output
+ 5, 5, 5, 5, // 32
+ 4, 4, 4, 4, 4, 4, 4, // 28
+ 3, 3, 3, 3, 3, 3, 3, // 21
+ 2, 2, 2, 2, 2, 2, 2, // 14
+ 1, 1, 1, 1, 1, 1, 1, // 7
+ 1 // 0
+ };
+
+ final static int[] LEADING_ZEROS_TO_U_VARLONG_SIZE = new int[] {
+ // 64 bits, and each 7-bits adds one byte to the output
+ 10, // 64
+ 9, 9, 9, 9, 9, 9, 9, // 63
+ 8, 8, 8, 8, 8, 8, 8, // 56
+ 7, 7, 7, 7, 7, 7, 7, // 49
+ 6, 6, 6, 6, 6, 6, 6, // 42
+ 5, 5, 5, 5, 5, 5, 5, // 35
+ 4, 4, 4, 4, 4, 4, 4, // 28
+ 3, 3, 3, 3, 3, 3, 3, // 21
+ 2, 2, 2, 2, 2, 2, 2, // 14
+ 1, 1, 1, 1, 1, 1, 1, // 7
+ 1 // 0
+ };
+
/**
* Number of bytes needed to encode an integer in unsigned variable-length
format.
*
* @param value The signed value
*/
public static int sizeOfUnsignedVarint(int value) {
- int bytes = 1;
- while ((value & 0xffffff80) != 0L) {
- bytes += 1;
- value >>>= 7;
- }
- return bytes;
+ int leadingZeros = Integer.numberOfLeadingZeros(value);
+ return LEADING_ZEROS_TO_U_VARINT_SIZE[leadingZeros];
Review comment:
Sorry to chime in late here, but I'm wondering if a lookup table may
still be faster. You can see these two instructions in your lookup table
implementation above,
```
1.26% │ 0x00007fde904792b1: cmp $0x21,%r10d
2.65% │ 0x00007fde904792b5: jnb 0x7fde90479332
```
this is comparing if the calculated number (leading zeroes) fits within the
size-33 array. Does Java have the concept of a 256 sized array, and uint8
numbers?
In my Go implementation (which uses bits.Len, i.e., the opposite of leading
zeroes), using a size-256 string and a uint8 allows for no bounds check,
resulting in even fewer instructions and shaving a fraction of a nanosecond of
time in comparison to the pure math version.
In this benchmark,
```go
func BenchmarkUvarintLen(b *testing.B) {
z := 0
for i := 0; i < b.N; i++ {
z += UvarintLen(uint32(i))
}
b.Log(z)
}
```
Comparing the existing code to this code:
```go
func UvarintLen(u uint32) int {
lz := bits.LeadingZeros32(u)
return (((38 - lz) * 0b10010010010010011) >> 19) + (lz >> 5)}
```
Top is math version, bottom is lookup table:
```
BenchmarkUvarintLen-8 1000000000 1.179 ns/op
BenchmarkUvarintLen-8 1000000000 0.7811 ns/op
```
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