[dev.link] cmd/link: make symbol's global index unique
Currently, when mapping symbol's local index to global index, for
duplicated and overwriting/overwritten symbols, each appearance
of the symbol gets a global index, with one being the "primary",
and others "redirect" to it through the overwrite map. Basically,
the local-global index mapping is one to one, with overwrite/
dedup happening in global index level.
This has a few drawbacks:
- All symbol accesses effectively need to query the overwrite
map. This may hurt performance.
- For multi-level overwrites, (Y overwrites X, Z overwrites Y),
this can get quite complicated, and we have to follow the
redirection recursively.
- Failed to follow or to update the overwrite map leads to bugs.
In this CL, we change the index mapping mechanism so that each
symbol get a unique global index. Multiple appearances of the
same symbol get the same index. Now the local-global index
mapping is N to one. Overwrite/dedup happens directly in the
local-global mapping.
We keep both mapping directions in arrays. Each object carries
an array for its local-global mapping. The loader carries an
array mapping global index to the "primary" local index, which is
the one we should load from. This way, we can get rid of the
overwrite map, and index conversions are simply array accesses.
TODO: we still make reservation of the index space upfront, and
leave holes for dup symbols. Maybe get rid of the reservation and
holes.