explaination of some gcc functions

[kernel coder]

hi,
    I'm trying to understand the backend code of gcc for MIPS
architecture.I'm having some trouble while understanding following
functions.

1:  push_topmost_sequence();
2: emit_insn_after(seq,get_insns());
3: pop_topmost_sequence();
4: emit_insn_before

Would you please explain what's the role of each function.