The following pattern can be seen throughout the JVM, but mostly the compiler code:

BitMap map(size); // The bitmap is cleared here
map.clear(); // Then here again

The reason for this is that resource allocated bitmaps used to not be cleared in the constructor.

When G1 was introduced, the code was changed to clear all bitmaps when the backing storage was created, but the compiler code wasn't updated accordingly.

See:
http://hg.openjdk.java.net/jdk9/hs/hotspot/diff/37f87013dfd8/src/share/vm/utilities/bitMap.cpp