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

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From Fedora commit ff647eda53.
This leaves a stale reference to __brp_llvm_compile_lto_elf,
which will come alive with #2022522.

Related: #2005080
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Florian Weimer 2021-11-16 13:01:45 +01:00
parent 6ec24c5e1f
commit 33f39cb8c3
1 changed files with 67 additions and 1 deletions
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@ -69,7 +69,7 @@ For other considerations involving shared objects, see:
* [Fedora Packaging Guidelines: Shared Libraries](https://docs.fedoraproject.org/en-US/packaging-guidelines/#_shared_libraries) * [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 It is possible to set RPM macros to change some aspects of the
compiler flags. Changing these flags should be used as a last compiler flags. Changing these flags should be used as a last
@ -213,6 +213,72 @@ it is possible to add `-fcommon` to the flags by defining `%_legacy_common_suppo
Properly fixing the failure is always preferred! 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 # Individual compiler flags
Compiler flags end up in the environment variables `CFLAGS`, Compiler flags end up in the environment variables `CFLAGS`,