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[WIP][DebugInfo] Lazily parse debug_loclist offsets

Authored by dblaikie on Aug 14 2020, 7:56 AM.

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[WIP][DebugInfo] Lazily parse debug_loclist offsets

Parsing DWARFv5 debug_loclist offsets when a CU is parsed is weighing
down memory usage of symbolizers that don't need to parse this data at
all. There's not much benefit to caching these anyway - since they are
O(1) lookup and reading once you know where the offset list starts (and
can do bounds checking with the offset list size too).

In general, I think it might be time to start paying down some of the
technical debt of loc/loclist/range/rnglist parsing to try to unify it a
bit more.

eg:

  • Currently DWARFUnit has: RangeSection, RangeSectionBase, LocSection, LocSectionBase, LocTable, RngListTable, LoclistTableHeader (be nice if these were all wrapped up in two variables - one for loclists, one for rnglists)
  • rnglists and loclists are handled differently (see: LoclistTableHeader, but no RnglistTableHeader)
  • maybe all these types could be less stateful - lazily parse what they need to, even reparsing rather than caching because it doesn't seem too expensive, for instance. (though admittedly so long as it's constantcost/overead per compilatiton that's probably adequate)
  • Maybe implementing and using a DWARFDataExtractor that can be sub-ranged (so we could slice it up to just the single contribution) - though maybe that's not so useful because loc/ranges need to refer to it by absolute, not contribution-relative mechanisms

Differential Revision: https://reviews.llvm.org/D86110

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