https://fedoraproject.org/wiki/Changes/DebugInfoStandardization == Summary == Fedora 18 implemented [[Features/DwarfCompressor]]. As the format did not get widespread and the tool is not much maintained it became burden to make existing debugging tools compatible with Fedora debug info. == Owner == * Name: [[User:jankratochvil| Jan Kratochvil ]] * Email: jan.kratochvil(a)redhat.com == Detailed Description == Debug info files *.debug contained in *-debuginfo.rpm are very big in general (x86_64 Fedora 32 distribution has debug/ directory of 82GB while all its other files are only 75GB). There exist several methods how to make the *-debuginfo.rpms at least a bit smaller. Fedora 18 started using DWZ tool (from [[Features/DwarfCompressor]]) while [https://gcc.gnu.org/pipermail/gcc-patches/2008-August/246281.html Google implemented] the same goal in a different way called -fdebug-types-section. Almost nobody uses existing Fedora DWZ (only Fedora/CentOS/RHEL and SuSE OSes) and so its support is missing in tools like [https://lldb.llvm.org/ LLDB], [[llvm-dwarfdumphttps://llvm.org/docs/CommandGuide/llvm-dwarfdump.html|llvm-dwarfdump]] or binutils readelf. -fdebug-types-section is used internally by Google (produced by clang). Debian does not store any debug info archives. Ubuntu uses neither -fdebug-types-section nor DWZ. * DWZ advantage: On the whole Fedora distro it saves 3.3% (5GB of the 157GB distribution size) ** If the 3.3% size increase is a concern I can implement a different optimization ([https://whova.com/embedded/session/llvm_202010/1193947/ talk (2)]) as a GCC post-processing phase which would require no changes in any DWARF consumers. * DWZ disadvantage: DWZ has currently less support across consumers (LLDB, llvm-dwarfdump, binutils readelf) * DWZ disadvantage: DWZ requires 8x times more complicated (LoC count) support in consumers than -fdebug-types-section. * DWZ disadvantage: DWZ cannot update LLVM .debug_names index which can be generated only by clang (it cannot be regenerated later for DWZ-compressed file) * DWZ disadvantage: DWZ DWARF-5 support is a work-in-progress. DWZ has been blocking DWARF-5 for Fedora for 3.5 years and only after I have now proposed to drop DWZ Mark Wielaard has started porting DWZ to DWARF-5. It can be expected next DWARF extensions will remain unsupported again. Even currently there is no plan to support DWARF-5 features used by clang which may need -fdebug-types-section for clang-built binaries or no size optimization of clang-built debug info at all. * DWZ disadvantage: Compilation (linking) requires for C++ up to 2x as big disk space (as DWZ is processing files after linker and DWZ is incompatible with -fdebug-types-section) * DWZ disadvantage: Compilation (linking) is slower This proposed DWARF format was originally submitted already for Fedora 18 as [[Features/DebugTypesSections]]. == Benefit to Fedora == * Better compatibility with existing debugging and tracing tools, primarily [https://lldb.llvm.org/ LLDB]. * Less resource-intensive rebuilds of C++ packages (in disk space, memory requirements and compilation time). == Scope == * Proposal owners: It affects all packages generating *-debuginfo.rpm, that is compiled (not scripted) languages. * Other developers: Report any possible debuginfo incompatibility (unexpected). * Release engineering: [https://pagure.io/releng/issues #Releng issue number] (a check of an impact with Release Engineering is needed) * Policies and guidelines: All the needed changes should be done in [https://src.fedoraproject.org/rpms/redhat-rpm-config redhat-rpm-config]. The [https://src.fedoraproject.org/rpms/dwz 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. == Upgrade/compatibility impact == As *-debuginfo.rpm have to exactly match NVRA of its binary package the compatibility is not relevant. Existing tools supporting DWZ will still support the DWZ file format in packages which have not been rebuilt. == How To Test == The change will update [https://src.fedoraproject.org/rpms/redhat-rpm-config redhat-rpm-config] by [https://people.redhat.com/jkratoch/redhat-rpm-config-fdebug-types-section... an -fdebug-types-section patch]. Then one can use rpmbuild to rebuild a package. For mock use -a|--addrepo with modified redhat-rpm-config.rpm (with increased NVRA). For packages already rebuilt in Koji nothing is needed. Test programs like lldb and gdb if they still can print source code, function parameters, variables etc. One should also verify integrated testsuites of tools like clang, lldb, gcc, binutils, gdb, elfutils or rpm are not regressing with the -fdebug-types-section option. One can also compare *.debug files built with/without DWZ and/or -fdebug-types-section using [https://src.fedoraproject.org/rpms/libabigail libabigail] utility dwdiff but that will be rather done by the change owner. == User Experience == No user visible change. This affects what tools can developers use. == Dependencies == none == Contingency Plan == * Contingency mechanism: Revert the change in [https://src.fedoraproject.org/rpms/redhat-rpm-config redhat-rpm-config]. Fedora can continue using DWZ, just some debugging/tracing tools will stay incompatible. * Contingency deadline: beta freeze * Blocks release? No * Blocks product? N/A == Documentation == * [http://www.dwarfstd.org/doc/DWARF5.pdf DWARF-5] E.2 Using Type Units * [https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#index-fdebug-ty... GCC -fdebug-types-section] -- Ben Cotton He / Him / His Senior Program Manager, Fedora & CentOS Stream Red Hat TZ=America/Indiana/Indianapolis