GIMPLE is a three-address representation derived from GENERIC by breaking down GENERIC expressions into tuples of no more than 3 operands (with some exceptions like function calls). GIMPLE was heavily influenced by the SIMPLE IL used by the McCAT compiler project at McGill University, though we have made some different choices. For one thing, SIMPLE doesn’t support goto.

Temporaries are introduced to hold intermediate values needed to compute complex expressions. Additionally, all the control structures used in GENERIC are lowered into conditional jumps, lexical scopes are removed and exception regions are converted into an on the side exception region tree.

The compiler pass which converts GENERIC into GIMPLE is referred to as the ‘gimplifier’. The gimplifier works recursively, generating GIMPLE tuples out of the original GENERIC expressions.

One of the early implementation strategies used for the GIMPLE representation was to use the same internal data structures used by front ends to represent parse trees. This simplified implementation because we could leverage existing functionality and interfaces. However, GIMPLE is a much more restrictive representation than abstract syntax trees (AST), therefore it does not require the full structural complexity provided by the main tree data structure.

The GENERIC representation of a function is stored in the DECL_SAVED_TREE field of the associated FUNCTION_DECL tree node. It is converted to GIMPLE by a call to gimplify_function_tree.

If a front end wants to include language-specific tree codes in the tree representation which it provides to the back end, it must provide a definition of LANG_HOOKS_GIMPLIFY_EXPR which knows how to convert the front end trees to GIMPLE. Usually such a hook will involve much of the same code for expanding front end trees to RTL. This function can return fully lowered GIMPLE, or it can return GENERIC trees and let the main gimplifier lower them the rest of the way; this is often simpler. GIMPLE that is not fully lowered is known as “High GIMPLE” and consists of the IL before the pass pass_lower_cf. High GIMPLE contains some container statements like lexical scopes (represented by GIMPLE_BIND) and nested expressions (e.g., GIMPLE_TRY), while “Low GIMPLE” exposes all of the implicit jumps for control and exception expressions directly in the IL and EH region trees.

The C and C++ front ends currently convert directly from front end trees to GIMPLE, and hand that off to the back end rather than first converting to GENERIC. Their gimplifier hooks know about all the _STMT nodes and how to convert them to GENERIC forms. There was some work done on a genericization pass which would run first, but the existence of STMT_EXPR meant that in order to convert all of the C statements into GENERIC equivalents would involve walking the entire tree anyway, so it was simpler to lower all the way. This might change in the future if someone writes an optimization pass which would work better with higher-level trees, but currently the optimizers all expect GIMPLE.

You can request to dump a C-like representation of the GIMPLE form with the flag -fdump-tree-gimple.