FIRM is a full graph based intermediate representation in SSA Form with a novel concept to model side effects. It allows fast, aggressive optimizations.

The internal representation of a program in firm is separated into five different modules:

• Firm Graphs representing the code of a program. (Subdirectory ir.) Firm Graphs are assembled out of several data structures: irprog: represents a program. Allows access to all types and all FIRM graphs for procedures and other global things. irgraph: represents a procedure. Allows access to the code of the procedure, the actual FIRM graph. irnode: A node of a FIRM graph. Nodes are typed with an opcode and a mode and represent instructions in a program. irop: The opcode of FIRM nodes. irmode: The mode of FIRM nodes. Most modes correspond to machine known data types (int, float, pointer).
• Entities representing program known objects. (Subdirectory tr.) All variables and procedures are entities.
• Types describing the type system for the program. (Subdirectory tr.)
• Target Values representing program known constants. (Subdirectory tv.)
• Identifiers representing any Strings used in the program. (Subdirectory ident.)

Further this library supplies functionality to build and optimize FIRM graphs and further functionality needed in a compiler. Finally there is more generic functionality to support implementations using firm. (Code generation, further optimizations).