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
--keep linker options.
Unused section elimination requires an entry point. Therefore, if no entry
point is specified for an image, use the
option to specify an entry point and permit unused section elimination to work, if it is
--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
- It is specified as the first or last input section, either by the
--lastoption or by a scatter-loading equivalent.
- It is marked as unremovable by the
- 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.
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.