Two fine points to consider

The compiler discards any unused code and data
This includes all static functions you don't call
And also data structures you don't refer to.
In the blatant case, a warning is reported
"function F defined but not used"
"unused variable V"

Constants known at build time help in this decision
This is why "if (0)" is good trick
No warning about "unused code/data" is reported
But still the function/data is discarded

Additionally, the linker eventually discards unused sections
This only if "--gc-sections" is requested

Thus, you need to protect precious data
Use "__attribute__((used))" in source files
Use KEEP(section) in the liker script