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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 ARMv8-M Security Extensions.
  • It is an SHT_INIT_ARRAY, SHT_FINI_ARRAY, or SHT_PREINIT_ARRAY section.
  • It is specified as the first or last input section, either by the --first or --last option or by a scatter-loading equivalent.
  • It is marked as unremovable by the --keep option.
  • 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.

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