Changes/DebugInfoLldbIndex

Debug Info LLDB Index

Summary

Provide .debug_names debug info index for LLDB for clang-built binaries using: clang -gdwarf-5 -gpubnames

Debuginfo index significantly accelerates loading of *.debug files by debugger. Fedora currently provides ELF section .gdb_index for GDB debugger. LLDB debugger cannot use .gdb_index (as it is missing DIE offsets for more effective processing by LLDB) but LLDB can use .debug_names index.

Owner

• Name: Jan Kratochvil
• Email: jan.kratochvil@redhat.com

Current status

• Targeted release: Fedora 34
• Last updated: 2020-10-07
• FESCo issue: #2487
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

There are currently 3 formats of debug info index:

• .gdb_index: It is currently produced in Fedora by GDB (/usr/bin/gdb-add-index), it is a part of rpmbuild process. It is compatible with GDB but incompatible with LLDB as it is missing essential DIE offsets needed by LLDB due to more effective (faster) reading of DWARF by LLDB.

• .debug_names from GDB (augmentation "GDB\x00"): It can be produced by GDB (/usr/bin/gdb-add-index -dwarf-5) but its format is non-conforming to DWARF-5 standard. LLDB expects DWARF-5 standard compliant .debug_names and therefore it is incompatible with this format. It can be expected GDB will fix the conformance in the future. Currently GDB .debug_names format has no advantage over GDB .gdb_index format.

• .debug_names from clang (augmentation "LLVM0700"): It can be produced by clang (clang -gdwarf-5 -gpubnames) for LLDB. It is conforming to DWARF-5 standard, one can expect GDB will be able to read it in the future.

It would be good to produce index from GCC by GDB and to produce index from clang by clang as the compatibility inside the same toolchain is best tested and supported. Using index across toolchains (index from GDB by LLDB or index from clang by GDB) should theoretically work but in practice there exist subtle differences in interpretation of more complicated DWARF constructs. It would be best to fix those but that will be always an afterthought.

Benefit to Fedora

• Faster startup of LLDB debugger using Fedora system *.debug files.

Scope

• Proposal owners: It affects all clang-built packages generating *-debuginfo.rpm.

• Other developers: none

• Release engineering: #Releng issue number (a check of an impact with Release Engineering is needed)

• Policies and guidelines: All the needed changes should be done in redhat-rpm-config. The dwz package can be then retired.

• Trademark approval: N/A (not needed for this Change)

• Alignment with Objectives: The size differences are only for *-debuginfo.rpm which is outside of scope of the listed objectives.

Currently the change will affect only packages using:

%global toolchain clang

Those are currently only these packages being built by clang and using this %toolchain framework: dotnet3.1 libcxxabi

FIXME: Which other Fedora packages are being built by clang?

Upgrade/compatibility impact

Existing tools not supporting .debug_names will just ignore the additional ELF section. The only issue is current GDB would get confused by the clang .debug_names as it expects .debug_names to be in its incompatible GDB format - FIXME: Provide a GDB bugfix patch.

Also each *-debuginfo.rpm has to exactly match NVRA of its binary package the Fedora change compatibility is not applicable.

How To Test

GDB should not get affected by the new .debug_names index from clang.

LLDB should load Fedora system *.debug files faster. LLDB functionality should not be affected by the index from clang (that is a part of LLVM development/testsuite).

"llvm-dwarfdump -debug-names *.debug" should show: Augmentation: 'LLVM0700'

User Experience

No user visible change. This affects what tools can developers use.

Dependencies

This Change is dependent on how is decided Changes/DebugInfoStandardization.

This Change is dependent on DWARF-5 F-34 feature expected to be filed by Mark Wielaard.

Mass rebuild is not required. Packages inherited from F-33 will just miss the LLDB index and LLDB will load them more slowly.

• .debug_names would need to be updated by DWZ but DWZ does not plan to support .debug_names (according to Mark Wielaard).
  • If DWZ is dropped (Changes/DebugInfoStandardization gets approved) then clang can normally produce .debug_names for LLDB.
  • If DWZ stays in use (Changes/DebugInfoStandardization gets rejected) then there are multiple options. LLDB currently cannot produce .debug_names (only clang can). GDB currently produces incompatible .debug_names format.
    • Changes/DebugInfoStandardization should be applied at least for clang-built packages, preferred by this proposal.
    • There was an idea DWZ would remove .debug_names. That would effectively reject this Change and make LLVM Toolchain slower due to a deficiency of the DWZ tool.
    • There isn't anyone willing to implement updating of .debug_names into DWZ, moreover DWARF-5 standard does not specify a needed format for combination of both DWZ and .debug_names. Also LLDB currently does not support the DWZ format anyway.
    • When GDB produces .debug_names compatible with DWARF-5 in the future it could theoretically produce .debug_names for LLDB. But that would mean the index for LLDB (and therefore interpretation of DWARF by LLDB) would be affected by interpretation of DWARF by GDB, that is a toolchain compatibility nightmare.
• clang produces more effective .debug_names (one per executable; not one per *.o compilation unit) when using -flto which is default since F-33 now.

Contingency Plan

• Contingency mechanism: Revert the change in redhat-rpm-config. LLDB can continue loading Fedora system *.debug files slightly slower.
• Contingency deadline: beta freeze
• Blocks release? No
• Blocks product? N/A

Documentation

• DWARF-5 6.1.1 Lookup by Name