androidfw: Add support for compact resource entries
Bug: 237583012
Given the large number of simple resources such as strings in
Android resources, their ResTable_entry and Res_value can be
encoded together in a compact way. This allows a significant
saving in both storage and memory footprint.
The basic observations for simple resources are:
* ResTable_entry.size will always be sizeof(ResTable_entry)
unless it's a complex entry
* ResTable_entry.key is unlikely to exceed 16-bit
* ResTable_entry.flags only uses 3 bits for now
* Res_value.size will always be sizeof(Res_value)
Given the above, we could well encode the information into
a compact/compatible structure.
struct compact {
uint16_t key;
uint16_t flags;
uint32_t data;
};
The layout of this structure will allow maximum backward
compatibility. e.g. the flags will be at the same offset,
and a
`dtohs((ResTable_entry *)entry->flags) & FLAG_COMPACT`
would tell if this entry is a compact one or not. For a
compact entry:
struct compact *entry;
entry_size == sizeof(*entry)
entry_key == static_cast<uint32_t>(dtohs(entry->key))
entry_flags == dtohs(entry->flags) & 0xff // low 8-bit
data_type == dtohs(entry->flags) >> 8 // high 8-bit
data_size == sizeof(Res_value)
data_value == dtohl(entry->data)
To allow minimum code change and backward compatibility,
we change 'struct ResTable_entry' to 'union ResTable_entry',
with an anonymous structure inside that's fully backward
compatible. Thus, any existing reference such as:
ResTable_entry *entry = ...
if (dtohs(entry->flags) & ResTable_entry::FLAG_COMPLEX) ...
would still work.
However, special care needs to be taken after an entry is
obtained, and when value needs to be extracted.
A compact entry will not encode a complex value, and hence
complex entries/values are handled the same way.
Change-Id: I15d97a4f5e85fab28c075496f7f0cf6b1fcd73e3
16 files changed
