kazuyukitanimura commented on a change in pull request #34611:
URL: https://github.com/apache/spark/pull/34611#discussion_r750833865
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File path:
sql/core/src/main/java/org/apache/spark/sql/execution/vectorized/WritableColumnVector.java
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@@ -179,6 +180,18 @@ public WritableColumnVector reserveDictionaryIds(int
capacity) {
*/
protected abstract void reserveInternal(int capacity);
+ /**
+ * Each byte of the returned value (long) has one bit from `bits`. I.e. it
is equivalent to
+ * byte[] a = {(byte)(bits >> 0 & 1), (byte)(bits >> 1 & 1),
+ * (byte)(bits >> 2 & 1), (byte)(bits >> 3 & 1),
+ * (byte)(bits >> 4 & 1), (byte)(bits >> 5 & 1),
+ * (byte)(bits >> 6 & 1), (byte)(bits >> 7 & 1)};
+ * return ByteBuffer.wrap(a).getLong();
+ */
+ protected final long toBitPerByte(int bits) {
+ return ((bits * 0x8040201008040201L) >>> 7) & 0x101010101010101L;
Review comment:
The purpose is for converting `0x000000FF` to `0x0101010101010101L` as
minimum ops as possible.
The simple shit mask would be something like
```((bits << 56) | (bits << 47) | (bits << 38) | (bits << 29) | (bits << 20)
| (bits << 11) | (bits <<2) | (bits >> 7)) & 0x101010101010101L```
that requires 16 ops (8 Shifts, 7 ORs, and 1 And.)
The equivalent
```((bits * 0x8040201008040201L) >>> 7) & 0x101010101010101L```
requires only 3 ops (1 Multiplication, 1 Shift, and 1 And.)
Multiplication with modern CPUs is as fast as 1 clock cycle. So this code
will save 13 ops.
I benchmarked several times. The variance between benchmark results was too
large to see the gain.
Given `bits * 0x8040201008040201L` is easy enough to understand, I kept it.
In theory, it should be faster.
Updated the comment, hopefully it is clearer now.
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