using -ffile-prefix-map

Hi, I'm using 10-2020-q4  release of the arm toolchain and got stuck trying to use -ffile-prefix-map. I'm trying to add it to a cmake debug build and it never seems to have any effect on the elf file produced. Has anyone succeeded in using it ? I'm trying to remove as many sources of build variability as I can. If some kind soul has a working cmake driven build that uses this option successfully and how it was added to CMakeLists.txt that would be great. 

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