buildflags.md: Document the post-processing on ELF files
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Florian Weimer
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@ -72,7 +72,7 @@ For other considerations involving shared objects, see:
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* [Fedora Packaging Guidelines: Shared Libraries](https://docs.fedoraproject.org/en-US/packaging-guidelines/#_shared_libraries)
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* [Fedora Packaging Guidelines: Shared Libraries](https://docs.fedoraproject.org/en-US/packaging-guidelines/#_shared_libraries)
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# Customizing compiler flags
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# Customizing compiler and other build flags
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It is possible to set RPM macros to change some aspects of the
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It is possible to set RPM macros to change some aspects of the
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compiler flags. Changing these flags should be used as a last
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compiler flags. Changing these flags should be used as a last
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@ -216,6 +216,72 @@ it is possible to add `-fcommon` to the flags by defining `%_legacy_common_suppo
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Properly fixing the failure is always preferred!
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Properly fixing the failure is always preferred!
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### Post-build ELF object processing
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By default, DWARF debugging information is separated from installed
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ELF objects and put into `-debuginfo` subpackages. To disable most
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debuginfo processing (and thus the generation of these subpackages),
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define `_enable_debug_packages` as `0`.
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Processing of debugging information is controlled using the
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`find-debuginfo` tool from the `debugedit` package. Several aspects
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of its operation can be controlled at the RPM level.
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* Creation of `-debuginfo` subpackages is enabled by default.
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To disable, undefine `_debuginfo_subpackages`.
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* Likewise, `-debugsource` subpackages are automatically created.
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To disable, undefine `_debugsource_subpackages`.
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See [Separate Subpackage and Source Debuginfo](https://fedoraproject.org/wiki/Changes/SubpackageAndSourceDebuginfo)
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for background information.
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* `_build_id_links`, `_unique_build_ids`, `_unique_debug_names`,
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`_unique_debug_srcs` control how debugging information and
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corresponding source files are represented on disk.
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See `/usr/lib/rpm/macros` for details. The defaults
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enable parallel installation of `-debuginfo` packages for
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different package versions, as described in
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[Parallel Installable Debuginfo](https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo).
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* By default, a compressed symbol table is preserved in the
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`.gnu_debugdata` section. To disable that, undefine
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`_include_minidebuginfo`.
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* To speed up debuggers, a `.gdb_index` section is created. It can be
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disabled by undefining `_include_gdb_index`.
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* Missing build IDs result in a build failure. To ignore such
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problems, undefine `_missing_build_ids_terminate_build`.
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* During processing, build IDs are recomputed to match the binary
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content. To skip this step, define `_no_recompute_build_ids` as `1`.
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* By default, the options in `_find_debuginfo_dwz_opts` turn on `dwz`
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(DWARF compression) processing. Undefine this macro to disable this
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step.
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* Additional options can be passed by defining the
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`_find_debuginfo_opts` macro.
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After separation of debugging information, additional transformations
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are applied, most of them also related to debugging information.
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These steps can be skipped by undefining the corresponding macros:
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* `__brp_strip`: Removal of leftover debugging information. The tool
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specified by the `__strip` macro is invoked with the `-g` option on
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ELF object (`.o`) files.
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* `__brp_strip_static_archive`: This is similar to `__brp_strip`, but
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processes static `.a` archives instead.
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* `__brp_strip_comment_note`: This step removes unallocated `.note`
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sections, and `.comment` sections from ELF files.
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* `__brp_strip_lto`: This step removes GCC LTO intermediate representation
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in ELF sections starting with `.gnu.lto_` and `.gnu.debuglto_`. Skipping
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this step is strongly discouraged because the tight coupling of LTO
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data with the GCC version. The underlying tool is again determined by the
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`__strip` macro.
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* `__brp_llvm_compile_lto_elf`: This step replaces LLVM bitcode files
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with object files, thereby removing LLVM bitcode from the installed
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files. This transformation is applied to object files in static `.a`
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archives, too.
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* `__brp_ldconfig`: For each shared object on the library search path
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whose soname does not match its file name, a symbolic link from the
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soname to the file name is created. This way, these shared objects
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are loadable immediately after installation, even if they are not yet
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listed in the `/etc/ld.so.cache` file (because `ldconfig` has not been
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invoked yet).
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# Individual compiler flags
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# Individual compiler flags
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Compiler flags end up in the environment variables `CFLAGS`,
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Compiler flags end up in the environment variables `CFLAGS`,
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