Bugzilla

checked self-hosted and ScriptPreloader usage (boot Firefox, wait 20 seconds, close)
in most case, almost all atoms are used.
  * use =  100% : 56% of scripts 
  * use >= 99% : 68% of scripts
  * use >= 98% : 81% of scripts
  * use >= 95% : 99% of scripts

then, lowest one is resource://activity-stream/vendor/Redux.jsm there 71% of atoms are used by stencil (163 / 228)

anyway, given index map with "source + dest" uses 2 index (8 bytes) for each used atom,
while index map with "dest or null" uses 1 index (4 bytes) for each atom in table,

so, if more than 50% atoms are used by stencil, index map with "dest or null" takes less space,
and all full-parse case are covered.

I'll add 2 mode for the index map

checked self-hosted and ScriptPreloader usage (boot Firefox, wait 20 seconds, close)
in most case, almost all atoms are used.
  * use =  100% : 56% of scripts 
  * use >= 99% : 68% of scripts
  * use >= 98% : 81% of scripts
  * use >= 95% : 99% of scripts

then, lowest one is resource://activity-stream/vendor/Redux.jsm there 71% of atoms are used by stencil (163 / 228)

anyway, given index map with "source + dest" uses 2 index (8 bytes) for each used atom,
while index map with "dest or null" uses 1 index (4 bytes) for each atom in table,
if more than 50% atoms are used by stencil, index map with "dest or null" takes less space,
and all full-parse case are covered.

I'll add 2 mode for the index map