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The FFE does not tell the GBE anything about a program unit until after the last statement in that unit has been parsed. (A program unit is a Fortran concept that corresponds, in the C world, mostly closely to functions definitions in ISO C. That is, a program unit in Fortran is like a top-level function in C. Nested functions, found among the extensions offered by GNU C, correspond roughly to Fortran's statement functions.)
So, while parsing the code in a program unit, the FFE saves up all the information on statements, expressions, names, and so on, until it has seen the last statement.
At that point, the FFE revisits the saved information (in what amounts to a second pass over the program unit) to perform the actual translation of the program unit into GBEL, ultimating in the generation of assembly code for it.
Some lookahead is performed during this second pass, so the FFE could be viewed as a "two-plus-pass" design.
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