The Middle-End of ["GCC"] performs ["SSA"] based optimizations on ["GIMPLE"], then converts the ["GIMPLE"] to ["RTL"] and does more optimizations. Finally it hands it off the optimized RTL to the BackEnd.
Work has started on performing ["Interprocedural optimizations"] in GCC. See also the ["ipa-branch"] and the -fwhole-program compilation flag (useful when passing a whole program source as several compilation units passed together to the compiler).
middle end passes
The following passes are done on the GIMPLE SSA representation. We are supposing -O2 optimization level (and perhaps more, even -fwhole-program)
See chapter 8 [http://gcc.gnu.org/onlinedocs/gccint/Tree_002dSSA-passes.html Tree-SSA passes] of the GCC internal documentation for an overview
please check and complete this list
Many passes are working on trees before the SSA, in particular for the call flow graph (the pass_build_cfg is building it in file gcc/tree-cfg.c) and tree inlining (pass_ipa_inline in file gcc/ipa-inline.c)
the pass_build_ssa (in file gcc/tree-into-ssa.c) is building the SSA representation
the pass_lower_vector and pass_lower_vector_ssa (file gcc/tree-vect-generic.c) are expanding vector to scalar operations (???)
The pass_may_alias (from gcc/tree-ssa-alias.c) computes may-alias information
The pass_return_slot (from gcc/tree-nrv.c) implements return value optimizations for functions which return aggregate types
The pass_rename_ssa_copies (from gcc/tree-ssa-copyrename.c) implements the SSA copy renaming phase
The pass_fre (from gcc/tree-ssa-pre.c) makes some full redunduncy elimination (?)
The pass_ccp (from gcc/tree-ssa-ccp.c) does conditional constant propagation
The pass_dce (from gcc/tree-ssa-dce.c) for dead code elimination
The pass_forwprop (from gcc/tree-ssa-forwprop.c) for forward propagation of expressions for single use variables
The pass_copyprop (from gcc/tree-ssa-copy.c) is the copy propagator
The pass_merge_phi (from gcc/tree-cfgcleanup.c) is a merging CFG cleanup
The pass_vrp (from gcc/tree-vrp.c) is for value range propagation
The pass_dce (in gcc/tree-ssa-dce.c) is dead code elimination on SSA
The pass_dominator and pass_phi_only_cprop (both in gcc/tree-ssa-dom.c) implement optimizations on the dominator tree.
The pass_phiopt (in gcc/tree-ssa-phiopt.c) optimizes PHI nodes by converting them into straightline code
The ["tree folder"] is also considered part of the middle-end, but the FrontEnds use it for constant folding also.