Inlining

Inlining is an optimization applied only at optimization level -O4 and only for functions contained within one file. That is, if one file contains 17 Fortran functions, 16 of those can be inlined into the first function, and you compile at -O4, then the source code for those 16 functions may be copied into the body of the first function. Then, when further optimizations are applied, it becomes difficult to determine which loop on which source line number was subjected to which optimization.

If the compiler hints seem particularly opaque, consider compiling with -O3 -parallel -Zlp, so that you can see what the compiler says about your loops before it tries to inline any of your functions.

In particular, "phantom" loops--that is, loops that the compiler claims exist, but you know do not exist in your source code--could well be a symptom of inlining.