Github user davies commented on a diff in the pull request:
https://github.com/apache/spark/pull/10068#discussion_r46378463
--- Diff: unsafe/src/main/java/org/apache/spark/unsafe/Platform.java ---
@@ -107,12 +107,27 @@ public static void freeMemory(long address) {
public static void copyMemory(
Object src, long srcOffset, Object dst, long dstOffset, long length) {
- while (length > 0) {
- long size = Math.min(length, UNSAFE_COPY_THRESHOLD);
- _UNSAFE.copyMemory(src, srcOffset, dst, dstOffset, size);
- length -= size;
- srcOffset += size;
- dstOffset += size;
+ // Check if dstOffset is before or after srcOffset to determine if we
should copy
+ // forward or backwards. This is necessary in case src and dst overlap.
+ if (dstOffset < srcOffset) {
--- End diff --
In JVM, two objects (or memory block) can't overlap each other (assuming
the two offsets are in valid range). The only case that src and dst could
overlap is they are exactly the same value (pointer), so we can only compare
the offsets.
After this patch, we can use copyMemory() without worrying about the src
and dst are overlapped or not, it always works.
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