[Lldb-commits] [PATCH] D78489: [lldb/DWARF] Trust CU DW_AT_low/high_pc information when building address tables

[Pavel Labath via Phabricator via lldb-commits]

labath created this revision.
labath added reviewers: JDevlieghere, aprantl, clayborg.
Herald added a project: LLDB.

The code in DWARFCompileUnit::BuildAddressRangeTable tries hard to avoid
relying on DW_AT_low/high_pc for compile unit range information, and
this logic is a big cause of llvm/lldb divergence in the lowest layers
of dwarf parsing code.

The implicit assumption in that code is that this information (as opposed to
DW_AT_ranges) is unreliable. However, I have not been able to verify
that assumption. It is definitely not true for all present-day
compilers (gcc, clang, icc), and it was also not the case for the
historic compilers that I have been able to get a hold of (thanks Matt
Godbolt).

All compiler included in my research either produced correct
DW_AT_ranges or .debug_aranges entries, or they produced no DW_AT_hi/lo
pc at all. The detailed findings are:

- gcc >= 4.4: produces DW_AT_ranges and .debug_aranges
- 4.1 <= gcc < 4.4: no DW_AT_ranges, no DW_AT_high_pc, .debug_aranges present. The upper version range here is uncertain as godbolt.org does not have intermediate versions.
- gcc < 4.1: no versions on godbolt.org
- clang >= 3.5: produces DW_AT_ranges, and (optionally) .debug_aranges
- 3.4 <= clang < 3.5: no DW_AT_ranges, no DW_AT_high_pc, .debug_aranges present.
- clang <= 3.3: no DW_AT_ranges, no DW_AT_high_pc, no .debug_aranges
- icc >= 16.0.1: produces DW_AT_ranges
- icc < 16.0.1: no functional versions on godbolt.org (some are present but fail to compile)

Based on this analysis, I believe it is safe to start trusting
DW_AT_low/high_pc information in dwarf as well as remove the code for
manually reconstructing range information by traversing the DIE
structure, and just keep the line table fallback. The only compilers
where this will change behavior are pre-3.4 clangs, which are almost 7
years old now. However, the functionality should remain unchanged
because we will be able to reconstruct this information from the line
table, which seems to be needed for some line-tables-only scenarios
anyway (haven't researched this too much, but at least some compilers
seem to emit DW_AT_ranges even in these situations).

In return, we simplify the code base, remove some untested code (the
only test changes are recent tests with overly reduced synthetic dwarf),
and increase llvm convergence.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D78489

Files:
  lldb/source/Plugins/SymbolFile/DWARF/DWARFCompileUnit.cpp
  lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugInfoEntry.cpp
  lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugInfoEntry.h
  lldb/test/Shell/SymbolFile/DWARF/DW_AT_loclists_base.s
  lldb/test/Shell/SymbolFile/DWARF/debug_loc.s

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