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buildflags.md: Document the post-processing on ELF files

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Florian Weimer 2021-11-16 13:01:45 +01:00
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@ -72,7 +72,7 @@ For other considerations involving shared objects, see:
* [Fedora Packaging Guidelines: Shared Libraries](https://docs.fedoraproject.org/en-US/packaging-guidelines/#_shared_libraries)
# Customizing compiler flags
# Customizing compiler and other build flags
It is possible to set RPM macros to change some aspects of the
compiler flags. Changing these flags should be used as a last
@ -216,6 +216,72 @@ it is possible to add `-fcommon` to the flags by defining `%_legacy_common_suppo
Properly fixing the failure is always preferred!
### Post-build ELF object processing
By default, DWARF debugging information is separated from installed
ELF objects and put into `-debuginfo` subpackages. To disable most
debuginfo processing (and thus the generation of these subpackages),
define `_enable_debug_packages` as `0`.
Processing of debugging information is controlled using the
`find-debuginfo` tool from the `debugedit` package. Several aspects
of its operation can be controlled at the RPM level.
* Creation of `-debuginfo` subpackages is enabled by default.
To disable, undefine `_debuginfo_subpackages`.
* Likewise, `-debugsource` subpackages are automatically created.
To disable, undefine `_debugsource_subpackages`.
See [Separate Subpackage and Source Debuginfo](https://fedoraproject.org/wiki/Changes/SubpackageAndSourceDebuginfo)
for background information.
* `_build_id_links`, `_unique_build_ids`, `_unique_debug_names`,
`_unique_debug_srcs` control how debugging information and
corresponding source files are represented on disk.
See `/usr/lib/rpm/macros` for details. The defaults
enable parallel installation of `-debuginfo` packages for
different package versions, as described in
[Parallel Installable Debuginfo](https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo).
* By default, a compressed symbol table is preserved in the
`.gnu_debugdata` section. To disable that, undefine
`_include_minidebuginfo`.
* To speed up debuggers, a `.gdb_index` section is created. It can be
disabled by undefining `_include_gdb_index`.
* Missing build IDs result in a build failure. To ignore such
problems, undefine `_missing_build_ids_terminate_build`.
* During processing, build IDs are recomputed to match the binary
content. To skip this step, define `_no_recompute_build_ids` as `1`.
* By default, the options in `_find_debuginfo_dwz_opts` turn on `dwz`
(DWARF compression) processing. Undefine this macro to disable this
step.
* Additional options can be passed by defining the
`_find_debuginfo_opts` macro.
After separation of debugging information, additional transformations
are applied, most of them also related to debugging information.
These steps can be skipped by undefining the corresponding macros:
* `__brp_strip`: Removal of leftover debugging information. The tool
specified by the `__strip` macro is invoked with the `-g` option on
ELF object (`.o`) files.
* `__brp_strip_static_archive`: This is similar to `__brp_strip`, but
processes static `.a` archives instead.
* `__brp_strip_comment_note`: This step removes unallocated `.note`
sections, and `.comment` sections from ELF files.
* `__brp_strip_lto`: This step removes GCC LTO intermediate representation
in ELF sections starting with `.gnu.lto_` and `.gnu.debuglto_`. Skipping
this step is strongly discouraged because the tight coupling of LTO
data with the GCC version. The underlying tool is again determined by the
`__strip` macro.
* `__brp_llvm_compile_lto_elf`: This step replaces LLVM bitcode files
with object files, thereby removing LLVM bitcode from the installed
files. This transformation is applied to object files in static `.a`
archives, too.
* `__brp_ldconfig`: For each shared object on the library search path
whose soname does not match its file name, a symbolic link from the
soname to the file name is created. This way, these shared objects
are loadable immediately after installation, even if they are not yet
listed in the `/etc/ld.so.cache` file (because `ldconfig` has not been
invoked yet).
# Individual compiler flags
Compiler flags end up in the environment variables `CFLAGS`,