Ada bareboard custom linker section garbage collected: MSE not bumped, fix with KEEP()

Bareboard Ada/SPARK projects using a GNU ld linker script (for example an embedded-stm32f429disco runtime copy of ld_user/common-ROM.ld) built with section garbage collection (--gc-sections, the usual gprbuild/bareboard default), where a new custom input section holds data that nothing in the program references directly.

Confirm the section is really being dropped, not misplaced. Build with a map file (for example -Wl,-Map=map.txt in the linker switches) and look for the custom section in it. Success check: the output section is listed at size 0, or the input section appears under Discarded input sections, which proves garbage collection rather than an address problem.

Open the linker script (in the bareboard case a copy of ld_user/common-ROM.ld) and find the output section you added after .data, the one collecting the custom input section by name. Success check: the input section line is a plain wildcard match with no KEEP() around it, which matches the diagnosis.

Wrap that input section in KEEP() so the linker retains it even though nothing references it. Change the plain wildcard line for the section to the same line wrapped in KEEP, keeping the output section name, address, and any start/end symbols exactly as they were. Success check: the script diff is only the KEEP wrapper on that one line.

Rebuild from clean so the linker script change is actually picked up (a stale object or cached link can hide it). Success check: the build reruns the link step with the edited script.

Check the map file again. Success check: the custom output section now has its real size (4 bytes for a single 4-byte object), it is no longer in the discarded list, and the section end symbol has advanced by that size.

Verify on the board in gdb: print the end symbol again. Success check: MSE now reads 4 bytes higher than the run without the custom object (0x20000838 in the original STM32F429 Disco case instead of 0x20000834), and the custom object reads back its initialised value.

In the Ada forum thread 4932 (OP sidisyom, Nov 10, 2026): a custom .mysectiondata output section declared after .data in a copy of lduser/common-ROM.ld for the embedded-stm32f429disco runtime does not change MSE (gdb prints MSE as system.address 0x20000834) after including a 4-byte MySectionData object; the OP asks whether the section is being garbage collected and whether KEEP is needed, and the posted script has no KEEP on the input section. One reply (damaki, Nov 11) asks for the linker map file and gives no fix. Source: https://forum.ada-lang.io/t/custom-linker-entry-in-bareboard-runtime/4932