The GNU Fortran Compiler: Evaluation of logical expressions

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### 4.3 Evaluation of logical expressions

The Fortran standard does not require the compiler to evaluate all parts of an expression, if they do not contribute to the final result. For logical expressions with `.AND.` or `.OR.` operators, in particular, GNU Fortran will optimize out function calls (even to impure functions) if the result of the expression can be established without them. However, since not all compilers do that, and such an optimization can potentially modify the program flow and subsequent results, GNU Fortran throws warnings for such situations with the `-Wfunction-elimination` flag.