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Compilers, what they might do ...

By FANG LU posted Tue March 24, 2020 07:37 PM


Ever wonder what compiler does to your code? I suppose this is of little importance if binary is generated quickly, it executes fast and yields correct result. I regard compiler as an interactive tool. Given that the destination is the same (the executable), compiler provides many routes for your source code to reach it.

Whether you are invoking the compiler via a JCL, under TSO, or ISPF panel, or you are using c89/xlc commands in z/OS USS environment, you can utilize a variety of compiler options that can affect how the source is treated during the compilation process.

For example, if your source is in C++ and declares/defines a whole bunch of generic types, you may want to use one of the C++ template options, FASTTEMPINC, TEMPINC, TEMPLATERECOMPILE, and etc. Or, if you are planning to debug a run-time problem, you may want to pass DEBUG (Batch/TSO) or –g (z/OS USS) to the compiler. Or you want to enable optimization, then you specify OPTIMIZE.

But, let’s say you are in a similar situation as I was this past week, and want to disable optimization at the subprogram level. I wanted the entire compilation unit, CU, to be compiled at O3 except one, non-overloaded function. I used* #pragma option_override* to bump down the optimization level to O2 for that one function.

In summary, compilers may do a lot of things to your code, but at the same time they allow you full control.