Here are the data structures with brief descriptions:

backend_params	This structure contains parameters that should be propagated to the libFirm parameter set
hook_entry_t	Hook entry
i_call_record	An intrinsic call record
i_instr_record	An intrinsic instruction record
i_record	An intrinsic record
ir_asm_constraint	A input/output constraint attribute
ir_op_ops	Io_op Operations
ir_settings_arch_dep_t	A parameter structure that drives the machine dependent Firm optimizations
loop_element	Loop elements: loop nodes and ir nodes
plist_element_t	An element in the pointer list
plist_t	The plist data type
pmap_entry	A key, value pair
pset_entry	The entry of a pset, representing an element pointer in the set and its meta-information
runtime_rt	A runtime routine description
set_entry	The entry of a set, representing an element in the set and its meta-information
symconst_symbol	SymConst attribute
tarval_mode_info	This structure contains helper information to format the output of a tarval of a mode
type_or_ent	A data type to treat types and entities as the same
vrp_attr	VRP information for a single node