[GitHub] [pinot] richardstartin commented on a change in pull request #7708: optimise getUnpaddedString with SWAR padding search

[GitBox <gi...@apache.org>]

richardstartin commented on a change in pull request #7708:
URL: https://github.com/apache/pinot/pull/7708#discussion_r743986902



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File path: pinot-segment-local/src/main/java/org/apache/pinot/segment/local/io/util/FixedByteValueReaderWriter.java
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@@ -56,14 +57,28 @@ public String getUnpaddedString(int index, int numBytesPerValue, byte paddingByt
     assert buffer.length >= numBytesPerValue;
 
     long startOffset = (long) index * numBytesPerValue;
-    for (int i = 0; i < numBytesPerValue; i++) {
-      byte currentByte = _dataBuffer.getByte(startOffset + i);
-      if (currentByte == paddingByte) {
-        return new String(buffer, 0, i, UTF_8);
+    int written = 0;
+    long pattern = (paddingByte & 0xFFL) * 0x101010101010101L;
+    ByteBuffer wrapper = ByteBuffer.wrap(buffer);
+    for (int i = 0; i < ((numBytesPerValue >>> 3) << 3); i += 8) {
+      long word = _dataBuffer.getLong(startOffset + i);
+      wrapper.putLong(i, word);
+      long zeroed = word ^ pattern;
+      long tmp = (zeroed & 0x7F7F7F7F7F7F7F7FL) + 0x7F7F7F7F7F7F7F7FL;
+      tmp = ~(tmp | zeroed | 0x7F7F7F7F7F7F7F7FL);
+      int end = Long.numberOfLeadingZeros(tmp) >>> 3;

Review comment:
       `Long.numberOfLeadingZeros` has an intrinsic implementation which compiles to an instruction with 1 cycle latency and reciprocal throughput of 1/2, it's really not worth guarding with a branch.




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