Elimination of unused sections
Elimination of unused sections is the most significant optimization on image size that the linker performs.
Unused section elimination:
- Removes unreachable code and data from the final image.
- Is suppressed in cases that might result in the removal of all sections.
To control this optimization, use the --remove, --no_remove, --first, --last, and --keep linker options.
Unused section elimination requires an entry point. Therefore, if no entry
point is specified for an image, use the --entry linker
option to specify an entry point and permit unused section elimination to work, if it is
enabled.
Use the --info unused linker option to instruct the linker to generate a
list of the unused sections that it eliminates.
An input section is retained in the final image when:
- It contains an entry point or an externally accessible symbol, for example, an entry function into the secure code for Arm®v8‑M Security Extensions.
- It is an
SHT_INIT_ARRAY,SHT_FINI_ARRAY, orSHT_PREINIT_ARRAYsection. - It is specified as the first or last input section, either by the
--firstor--lastoption or by a scatter-loading equivalent. - It is marked as unremovable by the
--keepoption. - It is referred to, directly or indirectly, by a non-weak reference from an input section retained in the image.
- Its name matches the name referred to by an input section symbol, and that symbol is referenced from a section that is retained in the image.
Note
Compilers usually collect functions and data together and emit one section for each category. The linker can only eliminate a section if it is entirely unused.
You can also use the -ffunction-sections compiler command-line option to
instruct the compiler to generate one ELF section for each function in the source
file.