A substantial part of ICU's data is in collation resource bundles. In an almost-ICU 4.2 build, they represent about a quarter of the total data size of 15.2MB.
genrb recognizes a collation tailoring rule, creates a Collator from it, and adds its binary data into the resource bundle.
We have several dozen files with very simple rules that result in 14kB or larger binaries. One could omit these binaries, if the runtime code has the builder code and the necessary data files (especially invuca.icu). This could be done manually in a makefile with an existing genrb command-line option, or a new option could decide based on the length or complexity of the rules string whether to store the binary. A build (makefile) could always supply this new option, and genrb manages when to include the binary. This would save about 800kB. Cost: Runtime code needs to include the collation builder and invuca.icu, slower loading of a collator, more heap memory usage.
We could review and modify the data format for collation binaries.
With data for Latin-1 but very few other characters the size of the binary data should be closer to 6 or 7kB not 14kB.
I believe that we copy the Latin-1 data from the UCA table into the tailoring table, for Latin-1 characters that would fall back to the UCA.
I believe that the collation runtime code already allocates and fills a Latin-1 CE table which changes depending on the runtime attributes. If this is the case, then copying the Latin-1 data from the UCA table may be unnecessary.
If there is no data outside of the Latin-1 range, then we may not need the UTrie at all. Or, we could have a canned UTrie/UTrie2 index array that is used when only a certain set of characters is tailored, and in that case we do not store a complete UTrie(2) but only its data32 array. Problem: If we adopt the European Ordering Rules (EOR) and they tailor many non-Latin-1 characters, then many of these tailorings will not fit these optimizations.
Investigate how dense or sparse the CJK data is in the UTrie. Is there an opportunity for a different, CJK-specific data structure? For example, storing only 16-bit primary weights for Han characters? (With a separate 16-bit UTrie for characters that fit, or a flat table for Unihan and/or Hangul?)
We might be able to always store the top and bottom 16 bits of a CE in two separate 16-bit-value tries. This would eliminate special checks for size-optimized character ranges and whether there is a special trie, and it should not have a performance impact in string comparisons that are decided by primary differences. The bottom-16 trie would be very small if most of the CEs have default values for secondary and tertiary weights. Cost: Generating sort keys would be a bit slower, but we probably care more about sort key length (which would not change) than generation time. We would have to always store two tries, which adds overhead and probably negates the savings for small tailorings.
Note: CLDR 1.9/ICU 4.6 tailors many more characters in several CJK tailorings. While the range above [last regular] got larger again, some of the tailorings still require 3-byte primary weights.
In addition to the CE mappings (main trie, contraction and expansion tables) we also have additional data for certain operations, like bit sets to optimize backward iteration. We could omit these and build them at load time. Or we could omit them only if they are the same as the ones in the UCA data, and the runtime could point there.
Maybe we could store small and sparse binary results (output from the collation builder) in a simple data format that's much smaller than the full form, for example a list of mappings from strings to CE sequences. At runtime, we would need the UTrie2 builder and enough code to build expansion and contraction tables from precomputed data, but not the full collation builder nor the invuca.icu data, canonical equivalence code, etc. We could store this data under a separate bundle key to avoid format versioning. We could even store this data in a resource item tree (array of strings parallel with CE intvector, etc.) instead of a custom binary. This could save a significant amount of data. Cost: Runtime code needs UTrie and contraction/expansion table builders, somewhat slower loading of a collator, more heap memory usage.
Vladimir's idea: For languages with small scripts (e.g., Cyrillic or Thai) we might be able to use a short, linear table mapping a single range of code points to CEs, to avoid the trie. Or maybe use two or three ranges, especially if one of them is in redundant Latin-1 anyway and is only used at load time to build the special Latin-1 table. Use the linear table(s) directly at runtime, instead of building a trie at load time. Cost: Trades size for performance, although it might actually be faster than the trie if the collator sees characters in the linear range. It just adds yet another test into the chain of where-is-the-data-for-this-character. Explore using a function pointer, or C++ subclass with virtual functions, to switch between variants of how CEs are stored and accessed?
We should make incompatible changes at the same time that we switch from the old UTrie to the new UTrie2, which will anyway require a major-version change.
For some of the rule strings, ICU could adopt new syntax like what CLDR has, for a more compact representation of a sequence of characters with primary differences from a reset point. This should significantly reduce the CJK rule strings — by more than 50%! We would have to try it out to get exact numbers.
We have three near-identical copies of the Unihan collation rules, in the ja/ko/zh collation bundles. genrb could auto-detect their large, common suffix (starting with "&[last regular ]"), store it in a separate coll/unihan.res bundle and replace that suffix in the CJK bundles with something like "[unihan]". Cost: At runtime, when someone asks for the unihan rule string (which is rare), the string needs to be allocated/concatenated from the distinct pieces and the common suffix.
We could use the pre-ICU 1.8 syntax of ';' and ',' instead of "<<" and "<<<". (See http://userguide.icu-project.org/collation/customization)
Is it feasible to omit the rule strings and build them at runtime, on demand? That would remove all of the very large strings in ICU data. (Mark has some ideas for how this could be done. We would at least have to store some data for the reset points. Might only be worth doing for large rules.)
We could store separate, parallel trees of collation resource bundles: One only with binaries, one only with rule strings. It would be easy to not include one or the other.
The Unihan collation data is large, and mostly but not completely the same in the three CJK collation resource bundles. Is there any way to share the common data?
We have more than 180 empty files "so genrb doesn't issue warnings" (like the fallback warning). Removing these would save about 10kB. We would have to redesign how to determine whether we support a language/locale in collation.