Toolchains may not keep unused variables placed with BSP_PLACE_IN_SECTION

[renesas-jbrabender]

Issue

When a variable is placed in a linker section via the BSP_PLACE_IN_SECTION but is unused, the toolchains will typically discard it.
In FSP versions prior to v6.0.0, linker command files would often include a "keep" directive for memory sections which would sometimes keep unused variables. In FSP v6.0.0 and greater those keep directives are not emitted (as they were unreliable in performing the task).

Workaround

If the source code does not actually use, the section variable, you can often convince the toolchain to not optimize it out by declaring it volatile and referencing it in source code. For example:

volatile uint8_t flash_write_buffer[4096] BSP_PLACE_IN_SECTION(".data_flash") BSP_ALIGN_VARIABLE(4);
...
(void) flash_write_buffer[0];