GNU Compiler Collection (GCC) Internals: Gimplification pass

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### 9.3 Gimplification pass

Gimplification is a whimsical term for the process of converting the intermediate representation of a function into the GIMPLE language (see GIMPLE). The term stuck, and so words like “gimplification”, “gimplify”, “gimplifier” and the like are sprinkled throughout this section of code.

While a front end may certainly choose to generate GIMPLE directly if it chooses, this can be a moderately complex process unless the intermediate language used by the front end is already fairly simple. Usually it is easier to generate GENERIC trees plus extensions and let the language-independent gimplifier do most of the work.

The main entry point to this pass is `gimplify_function_tree` located in `gimplify.c`. From here we process the entire function gimplifying each statement in turn. The main workhorse for this pass is `gimplify_expr`. Approximately everything passes through here at least once, and it is from here that we invoke the `lang_hooks.gimplify_expr` callback.

The callback should examine the expression in question and return `GS_UNHANDLED` if the expression is not a language specific construct that requires attention. Otherwise it should alter the expression in some way to such that forward progress is made toward producing valid GIMPLE. If the callback is certain that the transformation is complete and the expression is valid GIMPLE, it should return `GS_ALL_DONE`. Otherwise it should return `GS_OK`, which will cause the expression to be processed again. If the callback encounters an error during the transformation (because the front end is relying on the gimplification process to finish semantic checks), it should return `GS_ERROR`.

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