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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.

Note

By default, unused section elimination is disabled if you are building DLLs with --dll, or shared libraries with --shared. Therefore, you must explicitly include --remove to re-enable unused section elimination.

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 mark a function or variable in source code with the __attribute__((used)) attribute. This causes armcc to generate the symbol __tagsym$$used for each of the functions and variables. A section containing a definition of __tagsym$$used is not removed by unused section elimination.

You can also use the --split_sections compiler command-line option to instruct the compiler to generate one ELF section for each function in the source file.

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