Currently we round non-static fields size to word and subclass non-static fields start from this aligned offset. As result it could increase total instance size:

Tom Rodriguez wrote:

If I have this hierarchy:

class MDecimal {
  Object _impl;
  bool _dirty;
}

class MByte extends MDecimal {
  byte _value;
}

This is the layout I'm getting:

 - ---- non-static fields (3 words):
 - protected transient '_impl' 'Ljava/lang/Object;' @8
 - protected transient '_dirty' 'Z' @12
 - private '_value' 'B' @16

which bumps the object size up by a double word.

Note: final changes for 6523674 allowed to compact small fields into gaps at the same subclass only.