Mangled code is code which has had all comments, except for some licensing comments, and unnecessary blanks removed.   In addition any name which can be treated as a local variable is replaced by in order of appearance, A, B, ..., Z, A0, ..., A9, AA, ..., AZ, ..., etc. in the case of Fortran, and starting with aA, ..., aZ, a0, ..., z9, ..., bA, ... for C.   It is not intended to be read by human eyes, but is equivalent to the regular source as far as a compiler is concerned.   The order of the initial lines is not changed.
Whenever we make changes in the library, the output from running the code generated from mangled source is compared with that from the clean source. There has yet to be a single bit of difference. Also, at least on our machine, the object files are the same.