Which part of optimization is most im****tant in a compiler?

Hi,all

   I haven't taken the compiler course at college. Because my job is
to optimize code on DSP, I hope to understand compiler deep and begin
to read a compiler textbook. There are a lot of materials in a book,
but the overview of advanced compiling is not given in the book.

Maybe every part is im****tant for a optimized compiler. I hope to know
which part of a compiler is most im****tant in a compiler. Instruction
scheduling, or register allocation?

Best Regards
Jogging