[PATCH] D153268: [DWARFLinkerParallel] Add limited functionality to DWARFLinkerParallel.

[Alexey Lapshin via Phabricator via llvm-commits]

avl added a comment.

In D153268#4435883 <https://reviews.llvm.org/D153268#4435883>, @clayborg wrote:

>> a) No common abbreviation table. Each compile unit has its own abbreviation table. Generating common abbreviation table slowdowns parallel execution(This is a resource that is accessed many times from many threads). Abbreviation table does not take a lot of space, so it looks cheap to have separate abbreviations tables. Later, it might be optimized a bit(by removing equal abbreviations tables).
>
> Can't we just let each compile unit generate its own abbrev table and then unique them into a single abbrev table at when we write out the .debug_info data? That way compile units can stay fast in parallel execution, and since we need to emit the compile units serially when generating the .debug_info, we can just have a map that maps CU specific abbrev codes to the global and final abbrev list? Then we end up with just one .debug_abbrev table in the end

The problem with this solution seems that when local abbreviation number would be replaced with global number(which is ULEB128 format) the DIEs offsets will be changed(because size of global number may differ from the size of local number). It will brake all sizes and references.

Also, currently CU is emitted(when we write out the .debug_info data) immediately. The global abbreviation table might be done in the very end after all CUs are emitted. That means we need to parse already emitted CU and update abbreviation numbers. It would probably slowdown execution.

The size of abbreviation table for clang binary having .debug_info 1.7G : 12.5K for DWARFLinker and 7.2M for DWARFLinkerParallel. i.e. new abbreviation table is approx 0.5% of overall .debug_info size. It looks relatively cheap. Though, any solution making .debug_abbrev shorter would certainly be good. Probably we might experiment with fixed length abbreviation numbers...


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