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

1 files changed, 4 insertions, 4 deletions

diff --git a/tools/aapt/ResourceTable.cpp b/tools/aapt/ResourceTable.cpp
index 47750fc11a6e..4e597fb3b30a 100644
--- a/tools/aapt/ResourceTable.cpp
+++ b/tools/aapt/ResourceTable.cpp
@@ -3743,15 +3743,15 @@ ssize_t ResourceTable::Entry::flatten(Bundle* /* bundle */, const sp<AaptFile>&
     size_t amt = 0;
     ResTable_entry header;
     memset(&header, 0, sizeof(header));
-    header.size = htods(sizeof(header));
+    header.full.size = htods(sizeof(header));
     const type ty = mType;
     if (ty == TYPE_BAG) {
-        header.flags |= htods(header.FLAG_COMPLEX);
+        header.full.flags |= htods(header.FLAG_COMPLEX);
     }
     if (isPublic) {
-        header.flags |= htods(header.FLAG_PUBLIC);
+        header.full.flags |= htods(header.FLAG_PUBLIC);
     }
-    header.key.index = htodl(mNameIndex);
+    header.full.key.index = htodl(mNameIndex);
     if (ty != TYPE_BAG) {
         status_t err = data->writeData(&header, sizeof(header));
         if (err != NO_ERROR) {