David Mollitor created SPARK-59382:
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Summary: Avoid redundant volatile reads of lazy caches in
UTF8String
Key: SPARK-59382
URL: https://issues.apache.org/jira/browse/SPARK-59382
Project: Spark
Issue Type: Improvement
Components: Spark Core
Affects Versions: 4.1.0
Reporter: David Mollitor
h2. Summary
{{UTF8String}} lazily computes and caches several values in {{volatile}}
fields: {{numChars}}, {{isFullAscii}}, {{isValid}}, and {{numBytesValid}}. The
accessor methods read the {{volatile}} field two (or three) times per call --
the "is it computed yet" check plus the return -- and two callers invoke
{{numChars()}} twice. A {{volatile}} read cannot be kept in a register or
hoisted by the JIT, and on AArch64 it compiles to a load-acquire ({{ldar}})
rather than a plain load. This reads each field once into a local, halving the
volatile reads on the warm path.
This is a small, behavior-preserving micro-optimization; no functional change.
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