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Elimination of unused sections

Elimination of unused sections is the most significant optimization on image size that is performed by the linker.

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 there is no entry point 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.
It is referred to, directly or indirectly, by a non-weak reference from an input section containing an entry point.
It is specified as the first or last input section by the --first or --last option (or a scatter-loading equivalent).
It is marked as unremovable by the --keep option.

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.